Indian geography 2

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