GCSE Geography

GCSE Geography - The Restless Earth

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The Earth's Structure

The earth has 4 main concentric layers:

1) Inner Core
Solid, mainly iron and nickel, incredibly hot (5500dC)
Radioactive processes occur here which give out huge amounts of heat energy

2) Outer Core
Liquid, also iron and nickel, nearly as hot as the core

3) Mantle
Liquid, made of molten rock (magma)

4) Crust
Solid, thin outer layer of the earth

  • The crust is broken up into 7 major pieces called tectonic plates
  • These plates move due to convection currents in the mantle caused by the heat from radioactive processes in the inner core
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Tectonic Plates

There are two main types of tectonic plate:

1) Oceanic

  • Made of basalt
  • Newer rock
  • Thinner crust (5-10km)

2) Continental

  • Made of granite
  • Older rock
  • Thicker crust (25-70km)

Oceanic rock is denser than continental crust because it has the weight of the ocean pressing down on it. This means that when the two types of plate meet, the oceanic is always subducted.

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Plate Margins

There are 3 main types of plate boundary:

1) Constructive 
Two plates moving away from each other
Mantle rises through the gap in the crust
Forms shield volcanoes, mid-ocean ridges and new igneous crust.

2) Destructive
Two plates moving toward each other
The oceanic plate gets subducted in the subduction zone and melts
The continental plate gets compressed to form fold mountains
Forms oceanic trenches and composite volcanoes.

3) Conservative
Plates move in different directions or at different speeds
Earthquakes are common here as the rocks lock together and pressure builds up and is released when the rock breaks.

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Fold Mountains

  • Fold Mountains are formed when continental plate is compressed at destructive plate boundaries
  • Occur at huge depressions in the earth of sedimentary rock, which was formed by water, called geocynclines
  • This rock is pushed together to form anticlines (upfolds) and synclines (downfolds)
  • Because oceanic crust is being melted in the subduction zone beneath them, sometimes magma can rise through a gap and form a volcano.

Some of the worlds largest range of fold mountains are : the Alps (Europe), the Himalayas (Asia), the Rockies (America) and the Andes (South America).

Human uses for fold mountains include: hiking, skiing, climbing tourism industry, high altitude farming, and Hydro-Electric Power (HEP) dams built at huge resivours high in the mountains.

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Earthquakes

  • Occur at ALL plate boundaries
  • The movement of two plates causes friction
  • This results in the plates locking together and pressure building up
  • Eventually the pressure becomes too great and the rocks break causing a sudden jolt of movement along each of the plates
  • The energy produced travels out in seismic waves

Focus - The point beneath the earth's surface at which the earthquake origionates
Epicentre - The point on the earth's surface directly above the focus

  • Seismometer measures the shock waves
  • The Richter Scale measures energy (1-10) and is logarithmic
  • The Mercalli Scale measures damage (1-12)
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Volcanoes

There are 2 main types of volcano:

1) Composite

  • Formed at destructive plate boundaries
  • Narrow base and steep sides, form a mountain shape
  • Is formed from layers of previously cooled lava and ash
  • Acid lava (800dC) which is thick, travels slowly and cools quickly
  • Infrequent explosive eruptions of pyroclastic flow

2) Shield

  • Formed at constructive plate boundaries
  • Wide base and gently sloping sides, form a hill shape
  • Basic lava (1200dC) which is runny and travels far before cooling
  • Many gentle eruptions
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Supervolcano

  • Supervolcanoes are extremely rare
  • Found at destructive plate boundaries or above hotspots
  • In the form of a depression in the earth called a caldera
  • Calderas usually have lakes and can be up to 40m wide
  • The lake is situated directly above a massive magma chamber
  • Last eruption was 7400 years ago in Indonesia
  • Largest supervolcano in the world is Yellowstone, Yellowstone National Park, Wyoming, USA
  • An eruption would be mainly of ash and pyroclastic flow, and would have devastating global impacts
  • However, an eruption is very unlikely.
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