World Geography

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%

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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

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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

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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

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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

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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%)

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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)

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2 December 2009 | Tags: atmosphere| No Comments »

Global Geography #3 | Study Material :: General Studies | IAS Help

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
  

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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

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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
  

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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)
  

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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
  

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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

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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
  

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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
    

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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
  

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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

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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

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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

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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