Global Geography IN A ONE EYE

COMPOSITION OF THE SOLAR SYSTEM

THE PLANETS

1. Mercury
   1. Closest planet to the Sun
   2. Smallest planet
   3. No satellites
2. Venus
   1. Very dense atmosphere
   2. Hottest planet (over 400 C)
   3. Large amount of greenhouse gases
3. Earth
   1. Hydrosphere unique among rock-based planets
      
   2. Only planet where plate tectonics observed
      
4. Mars
   1. Atmosphere made mainly of carbon dioxide
      
   2. Red colour comes from iron oxide
   3. Geological activity such as volcanoes as recently as 2 million years ago
      
5. Jupiter
   1. 2.5 times the masses of all other planets combined
      
   2. Composed mainly of hydrogen and helium
      
   3. Great Red Spot in atmosphere created by strong internal heat
   4. 63 known satellites. Ganymede, Callisto, Io and Europa show similarities to terrestrial planets such as volcanism and internal heating
      
   5. Ganymede is the largest satellite in the solar system. It is larger than mercury
      
   6. Has planetary ring system
      
6. Saturn
   1. Extensive ring system
      
   2. Rings made by small particles of water ice clumped together
      
   3. Rings first observed by Galileo
      
   4. Least dense planet in solar system
      
   5. 60 satellites
      
   6. Titan is the only satellite in the solar system with a substantial atmosphere
7. Uranus
   1. Orbits the sun on its side
   2. Very cold core, radiates little heat
      
   3. 27 satellites
      
   4. Has planetary ring system
      
8. Neptune
   1. Smaller in size but more massive and more dense than Neptune
      
   2. 13 known satellites
      
   3. Has planetary ring system
      

OTHER COMPONENTS

1. Asteroids
   1. Small objects composed of rocky and metallic minerals
      
   2. Small asteroids are called meteoroids
      
   3. Main asteroid belt occupies orbit between Mars and Jupiter
      
   4. Asteroid belt is sparsely populated. Spacecraft routinely pass through belt without incident
   5. Ceres is the largest body in the asteroid belt. Classified as a dwarf planet
2. Comets
   1. Small bodies composed of volatile ices
      
   2. Coma of a comet (tail) is observed when proximity to the sun causes the icy particles to sublimate and ionize
   3. Hale-Bopp comet was visible to the naked eye for 18 months. It was the most widely observed and brightest comet recorded
      
3. Interplanetary medium
   1. It Is the interplanetary atmosphere created by the stream of charged particles emitted by the Sun (called solar wind)
      
   2. Aka Heliosphere
      
   3. Stretches out to 100 AU
      
   4. Space weather is created by geomagnetic storms on the Sun’s surface which disturb the heliosphere
   5. Earth’s magnetic field prevents solar wind from stripping away the Earth’s atmosphere
4. Kuiper belt
   1. Ring of debris, similar to asteroid belt, but composed mainly of ice
      
   2. Present in the area beyond Neptune
      
   3. Pluto is the largest object in the Kuiper belt

GALACTIC CONTEXT OF THE SOLAR SYSTEM

1. Located in the Milky Way galaxy, a spiral galaxy. Milky Way diameter 100000 light years. Contains about 200 billion stars
   
2. Solar System resides in one of the outer arms, called Orion Arm
   
3. Sun lies around 25,000 light years from galactic centre. Completes one revolution every 250 million years (aka Cosmic Year)
   
4. Closest star is Alpha Centauri triple star system
   
5. Largest star close to Sun is Sirius
   
6. Closest sun-like star is Tau Ceti
   
7. Closest extra-solar planet is Epsilon Eridani b, which orbits the star.r Epsilon Eridani
   

IMPORTANT GEOLOGICAL FEATURES IN THE SOLAR SYSTEM

1. Venus
   1. Ishtar Terra and Aphrodite Terra: two continents on Venus
      
   2. Maxwell Montes: highest mountain on Venus
      
2. Mars
   1. Adirondack: first rock chosen to be explored by the first Mars rover Spirit in Jan 2004
      
   2. Gusev Crater: crater on Mars, site of landing of the Mars rover Spirit, Jan 2004
   3. Sleepy Hollow: circular, shallow depression on Gusev crater
      
   4. Olympus Mons: tallest volcano and mountain in the Solar System. About 3 times as tall as Mount Everest (88000 ft)
      
   5. Meridiani Planum: Landing site of second Mars rover Opportunity. May indicate the presence hot springs or liquid water in the past
      
3. Moon
   1. South Pole – Aitken basin: largest crater on the Moon, on the far side
      
   2. Shackleton crater: Site where the Moon Impact Probe from Chandrayaan landed and found water, Nov 2008.
      Located at the South Pole, where the rim gets continuous sunlight while the interior is in perpetual shadow. 
      
   3. Cabeus crater: Site where the LCROSS spacecraft landed and confirmed significant presence of water, Oct 2009.Located at the south pole
      

TIMELINE OF SOLAR SYSTEM EXPLORATION

1. 1957: Sputnik 1, first Earth Orbiter
2. 1961: Vostok 1, first manned Earth orbiter
3. 1966: Luna 9, first Lunar Lander
4. 1969: Apollo 11, first manned lunar landing
5. 1970: Luna 17/Lunokhod 1 – first Lunar Rover
6. 1971: Mars 3 – first Mars Lander
7. 1976: Helios 2 – closest Solar approach
8. 1977: Voyager 2 – first to leave Solar System
9. 1978: International Cometary Explorer – first comet flyby (comets Giacobini-Zinner and Halley)
10. 1996: Mars Pathfinder – first Mars Rover
11. 2001: Genesis – first Solar wind sample return

TIMELINE OF SOLAR SYSTEM ASTRONOMY

1. 2137 BCE: Chinese astronomers record solar eclipse
2. 2^ND millennium BCE: Heliocentric solar system, with Sun at the centre, proposed in the Vedic texts
3. 499 CE: Aryabhata, in his Aryabhatiya, propounds heliocentric solar system of gravitation, elliptical orbits for planets, and suggests that moon and planets shine due to reflected light
4. 500: Aryabhata accurately computes the earth’s circumference, solar and lunar eclipses and length of earth’s revolution around the Sun
5. 620: Brahmagupta recognizes gravity as a force of attraction and describes law of gravitation
6. 628: Brahmagupta calculates motion and position of various planets
7. 1150: Bhaskara, in the Siddhanta Shiromani, calculates longitudes and latitudes
8. 1514: Copernicus states his Heliocentric theory in Commentariolus
9. 1610: Galileo Galilee discovers Callisto, Europa, Ganymede and Io, sees Saturn’s rings
10. 1656: Christiaan Huygens: identifies Saturn’s rings as rings and discovers Titan
11. 1705: Edmund Halley predicts the periodicity of Halley’s Comet
12. 1755: Immanuel Kant formulates the Nebular Hypothesis of Solar System formation
13. 1930: Clyde Tombaugh discovers Pluto
14. 1946: American launch of a camera-equipped V2 rocket provides the first images of the Earth from space

STRUCTURE OF THE EARTH...........

Depth (in km)	Layer
0-35	Crust
35-60	Uppermost part of the mantle
35-660	Upper Mantle
660-2890	Lower mantle
2890-5150	Outer core
5150-6360	Inner core

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Crust

• Depth varies from 70 km under mountains to 5 km under oceans
• Thin oceanic crust is composed of dense iron, magnesium silicate rocks like basalt
• Thick continental crust is less dense, composed of sodium, potassium, aluminium silicate rocks like granite
• The boundary between crust and mantle is called Mohorovicic discontinuity. Signifies change in seismic velocity and rock composition
  

Mantle

Schematic view of the interior of Earth. 1. continental crust - 2. oceanic crust - 3. upper mantle - 4. lower mantle - 5. outer core - 6. inner core - A: Mohorovičić discontinuity - B: Gutenberg Discontinuity - C: Lehmann discontinuity Source: Wikipedia

• Thickest layer of the earth
• Composed mainly silicate rocks rich in iron and magnesium
  
• Temperature ranges from 500 C (near the crust) to 4000 C (near the core)
  
• Despite high heat, the mantle is primarily solid due to high pressures
  
• The mantle is slightly ductile and can flow, although only on slow, long timescales
  
• Motion of tectonic plates is an expression of convection in the mantle
• The mantle lies exposed without any crust covering on the floor of the Atlantic Ocean near the Caribbean Islands

Outer core

• Convection in the outer core gives rise to earth’s magnetic field. The mechanism of the magnetic field is explained by the Dynamo Theory, which was proposed by Joseph Larmor in 1919 
  
• Liquid in composition
  

Inner core

• Believed to consist of an iron-nickel alloy
  
• Hottest part of the earth. Temperature may reach that of Sun’s surface i.e 5700 K
• Solid in composition
  
• Compressional waves can pass through it but not shear waves
  
• Inner core is younger than the age of the earth. Inner core: 2-4 billion years, earth: 4.5 years
  
• Inner core is cooling slowly (about 100 C per billion years)
  
• The inner core is too hot to hold a permanent magnetic field
  
• It has been speculated that the inner core may rotate slightly faster than the rest of the earth (about 0.3 to 0.5 degrees per year)
  

Lithosphere

• Includes the crust and uppermost parts of the mantle
  
• Constitutes the hard and rigid outer layer of the Earth
  
• Lithosphere is broken down into tectonic plates
  
• Is rigid and deforms through brittle failure, causing faults
• Lithosphere is thought to float or move around on the Asthenosphere, creating plate tectonics
  

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Asthenosphere

• Lies below the lithosphere
  
• Constitutes the weaker, hotter and deeper part of the upper mantle
  
• Involved in plate movements
  
• Deforms viscously and accommodates strain through plastic deformation
  
• Due to high temperature, rock becomes ductile, leading to convection currents
  
• Boundary between Lithosphere and Asthenosphere is defined by a change in seismic velocity: in asthenosphere seismic waves pass relatively slowly and hence it is called a low-velocity zone
  

Discontinuities in the Earth’s structure

Discontinuity	Depth	Boundary	Other notes
Mohorovicic discontinuity	30-50 km (continents) 7 km (ocean floor)	crust-mantle	Observed by abrupt change in seismic wave velocity Identified by Andrija Mohorovicic (Croatia) in 1909
Gutenberg discontinuity	2900 km	Core-mantle	Observed by difference in seismic wave velocity
Lehmann discontinuity	220 km		Appears beneath continents but not oceans

PLATE TECTONICS

Overview

• Plate tectonics is a theory that describes large scale motions of the earth’s lithosphere
  
• Proposed by Harry Hess in 1962. Builds on the concepts of continental drift, proposed by Alfred Wegener in 1915.
• Tectonic plates move because lithosphere has higher strength and lower density than the athenosphere. Thus the lithosphere rides on the athenosphere
• Tectonic plates on the earth move in relation to each other
  
• Movement of plates is typically 50 – 100 mm annually
• Earthquakes, volcanic activity, mountain building and ocean trench formation occur along plate boundaries
• Plate tectonics may exist on other terrestrial planets as well, especially Mars
  

Types of plate boundaries

• Transform boundaries:
  occur where plates slide past each other along transform faults. Eg: San Andreas Fault in California
  
• Divergent boundaries: occur where two plates slide apart from each other. Eg: Mid-Atlantic Ridge, Great Rift Valley (Africa)
  
• Convergent boundaries: occur where two plates slide towards each other forming either a subduction zone or a continental collision. Eg: Andes (South America), Japan, Himalayas
  
  • Subduction zones:
    occur where an oceanic plate is pushed underneath a continental plate. Eg ocean trenches. The descending end of the oceanic plate melts and creates pressure on the mantle, causing volcanoes
    
  • Obduction zones:
    occur where the continental plate is pushed underneath the oceanic plate. However, this is unusual as the relative densities of the plates favours subduction of the oceanic plate
    
  • Orogenic belts:
    occur when two continental plates collide and push upward to form large mountain ranges. Eg: Himalayas
    

Examples of Divergent boundaries

• East African Rift (Great Rift Valley), Africa
• Mid-Atlantic Ridge: separates the North and South American plates from the Eurasian and African plates
  
• Gakkel Ridge: a slow spreading ridge in the Arctic Ocean
  
• East Pacific Rise: extends from the South Pacific to the Gulf of California
  
• Carlsberg Ridge in the eastern Indian Ocean
  

Examples of Subduction zones

• The oceanic Nazca plate being subducted under the continental South American Plate forming the Chile-Peru Trench
• The Pacific Plate being subducted under the Eurasian and Philippine Sea Plates forming the Mariana Trench
• The Philippine Sea Plate subducting under the Philippine Mobile Belt forming the Manila Trench

Examples of Orogenic belts

• The belt between the Indo-Australian and Eurasian Plates giving rise to the Himalayas. This is the most dramatic Orogenic Belt in the world
• Interaction between the African and Adriatic Plates with the Eurasian Plate giving rise to the Alps
• Andes belt on the western margin of South America

Examples of Transform boundaries

• The San Andreas Fault in California. This arises due to the northwards movement of the Pacific Plate with respect to the North American Plate
• Motagua Fault between the North American Plate and the Caribbean Plate
• Dead Sea Transform fault which runs through the Jordan River Valley

Major and Minor plates

Major plates	Minor plates
African plate	Arabian plate
Antarctic plate	Caribbean plate
Australian plate	Juan de Fuca plate
Indian plate	Cocos plate
Eurasian plate	Nazca plate
North American plate	Philippine sea plate
South American plate	Scotia plate
Pacific plate

THE ATMOSPHERE OF EARTH............

COMPOSITION OF THE ATMOSPHERE

Compound	Distribution
Nitrogen	78%
Oxygen	21%
Argon	0.9%
Water vapour	0.4% (around 1% at the surface)
Carbon dioxide	0.03%

STRUCTURE OF THE ATMOSPHERE

1. Troposphere
   1. Begins at the surface and extends to between 7 km (at the poles) and 20 km (at the equator)
   2. Temperature in the troposphere decreases with altitude i.e. the lowest parts are the warmest
   3. The troposphere contains roughly 75% of the mass of the atmosphere and 99% of its water vapour
   4. The lowest part of the troposphere, where friction with the Earth’s surface influences air flow is called the planetary boundary layer. Usually extends from a few hundred metres to about 2 km
   5. The tropopause is the boundary between the troposphere and the stratosphere
2. Stratosphere
   

   
   

   Layers of the atmosphere
   1. Extends from the troposphere to about 51 km
   2. Temperature increases with height
   3. Restricts turbulence and mixing
   4. Commercial airliners usually fly within the stratosphere (10 km) to optimize jet fuel burn and to avoid atmospheric turbulence
   5. The stratopause is the boundary between the stratosphere and the mesosphere
3. Mesosphere
   1. Extends from stratosphere to about 80 km
   2. Upon entering the earth’s atmosphere, most meteors burn up in the mesosphere
   3. Temperature decreases with height
   4. The mesopause, the end of the mesosphere, is the coldest place on Earth with an average temperature of -100 C
4. Thermosphere
   1. Biggest layer of the atmosphere
   2. Extends from the mesosphere to about 500-1000 km
   3. Thermopause is a temperature boundary contained within the thermosphere
   4. Temperature increases up to the thermopause, then remains constant
   5. The temperature can reach 1500 C. However, despite the high temperature one would not feel warm because the atmospheric density is too low to enable heat transfer
   6. The International Space Station orbits in the thermosphere (320 – 380 km)
   7. The ionosphere is formed in this layer as a result of ionization caused by ultraviolet radiation
   8. The boundary between the thermosphere and the exosphere is called exobase
5. Exosphere
   1. Uppermost layer of the atmosphere
   2. It is a transitional zone between the Earth’s atmosphere and interplanetary space and does not fully fall within the atmosphere
   3. Extends to about 190,000 km. This is half the distance to the Moon, at which the influence of solar radiation becomes greater than the Earth’s gravitational pull
   4. The density is so low that molecules can travel hundreds of km without colliding with each other
   5. Composed mainly of the lightest gases such as hydrogen and some helium

OTHER LAYERS AND BOUNDARIES OF THE ATMOSPHERE

1. Ozone layer
   1. It is contained within the stratosphere at about 10 – 50 km above the Earth’s surface
   2. About 90% of the ozone layer is present in the stratosphere
   3. The ozone layer absorbs 93-99% of harmful ultraviolet light
   4. Ozone is formed when UV light strikes oxygen in the stratosphere to split the oxygen atoms, which then reform as ozone
   5. The ozone layer was discovered by the French physicists Charles Fabry and Henri Buisson in 1913
   6. British meteorologist GMB Dobson established a worldwide network of ozone monitoring stations between 1928 and 1958 that continues to operate today. He also developed a spectrophotometer (called the Dobsonmeter) to measure stratospheric oxygen from the ground. The Dobson unit, a measure of ozone density is named in his honour
2. Ionosphere
   1. Stretches from the thermosphere to the exosphere (100 km – 700 km)
   2. This is caused due to ionization by solar UV radiation
   3. Responsible for radio propagation by reflecting radio waves back to the Earth’s surface thereby enabling long-distance communication
   4. Plays an important part in atmospheric electricity (like lightning)
   5. Responsible for auroras
3. Homosphere and Heterosphere
   1. Homosphere is the part of the atmosphere where gases are well mixed due to turbulence
   2. This includes the troposphere, stratosphere and mesosphere
   3. Heterosphere is the part of the atmosphere where gases are not well mixed
   4. This usually happens above the turbopause (100 km) where distance between particles is large due to low density
   5. This causes the atmosphere to stratify with heavier gases like oxygen and nitrogen present in the lower layers and lighter gases like hydrogen and helium in the upper layers
4. Planetary boundary layer
   1. Part of the troposphere closest to the Earth’s surface and most influenced by it
   2. Friction with the earth’s surface causes turbulent diffusion
   3. Ranges from 100 m to about 2 km
5. Magnetosphere
   1. A mix of free ions and electrons from solar wind and the Earth’s atmosphere
   2. It is non-spherical and extends to more than 70,000 km
   3. It protects the Earth from harmful solar winds
   4. Mars is thought to have lost most of its former oceans and atmosphere to space due to the direct impact of solar winds. Similarly Venus is thought to have lost its water due to solar winds as well
6. Karman line
   1. Defines the boundary between the Earth’s atmosphere and outer space
   2. Lies at an altitude of 100 km above mean sea level
   3. At this altitude the atmosphere becomes too thin for aeronautical purposes
   4. However, there is no legal demarcation between a country’s air space and outer space
7. Van Allen Belt
   1. It is a region of energetic charged particles (plasma) around the Earth held in place by the Earth’s magnetic field
   2. Extends from about 200 km to 1000 km
   3. Has important implications for space travel because it causes radiation damage to solar cells, integrated circuits, sensors and other electronics

PHYSICAL PROPERTIES OF THE ATMOSPHERE

1. Pressure and thickness
   1. Atmospheric pressure at sea level is 1 atmosphere (around 14.7 psi)
   2. 50% of atmospheric mass is below an altitude of 5.6 km
   3. 90% of atmospheric mass is below 16 km
   4. 99.99% of atmospheric mass is below 100 km
2. Density and mass
   1. Atmospheric density decreases with height
   2. Density at sea level is about 1.2 kg/cu.m

OPTICAL PROPERTIES OF THE ATMOSPHERE

1. Scattering
   1. When sun’s rays pass through the atmosphere, photons in light interact with the atmosphere to produce scattering
   2. Eg: on overcast days there are no shadows because light reaching the surface is only scattered, indirect radiation, with no direct radiation reaching the earth
   3. Scattering is responsible for blue appearance of the sky, and for red appearance of sunset
2. Absorption
   1. The atmosphere absorbs radiation of different wavelengths, allowing only certain ranges (UV to IR) to pass on to the earth’s surface
3. Emission
   1. The atmosphere absorbs and emits IR radiation
   2. Earth cools down faster on clear nights than on cloudy nights because clouds absorb IR radiation from the Sun during the day and emit IR radiation towards the Earth at night
   3. Greenhouse effect is directly related to emission, where certain greenhouse gases (carbon dioxide) prevent IR radiation from the earth’s surface to exit back to space

WATER VAPOUR IN THE ATMOSPHERE

• 99.9% of water vapour is contained in the troposphere
• Condensation of water vapour into liquid or ice is responsible for rain, snow etc
• The latent heat released during condensation is responsible for cyclones and thunderstorms
• Water vapour is also a potent greenhouse gas
• Water vapour is most common gas in volcanic emissions (around 60%)

CARBON DIOXDE IN THE ATMOSPHERE

• It is an important greenhouse gas
• Natural sources of carbon dioxide in the atmosphere include volcanic activity, combustion of organic matter, respiration, decay of forests etc
• Current carbon dioxide levels (0.0384%) are around 35% higher than the levels in 1832
• The concentration of carbon dioxide is higher in the northern hemisphere because it has greater land mass and plant mass than the southern hemisphere
• Carbon dioxide concentrations peak in May (just after the end of winter in the Northern Hemisphere) and reach a minimum in October (at the end of summer in Northern Hemisphere, when the quantity of plants undergoing photosynthesis is greatest)

ATMOSPHERIC ELECTRICITY.........

Overview

Aurora Borealis seen over Canada

• The Earth’s surface, the atmosphere and the ionosphere combine to form a global atmospheric electrical circuit
• Free electricity is always present in the atmosphere. It is usually positive
• The intensity of atmospheric electricity is usually greater in the middle of the day than in the morning or at night. Also, it is greater in winter than in summer
• Atmospheric electricity increases with altitude
• The Earth’s surface is negatively charged, while the atmosphere is positively charged
• Benjamin Franklin was the first to prove electrical phenomena of the atmosphere in 1752

Variation of atmospheric electricity

• The primary cause of variation in atmospheric electricity is the thermodynamics of radiation
• Atmospheric electricity is maximum in January and minimum in June
• Humidity increases atmospheric electricity in the cold months but decreases it in hot months

PHENOMENA OF ATMOSPHERIC ELECTRICITY

1. Auroras
   1. Auroras are natural light displays observed in the night sky, especially in polar regions
   2. Auroras occur when the Earth’s magnetic field traps solar wind in the atmosphere resulting in a collision between the solar wind and atmospheric molecules leading to release of energy
   3. They are most prominent closer to the magnetic poles because of longer periods of darkness and strength of the Earth’s magnetic field
   4. The Aurora Borealis refers to auroras in the northern hemisphere. The corresponding auroras in the southern hemisphere are called Aurora Australis
   5. Auroras occur most often near the seasonal equinoxes: from September to October and from March to April
   6. Auroras have maximum intensity during the intense phase of solar cycle when coronal mass ejections increase the intensity of solar wind
2. Static electricity
   1. Static electricity is the build of electrical charge on the surface of objects
   2. The static charge remains on the object until it either bleeds off to the ground or is quickly neutralized by a discharge
   3. Lightning is caused by discharge of static electricity
3. St. Elmo’s Fire
   1. St. Elmo’s Fire is a bright blue or violet glow appearing from tall, pointed objects
   2. It is a phenomenon in which plasma is created when the electric field around the object causes ionization of air molecules
   3. Sharp objects tend to create more plasma because electrical fields are more concentrated in areas of high curvature
4. Lightning
   1. Lightning is an atmospheric discharge of electricity
   2. Occurs during thunderstorms, volcanic eruptions and dust storms
   3. The average lightning bolt can reach temperatures of 30,000 C (about 3 times the temperature of the sun) and carry around 100 million V of electricity
   4. This extreme temperature compresses surrounding air and creates a supersonic shock wave called thunder
   5. In addition to light, lightning has been shown to emit radio waves, X-rays and gamma rays

TYPES OF LIGHTNING

Lightning strikes can carry up to 100 million Volts and reach temperatures of 30,000 C

The lightning that is most commonly observed is called streak lightning. This is just the visible part of the lightning stroke – the majority of the lightning occurs inside clouds, so it is not visible from the Earth.

1. Cloud-to-ground lightning
   1. Second most common form of lightning
   2. b. Poses greatest threat to life and property since it strikes the ground
2. Cloud-to-cloud lightning
   1. Lightning occurring between two clouds is called inter-cloud lightning
   2. Lightning that occurs between two areas of the same cloud that have differing electric potential is called intra-cloud lightning
3. Ground-to-cloud lightning
   1. Lightning discharge between ground and cloud, in the upward direction
   2. Very rare
   3. Occurs when negatively charged ions from the Earth’s surface rise up and meet the positive ions in the cloud
4. Heat lightning: lightning that occurs too far away for the sound of thunder to be heard
5. Dry lightning
   1. Dry lightning is lightning that occurs without precipitation at the surface
   2. This is the most common natural cause of wildfires
   3. Occurs as a result of extreme surface temperatures when convection from the hot surface to cooler atmosphere leads to lightning
6. Positive lightning
   1. Occurs when positive charge is carried on the top of clouds
   2. Very rare
   3. Around 10 times more powerful and longer lasting than regular negative lightning
   4. Very dangerous to life and property
   5. At present, aircraft are not designed to withstand positive lightning
7. Sprite
   1. Large scale discharges occurring high above a thundercloud
   2. Occur about 80 km to 150 km above the Earth’s surface
   3. Reddish-orange or greenish-blue in colour
   4. May account for aircraft accidents at altitudes above thunderstorms
8. Blue jets
   1. Occur at lower altitudes than sprites, but still above thunderclouds
   2. Occur about 40 km to 80 km above surface
   3. Blue in colour
9. Elves
   1. ELVES stands for Emissions for Light and Very low frequency Electromagnetic pulse Sources
   2. Occur in the ionosphere, about 100 km above surface
10. Rocket-triggered lightning
    1. Lightning can be triggered by rockets carrying spools of wire into thunderstorms. When the wire unwinds, it provides a path for lightning to conduct to the surface
    2. Lightning can also be triggered by space shuttle launches and aircraft flight
11. Volcanically triggered lightning
    1. Extremely large volcano eruptions which eject gases and material high into the atmosphere can trigger lightning

LIGHTNING IN EVERYDAY LIFE

World map showing the frequency of lightning strikes. Lightning strikes most frequently in the Congo

Lightning conductor

• It is a metal rod or conductor used to protect a building from lightning
• It is mounted on the top of the building and connected to the ground using a wire
• When strikes, it will preferentially strike the rod and be conducted harmlessly to the ground
• Lightning rods are usually made from good conductors of electricity such as aluminium or copper

Lightning protection on aircraft

• On aircraft, an electrical circuit is established on the aircraft’s outer surface
• Aircraft made from aluminium naturally act as good conductors of electricity. When the aircraft is made of carbon composites, a layer of conductive fibre is embedded to ensure conductivity
• When the aircraft is struck by lightning, current travels on the outer surface of the aircraft, with the interior remaining unaffected
• Proper shielding is provided to ensure lightning does not affect cockpit electronics, fuel tanks and radar and other avionics
• Aircraft also use static dischargers to prevent buildup of static electricity

Trees and lightning

• Trees are natural conductors of lightning. They provide connection for lightning to reach the ground. However, the outer layer of trees (bark) is not a good conductor
• Trees get burnt from lightning because lightning travels on the outer surface of the tree, burning away the bark.
• Usually, trees can recover from damage to the bark. However, sometimes the damage is too severe for recovery.
• Oak and elm are two trees most frequently struck by lightning. Teak provides the best conducting connection for lightning
• By attracting lightning towards them, trees prevent damage to nearby buildings. However, for the same reason, it is not safe to seek shelter under trees during lightning

Shelter from lightning

• To get shelter from lightning, there needs to be an electrical connection through the exterior surface on to the ground. The connection must ensure that people do not get in contact with the electricity
• Best lightning shelters: houses, buildings, closed-roof cars, closed-cabin boats etc
• Worst lightning shelters: trees, tents, open barns, open-roof cars, open boats etc
• It is unsafe to use radios, cellphones etc during lightning strikes

MEASUREMENT OF ATMOSPHERIC ELECTRICITY

1. Electrometer
   1. Simple instrument for measuring atmospheric electricity at ground surface
   2. Developed in 1700s by Alessandro Volta
   3. Consists of a glass jar with a pointed metal rod, whose lower end is attached to two straws. Electricity in the atmosphere cause the two straws to recede from each other, the amount of divergence indicating the intensity of electricity
2. Weather balloons
   1. A balloon which carries instruments aloft to send back information regarding temperature, humidity etc
   2. The device that does the actual measuring is called radiosonde
   3. The radiosonde is an inexpensive device, and it is lost when the life of the balloon expires
3. Lightning rocket
1. It is a device that measures electrostatic and ionic charge in the atmosphere
2. Consists of a rocket launcher which is in communication with the detection device on the ground
3. This system controls the time and location of a lightning strike
4. Uses solid (cesium salts) or liquid (calcium chloride) propellants to produce exhaust gases that act as a conducting pathway between the clouds and the ground

WIND

Causes of wind

• Wind is caused by differences in pressure
• It always flows from high pressure to low pressure
• The two major driving factors of large scale atmospheric circulation are
  • Heating difference between the equator and the poles
  • Rotation of the planet, which leads to air being deflected according to the Coriolis effect. Coriolis effect is the apparent deflection of moving objects when viewed from a rotating reference frame
• Near the Earth’s surface, friction causes wind to be slower than it otherwise would be
• Away from the surface, large scale winds tend to approach a state of equilibrium called Geostropic Balance

Measurement of wind

A rock formation in Bolivia, sculpted by wind erosion

• Wind direction is reported based on the direction from which it originates. Eg: a northerly wind blows from the north to the south
• Wind direction is observed using weather vanes (atop buildings) andwindsocks (at airports)
• Wind speed is measured using anemometers
• Weather balloons and RADAR/LIDAR can also be used for measuring wind speed and direction
• Sustained winds are usually observed 10 m from the surface of the Earth
• Globally, wind speeds are reported over a 10 minute average. India reports winds over a 3 minute average

Wind categorization on the Beaufort scale

Beaufort scale	Wind speed (knots)	General term	Terminology of IMD (covers north Indian Ocean)
0 – 6	0 – 27	Breeze	Depression
7	28-33	Gale	Deep depression
8 – 9	34 – 47	Strong gale	Cyclonic storm
10 – 11	48 – 63	Storm	Severe cyclonic storm
12 – 16	64 – 120	Hurricane	Very severe cyclonic storm
17	> 120	Hurricane	Super cyclonic storm

AEOLIAN PROCESSES

Aeolian process refers to the action of wind in shaping the surface of the Earth.

1. Wind erosion
   1. Wind erodes the earth by deflation and abrasion. Deflation is the removal of fine, loosely grained particles while abrasion is the wearing down of surfaces by grinding action
   2. Regions that experience intense and sustained erosion are called deflation zones
   3. Desert rocks that have been exposed to wind for long periods of time exhibit a dark shiny stain called desert varnish
   4. Blowouts are hollows formed by the removal of particles by wind
2. Wind transport
   

   
   

   Suspension, saltation and creep
   1. Particles are transported by wind through the processes of suspension, saltation and creep
   2. Suspension is the holding of small particles in the atmosphere due to upward currents in air. Dust and haze are examples of suspension.
   3. Saltation is the movement of particles in jumps and skips by lifting up slightly from the surface. Examples of saltation include sand drift over deserts, soil blowing over fields.
   4. Creep is the slow downward progression of rock and soil down a low grade slope. Creep is responsible for the rounded shape of hillsides
      
   5. Other wind transport phenomena include dust storms and dust devils. Covered in detail below.
3. Wind deposition
   1. Wind-deposited bodes occur as sand sheets, ripples and dunes
   2. Sand sheets are flat, gently undulating surfaces of sand. They form about 40% of Aeolian deposition surfaces.
      
   3. Wind blowing on a sand surface also causes ripples, which form into crests and troughs. In ripples, the coarsest materials collect on the crests
   4. Sand dunes are hills of sand similar to ripples, except that they are larger and have the coarsest materials on the troughs

ROLE OF WIND IN NATURE

1. Desert dust migration
   1. Dust from deserts is carried across huge distances over to other continents
   2. Example: dust from the Sahara desert blows via the Caribbean to North America
   3. Desert dust migration can affect rainfall patterns
   4. It also causes the sky to change colour from blue to white
2. Effect on plants
   1. The dispersal of seeds through wind is called anemochory
   2. Examples of seeds that disperse through wind: dandelions, maples, weeds
   3. Wind also limits tree growth. The tree line is often lower in coasts and isolated mountains because high winds reduce tree growth
   4. Wind also causes soil erosion leading to uprooting of trees
3. Effect on animals
   1. Cattle and sheep are prone to wind chill when high wind speeds render their protective covering inffective
   2. For penguins, their flippers and feet are susceptible as well
   3. Bird migration and insect return tend to flow with wind patterns

WIND IN OUTER SPACE

1. Planetary wind
   1. The loss of gas from a planet to outer space is called planetary wind
   2. This happens when light elements such as hydrogen move up to the exobase (limit of atmosphere), and then reach escape velocity to escape to outer space
   3. The planet Venus is said to have its atmosphere due to planetary wind
2. Solar wind
   1. Solar wind is a stream of charged particles (plasma) ejected from the sun
   2. This plasma is ejected from the upper atmosphere of the sun at up to 400 km/s
   3. Solar wind creates the heliosphere, a vast bubble in interstellar medium
   4. Planets require large magnetic fields to reduce the ionization of their upper atmosphere by solar wind. Mars is said to have lost its atmosphere due to solar wind
   5. The surfaces of Venus and the Moon are bombarded directly by solar wind, resulting in high radiation levels

GEOLOGICAL PHENOMENA CAUSED BY WIND

1. Sand dunes
   1. Sand dunes are hills of sand built by wind
   2. Dunes are usually longer on the windward side and shorter on the leeward side
   3. Sand dunes can form in dry inland regions and also in coastal areas and underwater as well
   4. Dunes can move over tens of metres due to the consistent action of strong wind. Through saltation, wind picks up particles from the windward side and deposits it on the leeward side, gradually moving the dune
   5. The tallest sand dunes in the world are found in the Namib Desert
2. Erg
   1. An Erg is a large area of desert covered with wind-swept sand with little or no vegetative cover
   2. In essence, ergs are large dune fields
   3. Ergs are mainly found in Africa, central and western Asia and central Australia
   4. Ergs have been found on Venus, Mars and Titan as well
3. Loess
   

   
   

   The rich fertile soil of the Loess plateau has been the mainstay of Chinese agriculture for centuries
   1. Loess is a sediment formed by the accumulation of wind-blown silt, sand and clay, loosely cemented by calcium carbonate
   2. Loess deposits often occur in very thick layers, sometimes more than 100 m thick. It occurs as a blanket deposit covering areas of hundreds of square km
   3. Loess is highly prone to erosion
   4. Loess can occur from glacial or non-glacial soils. Example of glacial loess: Mississippi Valley, USA. Example of non-glacial loess: Shanxi, China
   5. Loess tend to develop into highly fertile soils
4. Wind waves
   1. Wind waves are surface waves that occur in oceans, lakes etc due to the action of wind
   2. Wind waves can range from ripples to more than 30 m in height
   3. A wind wave system generated by local winds in called wind sea. Wind wave system not generated by local winds is called swell
   4. Factors that influence the formation of waves include: wind speed, distance of open water, time duration and water depth
   5. Tsunamis and tides are specific types of waves caused not by wind, but by geological effects. They have longer wavelength than wind waves
      
   6. Waves can be measured using buoys that record the motion of the water surface
   7. A breaking wave is one whose one can no longer support its top causing it to collapse
   8. There are three main types of waves
      1. Spilling or rolling waves: Safest waves for surfing. Most common type of waves found at shores.
      2. Plunging or dumping waves: preferred by experienced surfers. Found where there is a sudden rise in ocean floor like a sandbar
      3. Surging waves: very dangerous for surfing. Tend to form on steep shorelines, where the depth results in waves not breaking as they approach the shore

GEOGRAPHICAL FEATURES SHAPED BY WIND

Feature	Location	Wind effect	Notes
Mississippi River Alluvial Valley	South-central USA	Glacial loess
Loess plateau	Shanxi, Northern China	Non-glacial loess	Thickest loess in the world (335 m) Most erodible soil on earth
Selima sand sheet	Egypt, Sudan	Wind deposition	Largest sand sheet
Namib desert sand dunes	Namibia, Angola	Wind deposition	Oldest desert in the world (55 million years)
Paha	Iowa, USA	Sand dunes
Badain Jaran desert	Inner Mongolia (China, Mongolia)	Sand dunes	Tallest stationary sand dunes in the world (500 m)
Great dune of Pilat	France	Sand dune	Largest sand dune in Europe
Merheb	UAE	Sand dune	Used for motor sports
Kelso dunes	USA	Sand dune
Cerro Blanco	Sechura Desert, Peru	Sand dune	Highest sand dune in the world

WIND CIRCULATION

Overview

The global circulation patterns of wind on Earth

• Winds that blow predominantly from a single direction over a particular point on the Earth’s surface are called Prevailing Winds
• The general trends in wind direction are called dominant winds
• In general regional winds can be divided into two groups
  • Global winds like easterlies, westerlies
  • Local winds like land breeze, sea breeze
• Prevailing winds greatly influence climate patterns such as rainfall gradients, where the windward side of mountains have high rainfall while leeward side experience desert conditions
• Wind rose is a graphic plotting tool that is used to describe the speed and direction of wind at a particular location
• Insects drift along prevailing winds, while birds are able to fly independent of them

Distribution of prevailing winds

• In general, easterly winds flow at low and high altitudes i.e. near the tropics and the poles
• Westerly winds flow at the mid-latitudes
• Directly under the subtropical ridge i.e. close to the equator, winds are lighter in intensity. These subtropical regions are called the doldrums, or horse latitudes
• The strongest winds are usually in the mid-latitudes, where cold air from the Artic meets warm air from the tropics
• Most of the earth’s deserts are found near the subtropical ridge, with high pressure leading to low humidity

GLOBAL WINDS

• Trade Winds
  • Trade winds are the prevailing easterly winds that blow across the tropics
  • They blow from the northeast in the northern hemisphere and from the southeast in the southern hemisphere
  • Trade winds act as the steering for tropical storms that form in the Atlantic, Pacific and south Indian Oceans. These storms make landfall in North America, Southeast Asia and India, respectively
    
  • Trade winds steer African desert dust across the Atlantic Ocean towards North America (esp. the Caribbean and Florida)
    
  • The weaker a trade wind becomes, the more rainfall it brings
  • Trade winds are stronger in winter than summer
  • The one region of the Earth where trade winds are absent is the north Indian Ocean
• Westerlies
  • Westerlies are the prevailing winds in the mid-latitudes i.e. between 35 and 65 degrees latitude
  • They blow from high pressure areas in the horse latitudes towards the poles
  • Westerlies blow from the southwest in the northern hemisphere and from the northwest in the southern hemisphere
  • Westerlies are instrumental in carrying warm equatorial winds towards the western coasts of continents
  • They are responsible for carrying desert dust from the Gobi Desert into North America
  • They are stronger in winter than in summer, and over regions that have less land to interrupt their flow. They are stronger in the Southern Hemisphere because of the vast ocean expanses uninterrupted by land mass
  • Westerlies are strongest in the Roaring Forties i.e. between 40 and 50 degrees latitude
• Polar Easterlies
  • Polar easterlies are the prevailing winds that blow at the north and south poles
  • They are cold and dry winds
  • They blow from high pressure areas near the poles towards low pressure areas within the mid-latitudes
  • They blow from the east to the west
  • Polar easterlies are often weak and irregular
  • They are also called Polar Hadley Cells, named after George Hadley who discovered them in 1753

LOCAL WINDS

Notable local winds

1. Sea and land Breeze
   1. Sea and land breezes are caused by the temperature differential between the sea and coastal areas
   2. Sea breeze occurs when the land gets heated during the day creating a low pressure, and cool air from the sea rushes in
   3. Land breeze occurs when the land cools off rapidly at night causing low pressure over the sea, and warm air flows from the land to the sea
   4. Sea breeze occurs during the day while land breeze occurs at night
2. Mountain winds
   1. In elevated surfaces heating of the ground exceeds heating of surrounding air, thereby changing wind circulation
   2. Hills and valleys significantly distort airflow by acting as physical barriers. This is known as barrier jet
   3. Jagged terrain results in unpredictable flow patterns and turbulence
   4. Passes in the mountain range experience lower pressure resulting in high wind speeds and erratic and turbulent air currents
   5. These conditions are dangerous to ascending and descending airplanes

MONSOON WINDS

Overview

• Monsoons are defined as seasonal reversing winds accompanied by seasonal changes in precipitation
• The major monsoon systems of the world are the West African and Asia-Australian monsoon systems
• The origin of monsoons about 15-20 million years ago has been linked to the uplift of the Tibetan Plateau after the collision of India and Asia 50 million years ago

Cause of monsoons

• Monsoons are caused by the larger amplitude of seasonal land temperature cycle compared to that of nearby oceans. This temperature differential arises because air over land warms faster and reaches higher temperature than the air over nearby ocean
• The hot air over land tends to rise creating a low pressure
• This creates a steady wind blowing from the ocean towards land, bringing moist air from the oceans
• In winter the land cools off quickly creating a high pressure that blows wind from land to sea
• In essence, monsoons are similar to sea and land breezes, except that they occur on a much larger scale

The Southwest Monsoon

The Southwest monsoon in India. It brings about 80% of India's annual rainfall

• The southwest monsoon occurs from June to September
• The southwest monsoon is caused by rapid heating of the Thar desert and north-central India in summer, creating a low pressure that is filled by moisture laden winds from the Indian Ocean
• The Himalayas prevent the wind from blowing towards Central Asia and redirect them inwards to cause rainfall
• The Arabian Sea branch of the southwest monsoon brings rainfall to the Malabar coast and central India
• The Bay of Bengal branch of the southwest monsoon picks up additional moisture in the Bay of Bengal and arrives at the eastern Himalayas, and then turns west towards the Indo-Gangetic plains
• Mawsynram in Shillong is the wettest place on Earth with about 12,000 mm of rainfall annually
• The traditional start date of the southwest monsoon is June 01
• The southwest monsoon accounts for 80% of rainfall in India

The Northeast Monsoon

• The northeast monsoon occurs October to December in India
• Around September, northern India begins to cool rapidly creating a high pressure zone
• This brings dry cold winds from the Himalayas towards the Deccan and into the Indian Ocean
• While travelling towards the Indian Ocean, the wind picks up moisture in the Bay of Bengal and pours it over southern peninsular India
• The northeast monsoon accounts for 50-60% of rainfall in Tamil Nadu

JET STREAMS

Overview

The polar and subtropical jet streams

• Jet streams are fast, narrow air currents in the atmosphere
• Jet streams are usually located near the tropopause (transition between troposphere and stratosphere)
• The main jet streams are westerly winds, flowing from the west to the east
• Jet streams are used for weather forecasting and aviation. It is hypothesised that they could be used as an energy source as well
  
• Jet streams have been observed in the atmosphere of Jupiter as well

Cause of jet streams

• Jet streams are caused by a combination of atmospheric heating and the rotation of the earth
• They form near boundaries of adjacent air masses with significant differences in temperature

Occurrence of jet streams

• The strongest jet streams are the polar jets (23,000-39,000 ft) and the somewhat weaker subtropical jets (33,000-52,000 ft)
• Other weaker jets also exist, especially over central USA
• There is one polar jet stream and one subtropical jet stream each in the northern and southern hemispheres
• The northern hemisphere polar jet is situated over the northern latitudes of North America, Europe and Asia, while the southern polar jet always circles Antarctica
• The northern and southern hemisphere jet streams have been found to be drifting towards the poles at a rate of 2.1 km per year
• Jet streams are typically a few hundred miles wide and about 3 miles thick vertically
• Wind speeds usually exceed 92 km/h, although speeds of over 398 km/h have been observed

Jet streams and aviation

• Jet streams are often as the preferred flight plans for commercial airliners
• Flying with the jet streams decreases travel time and reduced fuel consumption
• Conversely, flying against jet streams can add to travel time and increase fuel consumption. For this reason, flight plans use circuitous routes to avoid flying against jet streams
• Commercial use of jet streams began in 1952 on the Tokyo-Honolulu route cutting travel time from 18 hours to 11.5 hours

A FEW NOTEWORTHY LOCAL WINDS

Wind	Location	Description
Calima	Sahara to Canary Islands (west African coast)	Carries dust from the Sahara
Chinook	Rocky mountains	Warm, dry westerly winds
Elephanta	Malabar coast	South easterly wind Marks end of southwest monsoon
Nor’easter	North east USA	Strong storm winds from the northeast
Nor’wester	East coast of New Zealand	Warm dry winds
Santa Ana winds	Southern California	Strong, extremely dry winds Responsible for frequent wildfires
Sirocco	North Africa, Europe	Strong winds from the Sahara that cause dusty dry conditions in north Africa and cold wet conditions in Europe Reaches hurricane speeds, can last hours to days
Shamal	Persian Gulf	Strong Northwesterly wind Causes large sandstorms in Iraq

ROCKS

Overview

• Rocks are naturally occurring solid aggregates of minerals or mineraloids (a mineral-like substance that does not exhibit crystallinity)
• The Earth’s outer solid layer, the lithosphere, is made of rocks
• Rocks are generally classified into three types
  • Igneous rocks
  • Sedimentary rocks
  • Metamorphic rocks
• The structure and composition of rocks change over time, causing one type of rock to be reclassified as another
• The study of rocks is called petrology

IGNEOUS ROCKS

Overview

• Igneous rocks are rocks which form from the cooling and solidification of magma
• They are the results of volcanic processes
• The magma can be derived from melts of pre-existing rocks in either the crust or mantle. Typically, rocks melt under conditions of extremely high temperatures, low pressures or changes in composition
• Igneous rocks can be of two types:
  • Intrusive (plutonic) rocks
  • Extrusive (volcanic) rocks
• Igneous rocks make up about 90% of the Earth’s crust. However, they are hidden from the surface by a thin layer of sedimentary and metamorphic rocks
• Igneous rocks can be seen at mid ocean ridges, areas of volcanism and intra-plate hotspots
• They are crystalline and impervious
• They are resistant to erosion and weathering

Geological significance of igneous rocks

Crystallisation of magma leading to igneous rocks

• Since igneous rocks come from the mantle, the minerals and chemistry of igneous rocks give information about the composition of the mantle
• Their features are characteristic of a particular tectonic environment, allowing reconstitution of tectonic conditions
• They host important mineral deposits such as uranium, tungsten, tin, chromium, platinum

Mineralogical composition of igneous rocks

• Felsic rock: highest content of silicon with predominance of quartz and feldspar. These rocks are usually light coloured and have low density
• Mafic rock: lesser content of silicon, predominance of mafic minerals (manganese and iron). These rocks are usually dark coloured and have higher density than felsic rocks
• Ultramafic rocks: lowest silicon content, with more than 90% of mafic minerals
  

  	Felsic	Mafic	Ultramafic
  Intrusive	Granite	Gabbro	Peridotite
  Extrusive	Rhyolite	Basalt	Komatite

  
  

Intrusive igneous rocks (plutonic rocks)

• Intrusive igneous rocks are formed from magma that cools and solidifies within the crust
• These rocks are coarse-grained. Mineral grains in these rocks can be identified by the naked eye
• The central cores of most mountain ranges are made of intrusive rocks (usually granite). These large formations of intrusive rocks are called batholiths
  
• Examples of intrusive igneous rocks include granite and diorite

Extrusive igneous rocks (volcanic rocks)

• Extrusive igneous rocks are formed at the surface, from magma released into the surface from volcanic eruptions
• Extrusive rocks cool and solidify quicker than intrusive
• Extrusive rocks are fine grained in nature
• Examples of extrusive rocks include basalt and rhyolite

Large Igneous Province (LIP)

The Deccan Traps in the Western Ghats

• Large Igneous Provinces are extremely large accumulations of igneous rocks (both intrusive and extrusive)
• They refer to igneous rocks extending over 100,000 sq km, that formed in a short geological time scale of a few million years or less
• LIPs usually consist of basalt and rhyolite rocks
• When created, LIPs often have an area of few million sq km and volume on the order of a million cubic km. Majority of the LIP’s volume is emplaced in less than a million years.
• LIP’s are postulated to arise from hotspots of linear chains of volcanoes
• LIPs are often linked to mass extinction events. This is said to arise from the enormous quantities of sulphuric acid released into the atmosphere, the subsequent global cooling and absorption of oceanic oxygen.
  
• The Deccan Traps, one of the largest volcanic features on Earth, is an example of a Large Igneous Province. The Traps consist of multiple layers of basalt, more than 2 km thick and cover an area over 500,000 sq km, and were formed as a result of volcanic eruptions in the Western Ghats about 66 million years ago. It is believed that the enormous volcanic eruptions led to global cooling of around 2C, and were instrumental in the mass extinction of non-avian dinosaurs.
  

SEDIMENTARY ROCKS

Overview

• Sedimentary rock is the type of rock formed sedimentation of material. This sedimentation can occur on the Earth’s surface or within bodies of water
• Sedimentary rocks form the thin outermost layer of the earth’s crust, making up about 5% of the total volume of the crust
• Sedimentary rocks are deposited in strata called bedding
• Coal is a sedimentary rock
• Examples of sedimentary rocks include shale, sandstone, limestone

Geological significance of sedimentary rocks

Sedimentary rocks are the only rocks that contain fossils

• Study of sedimentary rocks provides information about subsurface, which is important in civil engineering for construction of roads, bridges etc
• Sedimentary rocks are also important sources of natural resources like fossil fuels, water, ores etc
• The study of sedimentary rock strata serves as the main source of scientific knowledge about the Earth’s geological history
  
• Sedimentary rocks are the only rocks that contain fossils.Sedimentary rocks contains fossils because, unlike igneous and metamorphic rocks, they form at temperatures and pressures that do not destroy fossils
  

Composition of sedimentary rocks

• Most sedimentary rocks contain either quartz or calcite
• Unlike igneous and metamorphic rocks, sedimentary rocks do not contain multiple major minerals
• Carbonate rocks contain carbonate minerals like calcite, aragonite or dolomite
• Siliclastic rocks contain silica-bearing minerals like quartz

Clastic sedimentary rocks

• Clastic rocks are composed of fragments, called clasts, of pre-existing rocks
• Clastic sedimentary rocks are those that are formed from rocks that have been broken down due to weathering, which are then transported and deposited elsewhere
  
• Clastic sedimentary rocks come in various grain sizes. They range from fine clay in shales, to sand in sandstone and gravel, cobbles and boulder size fragments in conglomerates and breccias
• Conglomerates are clastic sedimentary rocks with rounded fragments, while breccias consist of clasts with angular fragments. Both conglomerates and breccias contain clasts larger than sand (> 2 mm)
• Examples include shale, sandstone, siltstone
  

Organic sedimentary rocks

• Organic sedimentary rocks contain materials generated by living organisms
• They usually contain carbonate minerals generated by these organisms
• Examples include corals, chalk, coal and oil shale

Chemical sedimentary rocks

• Chemical sedimentary rocks are formed from minerals in solution that become oversaturated
• They usually occur as a result of evaporation
• Examples include limestone, barite, gypsum

METAMORPHIC ROCKS

Overview

• Metamorphic rocks form as a result of transformation of an existing rock, in a process called metamorphism. The existing rock is called protolith
• Metamorphic rocks are formed when the protoliths are subject to extreme temperatures and pressures
• They form from tectonic process, intrusion of magma, or simply by being deep beneath the earth’s surface (being subject to high temperatures and pressures of rock layers above)
• Much of the lower continental crust is metamorphic
• Examples of metamorphic rocks include gneiss, slate, marble

Composition of metamorphic rocks

• Metamorphic rocks are composed of metamorphic minerals
• Metamorphic minerals are those that form only at high temperatures and pressures. These include sillimanite, kyanite, andalusite, staurolite and garnet (all of which are silicates)
  
• Metamorphic rocks also contain smaller amounts of micas, feldspars and quartz. However, these are not products of metamorphism, and are instead leftovers from the protoliths

Contact metamorphic rocks

• Contact metamorphic rocks are those that form when magma is injected into surrounding rock
• The cooling magma leads to igneous rocks, and around this is a zone called contact metamorphism aureole where metamorphic rocks are formed
• The extreme temperatures cause sandstones to metamorphise into quartz, limestone into marble and shale into cordierite
• Igneous rocks are harder to transform than sedimentary rocks since they form at even greater temperatures

Regional metamorphic rocks

• Regional metamorphic rocks are those that form due to metamorphism over a wide area
• Regional metamorphism tends to make rocks foliated
• Regional metamorphic rocks tend to form at great depths simply under the temperature and pressures of upper layers of rock
• Continental crusts are examples of regional metamorphic rocks

IMPORTANT ROCK TYPES

Rock	Classification	Composition	Notes
Basalt	Igneous – extrusive volcanic	Feldspar, pyroxene	Present on moon, Mars, Venus Basalt rocks sustain microbial life Fine texture
Granite	Igneous (intrusive, felsic)	Quartz, feldspar	Coarse texture Massive, hard and tough Exhibit radioactivity (uranium)
Shale	Sedimentary (clastic)	Clay	Contain organic matter Contains multiple thin layers
Limestone	Sedimentary	Calcite (calcium carbonate)	Used in quicklime, mortar, cement, concrete Soluble in water Host of most cave systems
Sandstone	Sedimentary	Quartz, feldspar	Common building material Porous, allows water percolation Host of water aquifers and petroleum reservoirs
Slate	Metamorphic	Clay, volcanic ash	Used to make roofing, flooring It is an electrical insulator, used for switchboards Can host even microscopic amounts of fossils
Gneiss	Metamorphic	Garnet, biotite
Marble	Metamorphic	Calcite (calcium carbonate)	Comes from metamorphism of limestone Pure white marble comes from pure limestone Colours, swirls, veins come from mineral impurities Important source of calcium carbonate, used in toothpaste, paint
Quartzite	Metamorphic	Quartz	Comes from metamorphism of sandstone Used as a decorative stone Used for railway ballast

IMPORTANT ROCK FORMATIONS/STRUCTURES

For a full list of rock formations see here

For a list of fascinating rock formations, see here

Formation/structure	Location	Classification	Notes
Deccan Traps	Deccan Plateau, India	Large Igneous Province (LIP)	One of the largest volcanic features on earth
Siberian Traps	Siberia, Russia	LIP	One of the largest known volcanic events (250 million years ago)
Acasta Gneiss	Quebec, Canada	Metamorphic	Oldest known rock in the world (4.28 billion years)
Devil’s Tower	Wyoming, USA	Igneous	Monolithic rock that rises 1200 feet above surrounding terrain
Blue Lias	England	Limestone and shale	Rich in dinosaur fossils
Red Fort	Delhi	Sandstone
Hawa Mahal	Jaipur	Sandstone
Mahabalipuram sculptures	Mahabalipuram	Granite
Mount Augustus	Western Australia	Sandstone and conglomerate	Largest monolith in the world
Savandurga	Karnataka	Gneiss and granite	Largest monolith in India
Sphinx	Egypt	Limestone	Oldest known monumental sculpture Largest monolith statue in world
Phobos monolith	Mars	Igneous

MOUNTAINS

Overview

• A mountain is a large landform that stretches above the surrounding land in a limited area
• Mountains are sometimes referred to by the Greek name: montes or mons (singular)
• The highest mountain on earth is Mount Everest (8848 m)
• The highest mountain in the solar system is Olympus Mons on Mars (21,171 m)
• Mountains cover 24% of earth’s land mass
• The study of mountains is called Orology

Characteristics of mountains

• Mountains are colder than lower ground because the Sun heats the Earth from the ground up.
• When the Sun’s rays travel through the atmosphere and reach the ground, the earth absorbs the heat. In general air closest to the earth’s surface is warmest
• Air temperature usually decreases 1-2 C for every 300 m of altitude
• The flora and fauna in tall mountains tend to be isolated to one particular altitude zone. These isolated ecological systems are called sky islands.
• The peak shape of mountains is produced by glaciation and erosion through frost action
• As altitude increases, the atmospheric pressure decreases. Thus, although the percentage of oxygen remains constant (21%), the amount of oxygen decreases.
  
• Altitude sickness (aka Acute Mountain Sickness) is caused by lack of oxygen at high altitudes. Altitude sickness can lead to High Altitude Pulmonary Edema (HAPE) or High Altitude Cerebral Edema (HACE)
  
• Availability of oxygen decreases significantly over 3000 m (10,000 ft). for this reason, the cabin altitude in passenger aircraft is kept to 8000 ft
• Higher altitudes also mean lesser protection to UV radiation
  

Formation of mountains

• Mountains are usually produced by the movement of lithospheric plates
• Major mountains tend to occur along long linear arcs, indicating tectonic plate boundaries
• Compressional forces in continental collisions cause the compressed region to thicken and force the upper surface upwards
• Meanwhile, in order to balance the weight, much of the compressed rock is forced downwards as well, forming deep “mountain roots”. As a result, mountains form upwards as well as downwards.

Types of mountains

• Fold Mountains
  • Formed by the effects of folding on layers within the upper part of the earth’s crust
  • Fold mountains are generally formed on the less deformed areas adjacent to areas strongly affected by thrust tectonics
  • Most fold mountains are likely to relative young in geological terms since they will start to erode as soon as they are formed
  • Examples: Zagros mountains (Iran), Jura mountains (near the Alps i.e. France, Switzerland, Germany)
• Fault-block mountains
  • Formed when large areas of bedrock are broken up by faults creating large vertical displacements of continental crust
  • These mountains are formed by the crust being stretched and extended by tensional forces
  • The uplifted blocks are called block mountains or horsts. The intervening dropped blocks are called graben, and can form extensive rift valleys
  • Examples: Vosges (northeast France), Basin and Range (western USA)
• Volcanic mountains
  • Isolated mountains produced by volcanoes
  • Includes small islands that reach great heights beyond the ocean floor
  • Example: Mount Kilimanjaro (Tanzania)
• Inselberg (or Monadnock)
  • They are isolated hills or small mountains that rise abruptly from a surrounding plain
  • They arise when a rock resistant to erosion is enclosed within a softer rock like limestone. When the limestone erodes away to form the nearby plains, the resistant rock is left behind as an island-mountain
  • Example: sugarloaf mountain (Brazil), Pilot Mountain (USA)

The Seven Summits

• The Seven Summits are the highest mountain peaks of each of the seven continents
• The Seven Summits are
  • Africa: Mount Kilimanjaro – Tanzania
  • Antarctica: Vinson Massif – British Antarctic Territory
  • Australia: Kosciuszko – Australia
  • Asia: Mount Everest – Nepal, Tibet
  • Europe: Elbrus – Russia
  • North America: Mount McKinley (Denali) – Alaska
  • South America: Aconcagua – Argentina

Important Mountain ranges in the world

Mountain Range	Location	Length (km)	Notes
Mid ocean ridge		65,000	Underwater mountain range Longest mountain range in the world Demarcates boundary b/w tectonic plates Consists of seven ridges connected together: Gakkel Ridge, Mid Atlantic, Southwest Indian, Central Indian, Southeast Indian, Pacific Antarctic, East Pacific Rise
Andes	South America	7000	Longest continental mountain range Highest mountain range outside Asia
Rocky	North America	4800
Himalayas	Asia	3800	Highest mountain range on earth Includes Karakoram, Hindu Kush Separates Indian subcontinent from Tibetan plateau
Great Dividing Range	Australia	3700
Transantarctic Mountains	Antarctica	3500	Serve as division b/w East Antarctica from West Antarctica

Important Mountain peaks in the world

Mountain peak	Height (m)	Mountain Range	Location	Notes
Mount Everest	8848	Himalayas	Nepal/ Tibet	Highest mountain on earth
K2 (Mt. Godwen-Austen)	8611	Karakoram	Pakistan/China	Second highest mountain Second highest fatality rate (25%)
Kangchenjunga	8586	Himalayas	Nepal/India	Highest in India
Annapurna	8091	Himalayas	Nepal	Highest fatality rate (40%)
Aconcagua	6961	Andes	Argentina	Highest mountain outside Asia
Mt. Kilimanjaro	5895	Kilimanjaro	Tanzania	Highest volcanic mountain Highest in Africa
Mt Erebus	3794		Antarctica (Ross Island)	Southernmost active volcano
Mt Chimborazo	6268	Andes	Ecuador	Point on surface most distant from earth’s centre

Important Mountain ranges in India

Mountain range	Location	Notes
Himalayas
Aravalli	Rajasthan, Haryana, Gujarat	Were extremely tall in ancient times, now completely worn down due to weathering
Vindhyas	Gujarat, Madhya Pradesh	Earliest known fossil of eukaryotes discovered here (1.6 billion years)
Satpura	Gujarat, Maharashtra, Madhra Pradesh, Chattisgarh
Sivalik Hills	Sikkim, Nepal, Uttarakhand, Kashmir, Pakistan	Southernmost and geologically youngest of the Himalayan system
Eastern Ghats	West Bengal, Orissa, Andhra Pradesh, Tamil Nadu	Discontinuous range of mountains Older than Western Ghats
Western Ghats	Gujarat, Maharashtra, Karnataka, Kerala, Tamil Nadu	60% of Western Ghats located in Karnataka Rivers from Western Ghats drain 40% of India One of world’s ten “Hottest Biodiversity Spots”
Nilgiri Hills	Tamil Nadu	UNESCO World Heritage Site
Anamalai Hills (Western Ghats)	Kerala, Tamil Nadu	Under consideration for UNESCO World Heritage Site
Cardamom Hills (Western Ghats)	Kerala, Tamil Nadu	Under consideration for UNESCO WHS
Southwest Indian Ridge	Indian Ocean	Separates African Plate from Antarctic Plate
Central Indian Ridge	Indian Ocean	Boundary between African Plate and Indo-Australian Plate
Southeast Indian Ridge	Indian Ocean	Separates Indo-Australian Plate from Antarctic Plate

Important Mountain peaks in India

Mountain peak	Mountain range	Location	Notes
Kangchenjunga	Himalayas	Sikkim	Highest peak in India Third highest in the world
Nanda Devi	Himalayas	Uttarakhand	Highest peak entirely within India
Anamudi	Anaimalai Hills (Western Ghats)	Kerala	Highest peak in India outside the Himalayas
Mount Abu	Aravalli Hills	Rajasthan	Highest peak in the Aravallis

VOLCANOES

Overview

• A volcano is an opening in a planet’s surface or crust that allows hot magma, ash and gases to escape from below the surface
• Volcanoes erupt enormous quantities of toxic gases and water vapour, and can cause significant changes in global climate patterns
• The magma from volcanoes, upon cooling, solidifies into igneous rocks like basalt and granite
• Volcanism is mainly responsible for the formation of the earth’s atmosphere
• Active volcanoes are those that have erupted within the Holocene period (last 10,000 years)
• Dormant volcanoes are those that have not erupted in recent times, but might potentially erupt in the future.
• Extinct volcanoes are those that are not likely to erupt again, because the volcano no longer has a supply of lava. It is difficult to differentiate extinct volcanoes from dormant ones since many volcanoes that lie inactive for tens of thousands of years suddenly erupt without warning
  
• The explosiveness of a volcanic eruption is measured by the Volcanic Eruption Index (VEI). The index goes from 0 to 8, with 0 representing non-explosive eruptions and 8 representing mega-colossal eruptions from supervolcanoes
  

Occurrence of volcanoes

The 1991 eruption of Mt Pinatubo (Philippines) was the world's largest in living memory. The eruption sent an ash plume 19 km into the atmosphere and caused global temperatures to drop by 0.5 C

• Most volcanic activity occurs in the oceans, continuously forming new sea floor
• The most active volcanic belt is the Ring of Fire, which occurs along the boundaries of the Pacific Ocean
• In addition to the Earth, volcanoes occur on other planets as well
• Jupiter’s moon Io is the most volcanically active object in the solar system
• The tallest mountain in the solar system, the Olympus Mons on Mars (21 km tall), was built by volcanic activity

Volcanoes and plate tectonics

• Volcanoes are generally found tectonic plates are diverging or converging, but not where two tectonic plates slide past each other
• Divergent boundaries: At mid ocean ridges, tectonic plates diverge from one another. The release of pressure due to thinning of the crust leads to volcanism. Examples: deep sea vents, Iceland
  
• Convergent boundaries: when two tectonic plates, one subsides over the other, creating subduction zones. Water released from the subducting plate lowers the melting temperature of the other, creating magma. Examples: Mt. Etna, Pacific Ring of Fire
  
• Hotspots: Hotspots are not located at the boundaries of tectonic plates but above mantle plumes (narrow stream of hot mantle convecting up towards surface). The temperature of the plume causes crust to melt and form venting pipes. Examples: Hawaii, Yellowstone Caldera
  

Effects of volcanoes

• Earthquakes, hot springs, geysers etc often accompany volcanic activity
• Volcanoes typically emit large quantities of water vapour, carbon dioxide and sulphur dioxide
• Large explosive volcanic eruptions inject these gases into the stratosphere to heights of 16-32 km
• Conversion of the sulphur dioxide into sulphuric acid increases the earth’s albedo, increasing the reflection of radiation from the sun
• This leads to significant and protracted global cooling
• Gas emissions from volcanoes results in acid rain
• Volcanic activity releases about 0.13-0.23 giga tonnes of carbon dioxide every year (about 1% of amount released by human activity)
  

Decade volcanoes

• Decade Volcanoes are those volcanoes that have been identified by the International Association of Volcanology and Chemistry of the Earth’s Interior (IAVCEI) as being worthy of particular study
• Decade Volcanoes are bring particular attention due to their history of large destructive eruptions and their proximity to population areas
• They are named Decade Volcanoes because they were initiated as part of the UN International Decade for Natural Disaster Reduction (the 1990s)
• The Decade Volcanoes project encourages studies and public awareness activities with the aim of better understanding the volcanoes and the dangers they represent
• There are 16 recognised Decade Volcanoes. See list given
  

FEATURES OF VOLCANOES

Composition of Lava

Volcanoes release enormous quantities of gases into the atmosphere in an effect called Volcanic Injection

• Lava is the name given to magma once it has escaped to the surface
• Felsic lava: If the magma erupted contains a high percentage of silica (> 63%), the lava is called felsic lava
  • Felsic lava tends to be highly viscous and are erupted as domes or short stubbly flows.
  • They tend to form stratovolcanoes or volcanic domes
• Intermediate lava: silica content 52-63%
  • Generally occur at subduction zones
• Mafic lava: silica content 52-45%
  • These lavas have higher content of Magnesium and iron
  • Less viscous but much hotter than felsic lavas
  • They occur in mid ocean ridges, shield volcanoes and continental flood basalts
• Ultramafic lava: silica content less than 45%
  • Ultramafic lava flows are very rare
  • They have not erupted in millions of years
  • Ultramafic lavas were the hottest lavas

Pyroclastic flows

• Pyroclastic flows are fast moving currents of tephra (hot gas and rock), which travel from volcanoes at speeds up to 700 km/h
• Pyroclastic flows are a devastating result of explosive volcanic eruptions
• The gas can reach temperatures up to 1000 C
• Pyroclastic surges are flows where the proportion of gas is much higher than rock. This makes pyroclastic surges more turbulent and can rise above hills and ridges. Pyroclastic surges are even more devastating than pyroclastic flows and can reach speeds up to 1000 km/h
• Famous pyroclastic flows include the ones that engulfed the towns of Pompeii and Herculaneum in Italy in 79 CE

Calderas

• A caldera is a cauldron-like volcanic feature usually formed by the collapse of land following a volcanic eruption
• Calderas arise because the emptying of the magma chamber beneath the volcano, with the result that the emptied chamber is unable to support the weight of the volcanic material above it
• Calderas are formed as a result of a large volcanic eruption

TYPES OF VOLCANOES

Shield Volcanoes

Map of major volcanoes around the world

• Shield volcanoes are formed by the eruption of low viscosity lava
• The lava flows a great distance from the vent
• Shield volcanoes do not explode catastrophically
• They are more common in oceans than in continents
• Eg: Hawaii, Iceland

Mud volcanoes

• Mud volcanoes (not strictly volcanoes) are formations created by the geo-excretion of liquids and gases
• Temperatures in mud volcanoes are much cooler than in igneous processes
• Ejected material primarily consists of methane, carbon dioxide and water vapour (acidic)
• Mud volcanoes can reach 10 km in diameter and about 700 m in height

Submarine volcanoes

Barren Island, the only active volcano in India, as seen from the ISS

• Submarine volcanoes are underwater fissures in the earth’s crust from which magma can erupt
• Submarine volcanoes account for over 75% of the world’s magma releases
• Submarine volcanoes are mainly located near ocean ridges, where tectonic plate movement in maximum
• Due to the presence of water, lava from submarine volcanoes cools and solidifies quickly, turning into volcanic glass
• Submarine volcanoes are concentrated in the Ring of Fire in the Pacific Ocean
• The West Mata volcano, in the Pacific Ocean, is currently the deepest erupting submarine volcano (1100 m)

Subglacial volcanoes

• Subglacial volcanoes erupt beneath the surface of glaciers or ice sheets
• The rising lava causes the ice to melt and form a lake
• The rapid melting of ice into water due to lava can lead to glacial lake outburst floods
• Subglacial volcanoes are most common in Iceland and Antarctica

Stratovolcanoes

• Stratovolcanoes are tall conical volcanoes with many layers (strata) of hardened lava and ash
• Stratovolcanoes are characterised by steep slopes and explosive eruptions
• Stratovolcanoes are the most common type of volcanoes found
• They are common in subduction zones, forming chains of volcanoes along tectonic plate boundaries
• Stratovolcano explosions tend to result in destructive pyroclastic flows that have affected civilization through history
• The explosion of Tambora Volcano (Indonesia) in 1815, the most powerful eruption in recorded history, lowered global temperatures by about 3 C.
• Eg: Mt. Vesuvius (Italy), Mt. Fuji (Japan), Mt. St Helens (USA), Mt Pinatubo (Philippines)

Supervolcanoes

Fountain of lava about 10 m high issuing forth from a vent in Hawaii

• Supervolcanoes are volcanoes with ejected material greater than 1000 cubic km, which is millions of times larger than any volcanic event in known history
• Supervolcanoes can produce devastation on an enormous continental scale
• Supervolcanoes occur when magma rises to the crust in hotspots but is enable to break through the crust. Pressure build in the large and growing magma pool until the crust is unable to contain the pressure
• Super volcanic eruptions cause long lasting climate change (esp. global cooling) and directly result in the large scale extinction of species
• There are only seven known supervolcanoes: Yellowstone Caldera (USA), Long Valley Caldera (USA), Valles Caldera (USA), Lake Toba (Indonesia), Lake Taupo (New Zealand), Aira Caldera (Japan), Siberian Traps (Russia)
• Large igneous provinces are also considered supervolcanoes due to the amount of lava released, but they are non-explosive in nature
• There have been no supervolcanic eruptions in the Holocene period (10,000 yrs BP). The last supervolcano eruption was the Lake Taupo (New Zeland) about 26,500 yrs ago

LIST OF VOLCANOES

See here for a full list of volcanoes

List of Decade Volcanoes

S. No.	Volcano	Classification	Location	Notes
1	Avachinsky-Koryaksky	Active Stratovolcano	Kamchatka Peninsula, Russia
2	Colima’s Volcano	Active Stratovolcano	Mexico	One of the most active volcanoes in North America
3	Mount Etna	Active Stratovolcano	Sicily, Italy	Largest active volcano in Europe (3300 m) One of the most active volcanoes in the world Last eruption 2008
4	Galeras	Active Stratovolcano	Colombia
5	Mauna Loa	Active Shield volcano	Hawaii, USA	Largest volcano on Earth in terms of volume and area covered One of five volcanoes that constitute island of Hawaii Eruptions tend to be non-explosive
6	Mount Merapi	Active Stratovolcano	Indonesia
7	Mount Nyiragongo	Active Stratovolcano	Congo	Famous for its crater that contains a lake of lava
8	Mount Rainier	Dormant stratovolcano	USA	Has 26 glaciers and 35 sq miles of permanent snow fields and glaciers World’s largest volcanic glacier cave network
9	Sakurajima	Active stratovolcano	Japan	Eruption in 1914 cause former island to be connected to Osumi peninsula
10	Santa Maria	Active stratovolcano	Guatemala
11	Santorini	Dormant	Greece	Forms an archipelago of volcanic islands Site of one of largest eruptions in recorded history (3600 yrs ago destroyed Minoan civilization)
12	Taal volcano	Active stratovolcano	Philippines	Currently giving signs of activity since June 2009 Currently on a Level 1 alert
13	Mount Teide	Active stratovolcano	Canary Islands, Spain	Third largest volcano in the world
14	Ulawun	Active stratovolcano	Papua New Guinea
15	Mount Unzen	Active stratovolcano	Japan	Group of several overlapping volcanoes
16	Mount Vesuvius	Active stratovolcano	Italy	Most densely populated volcanic region in the world Famous for 79 AD eruption that destroyed cities of Pompeii and Herculaneum

List of supervolcanoes

Volcano	Location	Notes
Lake Taupo	North Island, New Zealand	Latest known supervolcanic eruption (26,500 yrs BP). Ejected 1170 cu km of material Last major eruption in 180 CE (ejected 100 cu km) Eruption of 180 CE was noticed as far away as China and Rome
Lake Toba	Sumatra, Indonesia	Largest volcanic lake in the world Supervolcanic eruption 74,000 yrs ago of VEI 8 (2800 cu km) Believed to be largest eruption on Earth in last 25 million yrs ago Eruption deposited ash layer 15 cm thick all over India, some parts up to 6 m thick Caused major extinctions of plant and animal species, including severely endangering human species
Whakamaru	North Island, New Zeland
Yellowstone Caldera	Wyoming, USA	Last eruption 640,000 years ago
Island Park Caldera	Idaho, USA	Last eruption 2.1 million years ago
Kilgore Tuff	Idaho, USA	Last eruption 4.5 million years ago
Blacktail Creek	Idaho, USA	Last eruption 6.6 million years ago
La Garita Caldera	Colarado, USA	Last eruption 28 million years ago Largest known explosive eruption in history of Earth (5000 cu km)

List of volcanoes in India

Volcano	Classification	Location	Notes
Barren Island	Active stratovolcano	Andaman Islands	Only active volcano in India Last eruption in July 2009
Baratang	Mud volcano	Andaman Islands	Last eruption in 2005
Narcondam	Potentially active stratovolcano	Andaman Islands	Thought to have been inactive, but recently mud and smoke activity in June 2005 Recent activity possibly related to 2004 Indian Ocean Earthquake Narcondam is famous for the Narcondam Hornbill, an endangered species Narcondam island is the eastern-most point of the Andaman & Nicobar Islands. It is claimed by Burma
Deccan Traps	Large Igneous Province	Deccan Plateau	One of the largest volcanic features on Earth Multiple layers of basalt more than 2 km thick Technically it may classified as a supervolcano

SEMI-VOLCANIC PHENOMENA

HOT SPRINGS

Overview

• Hot springs are springs that are produced by geothermally heated groundwater
  
  

  The Grand Prismatic Spring, a hot spring in the Yellowstone National Park (USA). The distinctive colour of the spring is due to thermophile - microorganims that thrive in high temperatures
• Hot springs range from tiny seeps to veritable rivers of hot water
• Hot springs are present all over the world, on all continents and even under the ocean
• The Dalhousie Springs in southern Australia are the largest hot springs in the world in terms of volume of water
• Water from hot springs has high mineral content, containing everything from calcium to lithium and even radium. For their high mineral content, hot spring are widely sought after spa destinations
• Thermophiles – organisms that thrive in temperatures 45-80 C – are found in hot springs and geysers

Sources of heat

• Hot springs are heated by geothermal heat
• Geothermal heat is the heat from the interior of the earth
• Geothermal heat can be produced by two natural phenomena: geothermal gradient and volcanic activity
• Geothermal gradient is the increase of temperature with depth inside the earth. Water that percolates deep inside the crust comes into contact with hot rocks and gets heated
• In areas of volcanic activity, water can also be heated by coming into contact with magma. The high temperature gradient near magma causes water to boil and even become superheated
• Hot springs in volcanic areas are almost always at or near the boiling point

List of important hot springs

Hot spring	Location	Notes
Aachen hot spring	Germany	The hottest springs in continental Europe (74 C)
Yangbaijing hot spring	Tibet	Several square km in size Probably the highest hot springs in the world Supplies a large part of the electricity for Lhasa
Icaria	Greece
Rio Hondo hot springs	Argentina	One of the most visited hot springs
Deildartunguhver	Iceland	Very high flow rate Very high water temperature (97 C) Used for heating neighbouring towns

GEYSERS

The Crystal Geyser in the Yellowstone National Park

• A geyser is a spring characterised by intermittent discharge of water ejected turbulently. The water discharge is accompanied by vapour (or steam) as well
• The word geyser comes from a spring in Haukadalur, Iceland called Geysir
• Geysers are temporary geological phenomena. The lifespan of geysers is a few thousand years at most.
• Generally all geyser sites are located close to active volcanic areas
• There are about a thousand known geysers in the world. Half of the world’s geysers are in the Yellowstone National Park, USA.
• Geysers are fragile phenomena and if conditions change, they can die. Many geysers have been destroyed by people throwing litter and debris into them, others have ceased due to dewatering by Geothermal power plants
• Geysers have been observed on the moons of other planets as well

Sources of activity

• Geysers are generally associated with volcanic areas, as the geyser effect is due to proximity of magma
• Surface water works its way down to a depth of about 2000 m, where it meets hot rocks of magma.
• The resultant boiling of the pressurised water results in the geyser effect of hot water and steam spraying out of the surface vent
• Geysers are relatively rare phenomena. They require a combination of three geological conditions
  • Intense heat: geysers need intense heat, which is provided by magma. The high pressures deep inside the earth raises the boiling point of water resulting in superheated water
  • Water: for geysers to exist, water must be available in the area. The water must be able to travel underground through deep pressurised fissures
  • Plumbing system: this includes a reservoir to hold the water while it is being heated and a vent on the surface for ejecting it. The plumbing system is made of a system of fissures, fractures, porous spaces that are essential for building up pressure before an eruption
    
    

    Distribution of major geysers in the world
• The geyser produces a material called geyserite that deposits onto the walls of the plumbing system making it pressure-tight. Geyserite is produced by rocks in the vicinity of the geyser, and consists mainly of silicon dioxide

Types of geysers

• Fountain geysers: erupt from pools of water in a series of intense violent bursts. Eg: Grand Geyser in Yellowstone National Park (USA)
• Cone geysers: erupt from cones or mounds of geyserite in steady jets that last from a few seconds to several minutes. Eg: Old Faithful Geyser in Yellowstone National Park (USA)

Important geysers in the world

Geyser	Location	Notes
Yellowstone National Park	Northwest USA	Home to half the world’s geysers World’s tallest predictable geyser: Grand geyser World’s tallest geyser: Steamboat geyser (300 ft) One of the most predictable geysers: Old Faithful
Valley of Geysers	Kamchatka Peninsula, Russia	Second largest concentration of geysers in the world Only geyser field in Eurasia
El Tatio	Chile	Very low height of eruptions (max 6 m)
Taupo Volcanic Zone	North Island, New Zealand	Was home to the world’s largest geyser: Waimangu Geyser (500 m) However, geothermal changes have changed the geyser field
Haukadalur Other places	Iceland	Geysers are distributed all over Iceland Famous geysers include Great Geysir and Stokkur

Cold water geysers

• Cold water geysers are similar to hot water geysers, except that carbon dioxide bubbles drive the eruption instead of steam
• In cold water geysers, carbon dioxide laden water lies in a confined aquifer trapped by less permeable overlying strata
• The column of water exerts enough pressure on the CO2 such that it remains in water in small bubbles
• When the pressure decreases due to formation of fissures, the CO2 bubbles expand and cause eruption
• CO2 laden water in cold water geysers are more white and frothy than hot water geysers
• Eg: Crystal Geyser in northwest USA, Geysir Andernach (Germany)
  
  

  A fumarole at the Halema Umau crater in Hawaii

Geysers in the solar system

• Geysers have been observed elsewhere in the solar system
• Unlike eruptions on earth, these geysers mainly consist of gas, dust and ice particles, without liquid
• Geysers have been observed on Saturn’s moon Enceladus, Neptune’s moon Triton and in the south pole of Mars

FUMAROLES

• A fumarole is an opening in the earth’s crust that emits steam and gases. Fumaroles occur in the neighbourhood of volcanoes
• Typical emitted gases include carbon dioxide, sulphur dioxide, hydrochloric acid and hydrogen sulphide
• Fumaroles occur when magma at shallow depths release gases or interact with groundwater releasing steam
• Fumaroles may persist for centuries if they occur over a persistent heat source, or disappear in weeks if they occur over fresh volcanic deposit that quickly cools off
  
  
  

  A mud pot in Namafjall, Iceland
• Eg: Valley of Ten Thousand Smokes (Alaska, USA), Yellowstone National Park (USA)

MUD POTS

• A mud pot is a hot spring or fumarole consisting of a pool of bubbling mud
• Mud pots form in high temperature geothermal areas where water is in short supply
• The little water that is available rises to the surface at a spot rich in volcanic ash, clay and other particles
• The mud takes the form of a viscous bubbling slurry
• Eg: Yellowstone National Park (USA)

EARTHQUAKES

Global distribution of earthquake epicentres, 1963-1998

• An earthquake is a result of a sudden release of energy in the earth’s crust that creates seismic waves
• Earthquakes are recorded with a seismograph and are reported on a magnitude on the Richter scale.
• In general, earthquakes of magnitude less than 3 are imperceptible, and more than 7 cause serious damage
• The intensity of an earthquake can also be measured on the Modified Mercalli (MM) scale. The MM scale quantifies the effect an earthquake has on humans, natural objects and man-made structures
• The shaking caused by earthquakes can result in landslides, and in volcanic activity as well. When a large earthquake occurs in the oceans, the ocean floor can suffer sufficient displacement to cause a tsunami
• Earthquakes are usually caused by rupture of geological faults, but can also be caused by volcanic activity, landslides, mine blasts and nuclear experiments
• The point of initial rupture of an earthquake is called its hypocentre, while the point on the surface directly above it is called the epicentre
• Earthquakes that occur under the ocean and of high magnitude can generate tsunamis (eg 2004 Indian Ocean tsunami)
  
• The most powerful earthquake ever recorded is the Valdivia earthquake in Chile in 1960. It measured 9.5 on the Richter scale
  

Mechanism of action

• Earthquakes can occur anywhere within the earth where there is stored elastic energy sufficient enough to drive fault propagation along a fault plane
• Tectonic plates move past each other smoothly only if there are no irregularities and asperities. Most plate boundaries do have asperities and this leads to stick-slip behaviour
• Once the boundary has locked into a relative stable position, continued relative motion between the plates leads to increased stress and stored strain energy
• This continues until the stress rises sufficiently to break through the relative stable position, suddenly sliding over the locked position of the fault and thereby releasing the stored energy
• The energy is released as a combination of elastic seismic waves, frictional heating of the surface and cracking of rock, thereby causing an earthquake
• This process of gradual build up of stress and sudden release of energy in the form of earthquakes is called elastic-rebound theory
• It is estimated that less than 10 % of the total energy of an earthquake is radiated as seismic energy. Most of the earthquake’s energy is used to power fracture growth or is converted as heat generated by friction
  

Occurrence of earthquakes

• Minor earthquakes occur nearly constantly. Most of these happen in places like California and Alaska in the US, as well as in Guatemala, Chile, Peru, Indonesia, Iran, Pakistan, Turkey, Greece, Italy, Japan and New Zealand. Larger earthquakes occur less frequently
• However, in general, earthquakes can occur almost anywhere (even away from plate boundaries)
• The relationship between frequency and intensity of earthquakes is roughly exponential i.e. for instance, there are roughly 10 times as many earthquakes of magnitude 4 as of magnitude 5
• Most of the world’s earthquakes occur in Pacific Ring of Fire seismic belt. Massive earthquakes occur along other plate boundaries too, such as the Himalayas
  

Induced seismicity

• While most earthquakes occur due to natural movement of the earth’s tectonic plates, human activity can produce earthquakes as well
• Four main human activities that contribute to earthquakes include
  • Large dams
  • Drilling and injecting liquids into wells
  • Coal mining
  • Oil drilling
• For instance, the 2008 Sichuan earthquake in China is believed to have been caused by the Zipingpu dam which caused the pressure of a nearby fault to fluctuate, increasing the movement of the fault and the magnitude of the earthquake

Earthquakes and volcanic activity

• Earthquakes often occur in volcanic areas
• They are caused both by tectonic faults and the movement of magma in volcanoes
• Such earthquakes can serve as early warning of impending volcanic eruptions. Eg: Mount St Helens eruption of 1980 (USA)
  

Seismic waves

• Seismic waves are waves of force that travel through the earth
• Earthquakes produces different types of seismic waves that travel through the earth at different velocities:
  • P waves (Pressure or Primary waves): they are longitudinal waves that travel fastest through solids, and are therefore the first waves to appear on a seismogram
  • S waves (shear or secondary waves): transverse waves that travel slower than P waves.
    
    They do not exist in fluids such as air or water
    
  • Surface waves (Rayleigh waves and Love waves): slower than P and S waves, but have much larger amplitude. These surface waves cause most damage during an earthquake
• The propagation velocity of the seismic waves depends on density and elasticity of the medium
• In solid rock, P waves travel at about 6-7 km/s (within the mantle about 13 km/s), while S waves travel at about 2-3 km/s (mantle 9 km/s)

Global tectonic plate movement

• Earthquakes can be recorded at great distances, since seismic waves travel through the whole of the earth’s interior
• The absolute magnitude of a quake is reported on the Moment Magnitude scale, while perceived magnitude is reported on the Modified Mercalli (MM) scale. The Richter scale is another scale that measures the absolute magnitude – it is no longer used in academic circles but is still used in popular parlance.
  
• As a rule of thumb, the distance to the earthquake epicentre is the number of seconds between the P and S waves multiplied by 8

Major earthquakes

S. No.	Date	Location	Magnitude
1	1960	Valdivia, Chile	9.5
2	Dec 2004	Sumatra, Indonesia	9.3
3	1964	Alaska, USA	9.2
4	1952	Kamchatka, Russia	9.0
5	1700	Cascadia Subduction Zone (Pacific Ocean rim)	9.0

OCEANS

Division of ocean depths

• The World Ocean is a global, interconnected continuous body of saline water. Approximately 71% of the earth’s surface is covered by the ocean
• For human convenience, the ocean has been divided into several smaller divisions known as oceans and seas
• There are five major divisions of the world ocean: Pacific Ocean, Atlantic Ocean, Indian Ocean, Arctic Ocean and Southern Ocean
• Evaporation of water from the oceans is the source of most rainfall, and ocean temperatures determine climate and wind patterns on land
• Life within the ocean evolved about 3 billion years prior to life on land. More than 230,000 marine life forms are currently known, but the actual number may be 10 times as much
  

Physical properties

• The total area of world ocean is 361 x 10⁶ sq km, and volume is approx 1.3 billion cu km
• The average depth of the ocean is 3790 m and maximum depth is 10,923 m
• The average density of sea water is 1.025 g/ml and has a freezing point of -2 C
• Sea water contains more dissolved ions than all types of freshwater, especially sodium and chloride. On average, sea water has a salinity of 3.5 %.
• The causes of high salinity of sea water include
  • River runoff causing concentration of sodium in the ocean
  • Sodium leaching out of the ocean floor when the ocean was formed
  • Chloride abundance due to volcanic activity and hydrothermal vents on the ocean floor
• Ocean salinity has been stable for billions of years

Sea level

• Mean Sea Level (MSL) is a measure of the average height of the ocean’s surface
• Mean Sea level is usually taken to be the half way point between mean high tide and mean low tide
• Sea level change can be measured in two ways
  • Local change: local mean sea level can be affected by vertical movement of land, and changes in atmospheric pressure, ocean currents and local ocean temperature
    
  • Eustatic change: is the alteration of global sea levels, such as changes in volume of water in the world oceans and changes in volume of ocean basins
    
• Short term changes in sea level can arise from tides, atmospheric pressure, storm surges, El Nino etc
  
• Medium term changes in sea level arise mainly from two factors: atmospheric temperature and the mass of water locked up as fresh water in rivers, lakes, glaciers, ice caps etc
  
• Geological changes in sea levels mainly arise from changes in the configuration of continents and sea floors due to plate tectonics and seafloor spreading
  
• On a geological time scale, long term sea level has always been higher than today (except at the Permian-Triassic boundary 250 million years ago). As a result, sea level is more likely to rise than fall today, even due to small changes in climate
• Over the past 100 years, sea level has been rising at an average of 1.8 mm per year. The majority of this rise is attributed to thermal expansion of ocean water due to increase in ocean temperatures

Ocean currents

• Ocean current is a continuous directed movement of ocean water generated by wind, Coriolis force, temperature and salinity gradients, and tides
  
  

  Major ocean currents of the world
• Ocean currents greatly affect earth’s climate by transferring heat from the tropics to polar regions, and transferring precipitation to coastal regions
• The most famous example of ocean currents is the Gulf Stream, which makes northwest Europe much more temperate than any other region at that latitude
• Surface ocean currents are generally driven by wind, and circulate in clockwise direction in the northern hemisphere and anticlockwise in the southern hemisphere (due to prevailing winds). Surface currents make up about 10% of all ocean currents
• Deep ocean currents, called thermohaline circulation, are driven by water density and temperature gradients. Also known as the world’s conveyor belts, these deep ocean currents supply heat to polar regions and thereby regulate sea ice formation. Deep ocean currents make up about 90% of all ocean currents
• Ocean currents are measured in Sverdrup (Sv), with 1 Sv being equivalent to a flow rate of 1 million cu. m per second

MARINE GEOGRAPHY

Oceanic basins

• Oceanic basins are large geologic basins (large scale rock strata) that are below sea level
• In a sense, oceanic basins are the complement to continents
• Ocean basins serve as sedimentary basins that collect sediments eroded from continents
• Ocean basins can be actively changing or inactive depending on plate tectonics. The Atlantic and Antarctic Ocean basins are actively growing while the Mediterranean is shrinking. Inactive ocean basins include the Gulf of Mexico, the Sea of Japan and the Bering Sea

Schematic of a continental shelf

• The continental shelf is the extended perimeter of a continent which is currently under sea
• Continental shelves were part of the continents during glacial periods (when sea levels were low) but are under sea during interglacial periods (like today)
• The continental shelf usually ends at a point of decreasing slope, called the shelf break. The sea floor below the shelf break is called the continental slope. Below the slope is the continental rise, which merges into the deep ocean floor (called abyssal plain)
• Due to the availability of sunlight in shallow waters, continental shelves teem with life, compared to the biotic deserts in the deep ocean abyssal plains
• Continental shelves consist of thick sediments from the continents
• Continental shelves extend on average about 80 km from the coast. The largest shelf, the Siberian Shelf in the Arctic Ocean, stretches to about 1500 km, while certain areas have no shelves at all such as the coast of Chile and the west coast of Sumatra (Indonesia)
• The United Nations Convention on the Law of the Sea (UNCLOS) defines the extent and regulates usage of continental shelves by sovereign nations
  • The continental shelf was defined as the natural prolongation of land to the continental margin’s outer edge, or 200 nautical miles from the coast, whichever is greater. 
    
  • However, the shelf is to not exceed 350 nautical miles, and it is to not exceed 100 nautical miles beyond the 2500m isobath
  • The coastal nations have the exclusive right to harvest mineral and non-living material in the subsoil of the continental shelf
  • Coastal states also have exclusive rights to living resources “attached” to the shelf, but not to creatures living there freely

Mid Ocean Ridges

• A mid ocean ridge is an underwater mountain range formed by plate tectonics
• Mid ocean ridges are caused by seafloor spreading i.e. magma rising through the crust and emerging as lava which then cools to form new oceanic crust
• A mid ocean ridge demarcates the boundary between two tectonic plates, and is called a divergent plate boundary
• The various mid-ocean ridges of the world are connected and form a single global mid ocean ridge system which covers every ocean. Thus, the mid ocean ridge system is the longest mountain range in the world (over 65,000 km)
• Mid ocean ridges are geologically active, with new magma constantly emerging onto the ocean floor

Ocean Trenches

• Ocean trenches are large scale long but narrow depressions on the sea floor. They are the deepest parts of the ocean floor
• Trenches are found at convergent plate boundaries, where one plate subducts (descends) beneath another. On average, oceanic crust moves into trenches at a rate of about 0.1 sq m per second
• They are usually located parallel to volcanic arcs at a distance of about 200 km
• Ocean trenches typically extend about 3-4 km below the level of the surrounding sea floor
• The deepest ocean depth known is the Challenger Deep point of the Mariana Trench in the Pacific Ocean (10,911 m)

Trench	Location	Depth	Notes
Mariana Trench	Western Pacific Ocean (near Philippines and Japan)	10,911 m	Deepest known part of the ocean Lowest elevation on the surface of the earth’s crust Maximum depth is recorded at Challenger Deep, a small valley at its southern end Formed by the subduction of Pacific plate under Mariana plate
Tonga Trench	Southern Pacific (near New Zealand)	10,882 m	Formed by subduction of Pacific plate under Tonga plate and Indo-Australian plate Fastest plate velocity recorded on earth (24 cm per year)
Kuril-Kamchatka Trench	Northern Pacific	10,542 m
Philippine Trench	Philippines (Pacific Ocean)	10,540 m
Kermadec Trench	New Zealand (Pacific Ocean)	10,047 m

Extraterrestrial oceans

• The earth is the only known planet to have liquid water on its surface
• However, liquid water is known to be present under the surface on Jupiter’s moons Europa, and possibly on Ganymede and Callisto
• It is believed that Venus once had liquid water and oceans on its surface, but they have now vanished
• Saturn’s moon Titan is thought to have subterranean water ocean under its crust (which consists of ice and hydrocarbons)

OCEANIC HABITATS

Corals are nocturnal feeders. Here, in the dark, the coral polyps extend their tentacles to feed on zooplankton

• Coral reefs are aragonite structures formed by living animal colonies.Aragonite is a carbonate mineral, one of the two major naturally occurring crystalline forms of calcium carbonate (the other being calcite)
• Reefs consist mostly of stony corals. These corals are built from polyps that secrete an exoskeleton of calcium carbonate
• Coral reefs grow best in shallow, clear, sunny waters. They are usually found in shallow depths in the tropics, but deep cold water reefs also exist although on a much smaller scale
• Coral reefs are some of the richest and most diverse ecosystems in the world. They occupy less than 1% of world ocean surface, but provide habitat to about 25% of all marine species
• Reefs are found in ocean waters containing few nutrients. High nutrient levels, such as found in agricultural runoff, can harm reefs by encouraging excess algae growth
• Coral reefs are under threat from climate change, ocean acidification, overfishing and overuse of reef resources

Coral reef	Location	Notes
Great Barrier Reef	Queensland, Australia (northeast Australia)	Largest coral reef system in the world World’s biggest structure made by living organisms Area of approx 344,000 sq km
Belize Barrier Reef	Belize	Second largest coral reef in the world Part of the Mesoamerican Barrier Reef that stretches along eastern coast of Central America from Mexico to Honduras
New Caledonia Barrier Reef	New Caledonia (southwest Pacific)	French territory in southwest Pacific Home to endangered dugong, and nesting site for green sea turtle
Andros (Bahamas) Barrier Reef	Bahamas (Caribbean)
Red Sea Coral Reef	Red Sea
Pulley Ridge	Florida, USA (southeast USA)	Deepest photosynthetic coral reef in the world (about 60-80 m)
Maldives	Indian ocean	Consists of about 1200 coral islands
Raja Ampat Islands	Indonesia	Contains the highest marine life diversity in the world

Deep sea and trenches

• As the ocean depth increases, sunlight decreases and water pressure increases.
• In general, sunlight is not able to penetrate the ocean water beyond a depth of 200 m. This depth is considered to be the beginning of aphotic zone (deep sea). Unusual and unique creatures inhabit these depths including giant squid, gluper eel, angler fish and vampire squid
• In the trenches, water pressure is extreme and sunlight is non existent. However, small flounder fish (family Soleidae) and shrimp have been observed even at these depths
• Seamounts (extinct undersea volcanoes that rise to shallow depths) provide natural habitats for fish and other species to spawn and feed
• Hydrothermal vents on the ocean floor support unique life forms, deriving essential nutrients from the chemicals released by volcanic activity

Open Ocean

• The open ocean is relatively unproductive due to lack of nutrients. However, simply due to its vastness, it possesses the largest number of life forms in total
• In the aphotic zone, energy for life forms is mainly supplied in the form of detritus, which is non living organic material consisting of dead organisms and fecal material
• The open ocean consists mainly of jelly fish and its predators like the mola mola

Intertidal and shores

• Intertidal zones are those areas close to the shore, which are constantly being covered and exposed by the tides
• These areas can be underwater anywhere from daily to very infrequently
• A huge array of life forms is found in this zone. This includes crabs, snails etc

FORESTS

Overview

• Forests are areas with a high density of trees
• Forests cover approximately 9.4% of the Earth’s surface i.e. about 30% of total land area, although they used to cover as much as 50% of land area
• Forests are differentiated from woodland by the extent of canopy coverage: in forests, the foliage of trees meet and interlock while in woodlands there is enough gap between trees allowing sunlight to penetrate to the ground
• Forests are one of the most important aspects of the Earth’s biosphere
• The functions of forests include
  • Habitat for organisms
  • Hydrologic flow modulation
  • Soil conservation
• Human factors affecting forest sustenance include logging, urban sprawl, agriculture, industries, human-induced forest fires etc. Natural factors affecting forests include forest fires, insects, diseases, weather etc
• Only about 20% of the world’s original forests remain in undisturbed forest. Of this, 75% are in Russia, Canada and Brazil

Distribution of forests

• In general, forests can be found in all regions capable of sustaining tree growth (at altitudes up to the tree line), except where natural disturbance is too high or human activity has altered the environment
• The areas between latitudes 10 N and 10 S are mostly covered in tropical rainforests, and between 53N and 67N have boreal forests (taiga)
• Forests can contain many species in a small area (like rainforests) or relatively few species in a large area (like taiga and montane coniferous forests)
• Forests have higher biomass per unit area compared to other vegetation types. Much of the forest biomass occurs below the ground in root systems and partially decomposed detritus
• The major types of forest systems are
  • Rainforests (both tropical and temperate)
  • Taiga
  • Temperate broadleaf forests
  • Tropical dry forests

Temperate rainforest in Washington, USA (northwestern USA)

Old growth forests

• Old growth forests (also called primary forests, ancient forests) are forests that contain trees which have attained great age
• Old growth forests typically contain large and old live trees, large dead trees and large logs
• Death of individual trees creates gaps in the canopy layer allowing light to penetrate and create favourable conditions for undergrowth
• Old growth forests are often home to rare and threatened species, making them ecologically significant. For instance, the Northern Spotted Owl is reliant on old growth forest
• The importance of old growth forests include
  • They contain rich communities of plants and animals due to the long period of forest stability
  • They serve as a reservoir for species that cannot thrive or regenerate in younger forests
  • They store large amounts of carbon both above and below the ground (either as humus or in wet soils as peat)
• Forests that are regenerated after disruptions must wait several centuries to millennia before they can reach the stable equilibrium that signifies old growth forests
• Due to increased human activity, old growth forests have been substantially destroyed over the last century. Of the old growth forests that still remain, 35% are in Latin America (Brazil), 28% in North America (mainly Canada) and 19% in northern Asia (Siberia)

Second growth forests

• Second growth forests (secondary forests) are forests that have re-grown after a major disturbance such as fire, insect infestation, logging, windthrow etc
• Second growth forests tend to have trees closer spaced than primary forests and have more undergrowth
• Second growth forests usually have less biodiversity than old growth forests, since the former have had lesser time to develop and reach stable equilibrium
• Secondary forests are common in areas under shifting agriculture, areas with forest fires, and forests that are recovering from harvesting and agriculture
• Secondary forests can several generations of trees (centuries) to resemble the original old growth forests. However, in some areas, secondary forests do not succeed due to soil nutrient loss and erosion (especially in tropical rainforests)
• Most of the forests of eastern North America and of Europe are secondary forests

Distribution of tropical rainforests in the world

Overview

• Rainforests are forests characterised by high rainfall, with minimum annual rainfall as high as 1700-2000 mm
• Rainforests are responsible for 28% of the world’s oxygen turnover.However, rainforests do not contribute much to the net oxygen additions to the atmosphere. Instead, they are vital in storing carbon in bio sequestration
• The Intertropical Convergence Zone (ITCZ), the area near the equator where winds originating in the northern and southern hemispheres meet, plays a significant role in creating the rainforests
• Despite the growth of vegetation, soil quality in a rainforest is poor.Most trees have roots near the surface due to lack of nutrients below the ground
• More than half the world’s species of plants and animals are found in rainforests

Tropical Rainforests

• Tropical rainforests are rainforests in the tropics, near the equator between the Tropic of Cancer and the Tropic of Capricorn
• Tropical rainforests are found in South America (Brazil), Central America (Yucatan Peninsula), Sub-Saharan African (Congo), Northeast India, Southeast Asia (Indo-Malaya, Indonesia, Papua New Guinea)
• Tropical rainforests are called ‘world’s largest pharmacy’, since over 25% of modern medicines originate from these plants
• Tropical rainforests are home to half of all the plant and animal species on earth
• Tropical rainforests are characterised by heavy rainfall, resulting in poor soil due to leaching of nutrients
• Temperatures range 15 C to 50 C. Rainfall ranges from 1250 mm to 6600 mm annually

Temperate Rainforests

• Temperate rainforests are rainforests that occur in the temperate zone and receive high rainfall
• Temperature range 4 to 12 C. Rainfall minimum is around 1400 mm annually
• Temperate rainforests are found in close proximity to oceans, and usually occur in coastal mountains. This is because temperate rainforests depend on the proximity to oceans to moderate seasons, creating milder winters and cooler summers. Coastal mountains increase rainfall on the ocean facing slopes
• Wildfires are uncommon in temperate forests due to the high moisture content in the forest
• Mosses are abundant in temperate rainforests
• Temperate rainforests sustain the highest levels of biomass of any terrestrial ecosystem
• Temperate rainforests are notable for trees of massive proportions, including coast redwood, coast douglas fir, sikta spruce etc
• Temperate rainforests are found in western North America, south-western South America, Norway, northern Spain, south-eastern Australia and New Zealand

Distribution of temperate rainforests in the world

• Rainforests are typically divided into four layers, each with different plants and animals adapted for life in that environment
• Emergent layer
  

  
  
  • The highest layer, formed by a small number of very tall trees that grow above the general canopy
  • They reach heights of 45-55 m, occasionally even 70-80 m
  • Need to be able to withstand high temperatures and strong winds
  • Eagles, butterflies, bats and some monkeys inhabit this layer
• Canopy layer
  

  
  
  • The canopy layer consists of the largest number of tall trees, which provides a more or less continuous cover of foliage by adjacent treetops
  • Usually reach heights of around 30-45 m
  • The canopy layer is the densest area of biodiversity in a rainforest. It is estimated that the canopy layer is home to about 50% of all plant species and 25% of all insect species
• Understory
  

  
  
  • The understory layer lies between the canopy and the forest floor
  • Leaves are much larger at this level
  • Only about 5% of sunlight incident on the rainforest reaches the understory layer
  • This layer is home to a number of birds, snakes, lizards, and predators like jaguar, boa constrictors etc
• Forest floor
  

  
  
  • The forest floor is the bottom most layer
  • The forest floor receives only about 2% of the sunlight. Only plants adapted to low light can grow in this region
  • Due to low sunlight penetration, forest floor is relatively clear of vegetation. This makes it possible to walk through a rainforest

Effect on global climate

• Rainforests emit and absorb massive quantities of carbon dioxide. Undisturbed rainforests usually have no net impact on atmospheric carbon dioxide levels
• However, rainforests play a vital role in other climatic effects such as cloud formation and water vapour recycling
• Deforestation caused by human activities and drought can cause rainforests to release massive amounts of carbon dioxide into the atmosphere

TROPICAL DRY FORESTS

Overview

• Tropical dry forests are located in the tropical and subtropical latitudes
• These forests occur in areas that are warm and receive plentiful rainfall (several hundred centimetres) but experience long dry seasons which last several months. These seasonal droughts have great impact on the forest
• Deciduous trees dominate in these forests
• Tropical dry forests are less biologically diverse than rainforests
• However, they are home to a wide variety of wildlife including monkeys, deer, parrots, large cats etc. Mammalian biomass tends to be higher in dry forests than in rainforests.

Characteristics

• During the drought season a leafless season occurs. The shedding of leaves allows trees like teak and ebony to conserve water during these dry periods
• When the trees enter the dry leafless season, the canopy layer opens up allowing sunlight to reach the ground, thereby enablinggrowth of thick undergrowth
• However, certain areas of tropical dry forests can have evergreen trees. This happens especially when the forests are on moisture sites or have access to groundwater
• Three tropical dry forest regions have evergreen forests:
  • East Deccan dry evergreen forests (India)
  • Sri Lanka dry zone evergreen forests (Sri Lanka)
  • Southeastern Indochina dry evergreen forests (Cambodia, Laos, Thailand, Vietnam)
• The forests of central India are tropical dry forests
• Dry forests are extremely sensitive to forest fires, overgrazing and deforestation. Restoration of dry forests is possible, but challenging

Distribution

• Dry forests tend to exist north and south of the equatorial rainforest belt, and south and north of the subtropical deserts
• They usually occur in two bands: one between 10 and 20 N latitudes and the other between 10 and 20 S latitudes
• The most diverse dry forests of the world are found in southern Mexico and Bolivia
• The dry forests of central India and Indochina are notable for their diverse and large vertebrate fauna
• Other tropical dry forests are found in New Caledonia,     Madagascar, south eastern Africa and the Pacific coast of South America

TEMPERATE BROADLEAF AND MIXED FORESTS

Overview

• Temperate broadleaf and mixed forests are a temperate and humid biome (ecological system)
• These forests typically have four layers
  • Canopy layer: contains mature trees 100-200 ft high
  • Understory: shade tolerant layer of trees that grow to about 30-50 feet shorter than the canopy
  • Shrub layer: low growing woody plants
  • Herbaceous layer: this is the ground cover, most diverse layer
• Characteristic broadleaf trees these forests include oaks, birches, beeches and maples. Mixed trees are basically coniferous trees such as pines, firs and spruces
• Areas of temperate broadleaf and mixed forests include northeast USA, northern India, eastern Australia, New Zealand, southwest China

Distribution

• Temperate broadleaf and mixed forests occur in areas with distinct warm and cool seasons, with moderate annual average temperature (5-15 C)
• They usually occur in moderately warm and rainy climates, sometimes with a distinct dry season
• Annual rainfall is typically over 600 mm and sometimes over 1500 mm

The Taiga, the world's largest terrestrial biome, is found throughout the high nothern latitudes

Overview

• Taiga is a biome characterised by coniferous forests
• The Taiga is the world’s largest terrestrial biome
• The taiga experiences relatively low precipitation (250mm – 750 mm), mostly in the form of fog, snow and summer rain. However, since evaporation is also low, there is enough moisture to enable dense vegetation growth
• Taiga soils tend to be young and nutrient-poor. The soil tends to be acidic and hence the forest floor only has lichens and mosses growing

Characteristics

• The taiga has harsh continental climate and large range of temperatures: -54 C to 27 C
• Except for the tundra and permanent ice caps, the taiga is the coldest biome on earth
• There are two main types of taiga:
  • Closed forest: closely spaced trees with mossy ground cover
  • Lichen woodland: trees that are farther apart and have lichen ground cover. More common in colder areas
• In the northern taiga areas, forest cover is not only sparse but also stunted
• The forests of taiga are mainly coniferous consisting of larch, spruce, fir and pine
• Taiga trees tend to have shallow roots to take advantage of thin soils.
• Since the sun is low on the horizon most of the time, it is difficult to photosynthesise. Pine and spruce do not lose their leaves in winter and can photosynthesise using their older leaves
• The adaptation of evergreen needles (on pines) limits water lost to transpiration and the dark green colour increases sunlight absorption

Distribution

• The taiga covers most of Canada, Alaska, Sweden, Finland, Norway, the Scottish Highlands and Russia. It is also found in parts of northern USA, northern Kazakhstan, northern Mongolia and northern Japan
• Large areas of Siberia’s taiga have been destroyed in recent years
• In Canada, less than 8% is protected development and more than 50% has been allocated for logging

Taiga in Alaska, USA

• The taiga is home to a large number herbivorous mammals and smaller rodents
• Some of the animals, like bears, eat in summer and hibernate in winter. Others have evolved layers of fur to insulate them from the cold
• Due to the climate, carnivorous diets are inefficient for obtaining energy.
• A significant number of birds like Siberian thrush, white throated sparrow, black throated green warbler migrate to the taiga to take advantage of long summer days

DESERTS

Overview

• A desert is a region that receives almost no rainfall. In general deserts are areas with a moisture deficit i.e. lose more moisture than receive
• Deserts are defined as areas with average precipitation less than 250 mm per year or where more water is lost by evaporation and transpiration than falls by precipitation
• Deserts are located where vegetation is sparse or nonexistent
  
• Deserts constitute about one third (33%) of the Earth’s land surface
• The largest desert on Earth is Antarctica

Classification of deserts

The world's largest deserts (excluding polar deserts)

• Hot deserts
  • This is the most common form of desert
    
  • They have large diurnal (daily) and seasonal temperature variation, with daytime temperatures reaching more than 45 C in the summer and dipping to 0 C at night in the winter
    
  • Water acts to trap IR radiation from both the sun and the ground, and dry desert air is incapable of blocking sunlight during the day or trapping heat at night
  • The largest hot desert is the Sahara Desert
• Cold deserts
  • Cold deserts (aka polar deserts) are deserts which occur in extremely cold regions. In cold deserts, the mean temperature during the warmest month is less than 10 C
    
  • Cold deserts form due to extreme lack of precipitation (in the form)
    
  • Cold deserts are covered in snow and ice. Due to lack of liquid water, cold deserts cannot support life
  • Instead of sand dunes, polar deserts have snow dunes (in areas where precipitation is locally available)
  • The largest cold desert is the continent of Antarctica
• Montane deserts
  • Montane deserts are deserts that occur at very high altitudes
  • Example: Ladakh, Tibet
  • These places are profoundly arid (low humidity) due to their large distance from the nearest available source of moisture
    
• Rain shadow deserts
  • Rain shadow deserts form when tall mountain ranges block clouds from reaching areas in the direction of the wind
  • As air moves over the mountains, air cools and moisture condenses, causing precipitation on the windward side of the mountain. When the air reaches the leeward side, it is dry since it has already lost all its moisture, resulting in a desert
    
  • Example: Tirunelveli area in southern Tamil Nadu
    

Flora and Fauna in deserts

The snow surface at Dome C in Antarctica is representative of most of the continent's surface

• Although deserts are generally thought to support little life, in reality deserts do have high biodiversity
• Animals in the desert include kangaroo rat, coyote, jackal, jack rabbit and lizards
• Most desert animals remain hidden during the daytime to control body temperature and limit moisture needs
• Animals that have adapted to live in deserts are called xerocoles. A particularly well-studied adaptation is the specialisation of mammalian kidneys shown by desert-inhabiting species
• Deserts typically have plant cover that is sparse but diverse
• Most desert plants are salt and drought tolerant, such as xerophytes
• Some desert plants store water in their leaves, stems and roots. Others have long taproots that penetrate deep into the ground to reach the water table, or have roots that spread over a wider area in order absorb moisture from the ground
• Another desert adaptation is the development of long spiny needle-like leaves that lose less moisture to transpiration
• The giant Saguaro cacti, which grow to about 15 m height, are commonly found in the Sonora desert in Arizona (USA). The Saguaro cacti grow slowly but live up to 200 years, provide nests for desert birds and serve as desert trees
  

Water in deserts

• Rain does fall occasionally on deserts, and when they do, desert storms are often violent
• Large storms in the Sahara deliver up to 1mm of rain per minute
• Normally dry streams, called arroyos or wadis, can quickly fill up following rain and cause dangerous flash floods
• A few deserts are also crossed by ‘exotic’ rivers – rivers that originate elsewhere but run through desert areas. These rivers lose enormous quantities of water to evaporation while journeying through the desert, but have sufficient volume to ensure continuous flow. Examples: Nile, Colorado and Yellow rivers
• Desert lakes can form where rainwater or meltwater in interior drainage basins is sufficient. Desert lakes are usually salty, shallow and temporary.
• Since they are shallow, wind stress can make the lake waters move over several sq km.
• When desert lakes dry up, they leave a salt crust or hardpan. This flat area of clay, silt and sand encrusted by sand is called a playa or sink. The flat terrains of playas and hardpans makes them excellent speedways and natural runways for aircraft
• Examples of desert lakes: Great Salt Lake (Utah, USA)
• The Atacama Desert in Chile is the driest place on Earth. Blocked from moisture on both sides by the Andes and the Chilean coastal range, the Atacama is virtually sterile and devoid of all life. The average rainfall in the region is 1 mm per year. Some weather stations in the desert have never received rain.
  

Mineral resources in deserts

The Saguaro Cactus tree in the Sonoran Desert (USA) can grow up to 15 m and live up to 200 years

• Deserts may contain a great amount of mineral resources over their entire surface
• The red colour of many sand deserts is due to the occurrence of laterite. Laterite, rich in iron and aluminium, is commonly used in making bricks
• Evaporation enriches mineral accumulation in desert lakes, including gypsum, sodium salts and borates
• The Great Basin Desert (USA) has been extensively used to mine borates, which are used in the manufacture of glass
• The Atacama Desert (Chile) is abundant in saline minerals. Sodium nitrate for fertilisers and explosives has been mined from the Atacama since the middle of the 19^th century
• Significant petroleum deposits are found in desert regions. However, these oil fields were originally formed when the areas were shallow marine environments.Subsequent climate change has rendered these regions arid
• Deserts are also increasingly seen as sources of solar energy. It is estimated that all the world’s electricity needs could be met by 10% of the solar energy tapped from the Sahara Desert

Oasis

• An oasis is an isolated area of vegetation in a desert, usually surrounding a spring or similar water source
• Oases provide natural habitats for animals, plants and even humans
• Oases are formed from underground rivers or aquifers, where water reaches the surface by natural pressure

List of important deserts

S. No.	Desert	Location	Notes
1	Antarctica	Antarctica	Largest desert on earth
2	Arctic	Arctic	Second largest desert
3	Sahara	Northern Africa (Egypt, Libya, Sudan, Morocoo, Algeria)	Largest hot desert Third largest desert
4	Arabian desert	Arabia (Saudi Arabia, UAE, Yemen)
5	Gobi desert	Mongolia, China
6	Kalahari desert	Southern Africa (Botswana, parts of Namibia, South Africa)	Supports plants and animals since much of it is not a true desert Receives about 75-200 mm of rainfall per year
7	Patagonian desert	Argentina	Cold weather desert
8	Great Victoria Desert	Australia

DESERTIFICATION

Overview

• Desertification is the extreme deterioration of land in arid and dry areas due to loss of vegetation and soil moisture
• Desertification results mainly from human activities but is influenced by climatic variations
• Desertification directly results in biodiversity loss and loss of productive capacity

Causes of desertification

• The primary reasons for desertification are
  • overgrazing
  • over-cultivation
  • increased fire frequency
  • water impoundment
  • deforestation
  • overdraft of ground water
  • increased soil salinity
  • climate change
• Droughts by themselves do not cause desertification. However, continued land abuse during droughts increases land degradation leading to desertification.
• Nomadic lifestyles with slash and burn agriculture can directly lead to desertification

Historical and current desertification

The Atacama Desert (Chile), the driest place on Earth, is almost completely sterile and devoid of all life. The only such place on Earth, it has often been compared to planet Mars.

• Desertification is a historic phenomenon: the world’s largest deserts were formed by natural processes over long intervals of time. 
  
• Dated fossil pollen indicate that the Sahara has been changing between desert and fertile savanna. The Sahara is currently expanding southward at a rate of 48 km per year
  
• Drought and overgrazing in the 1930s transformed parts of the Great Plains in the US into the Dust Bowl
• Slash and burn agriculture in Madagascar has caused almost 10% of the country to become barren, sterile land

Countering desertification

• Counter-desertification techniques usually focus on two major aspects
  • Provisioning of water
  • Fixating and hyper-fertilising soils
• Fixating of soils is done by means of shelter belts, woodlots and windbreaks. Made from trees and bushes, these reduce soil erosion and evapotranspiration
• Soil fertilisation and enrichment is often achieved using leguminous plants (which extract nitrogen from air and fix into soil). Grains, barley, beans and dates are used for this purpose
• Stacking stones around the base of trees and artificial groove digging can also help plant survival by collecting morning dew and retaining soil moisture
• Desertification can also be temporarily forestalled by using sand fences (using bushes and trees), which decrease wind velocity and hence soil erosion and moisture loss
• The Green Wall project in Africa aims to plant trees in a 15 km strip from Senegal in the west to Djibouti in the east. The project aims to counter desert progression while also providing economic opportunities to the local populations
  

United Nations Convention to Combat Desertification

• The UN Convention to Combat Desertification (UNCCD) aims to combat desertification and mitigate the effects of drought
• The Convention was adopted in Paris in 1994 and came into effect in 1996. The UNCCD has 193 member nations including India
• The Convention seeks to achieve its goals through national-level action programmes that incorporate long term strategies supported by international cooperation
• It is the first and only legally binding framework to address the problem of desertification
• The nodal agency for implementing the UNCCD in India is the Ministry of Environment and Forests