geo

• Plate tectonics


  Theory that explains the structure and movement of the Earth's surface
• Structure of the Earth

  • Crust (thin solid outer layer up to 70 km thick, with oceanic and continental types)
  • Mantle (very thick layer extending to 2,900 km, mostly dense and solid with a thin semi-liquid layer in the upper mantle)
  • Core (can be divided into a liquid outer core and an iron-rich solid inner core, temperatures up to 5,500°C)
• The centre of the Earth is 6,400 km below the surface
• Oceanic crust is continuous around the Earth's surface, mostly made of basalt where silica (si) and magnesium (ma) are dominant
• Fossil evidence supporting continental drift

  • Lystrosaurus, a pig-sized lizard that lived about 250 million years ago and was found in Africa, Antarctica and India
• Earth's core


  Can be divided into a liquid outer core and an iron-rich solid inner core. Temperatures here are extremely hot (5,500°C).
• Components of the Earth

  • Continental crust
  • Oceanic crust
  • Mantle
  • Outer core
  • Inner core
• Oceanic crust (sima)


  • Continuous around the Earth's surface, mostly made of basalt where silica (si) and magnesium (ma) are dominant
• Continental crust (sial)


  • Discontinuous around the Earth and less dense, comprising rocks such as granite where silica (si) and aluminium (al) dominate
• The Earth's crust is broken into several large sections called plates
• Each plate is about 100 km thick and is made up of the crust and the upper part of the mantle
• The solid plates rest on a semi-liquid layer in the upper mantle and this enables them to move in relation to each other
• Types of plate boundaries

  • Divergent
  • Convergent
  • Collision
  • Transform
• What causes the plates to move

  1. Deep within the Earth the temperatures are extremely high

  2. Heat spreads out and flows towards the surface forming convection cells

  3. Convection currents rise from deep in the mantle and move towards the surface

  4. They then spread below the crustal plates, cool and finally descend back down into the mantle
• Some 250 million years ago the continents of the world were joined together to form one enormous continent called Pangaea
• Since then the continents have drifted apart to their current position
• Evidence for continental drift


  • Identical rocks in South America and West Africa
  • Continents fit together like a jigsaw puzzle
  • Identical land-based fossils found in separated continents
  • Rocks on the ocean floor become steadily older the further they are from the middle of the ocean
• During the 1960s and 1970s further scientific evidence led to a greater understanding of convection currents and of the mechanisms involved in plate tectonics
• Patterns of earthquakes, volcanoes, mountain ranges and deep ocean trenches provided evidence of the precise location of the active plate boundaries
• Scientists are still developing their understanding of the theory of plate tectonics in the light of recent earthquakes and volcanic activity
• Divergent plate boundary


  Two plates are moving away from each other, new crust is being formed
• Divergent plate boundary


  • Rising convection current causes the crust to crack, allowing molten rock (magma) to pass through
  • Magma escapes to the surface as lava erupted from underwater volcanoes, forming a mid-oceanic ridge
  • New plate material forms at the mid-oceanic ridge, plates extend outwards and diverge
• Convergent plate boundary


  Two plates are moving towards one another, one plate dives down beneath the other plate to form a subduction zone
• Convergent plate boundary


  • Subducting plate triggers earthquakes
  • Melting of subducting plate forms magma that rises to the surface as volcanoes
  • Crumpling of sea-floor sediments forms fold mountains
• Collision plate boundary


  Two plates of continental crust converge, no subduction occurs, layers in the seabed crumple to form fold mountain ranges
• Collision plate boundary


  • Powerful earthquakes due to the cracking of rocks
  • No magma or volcanoes due to lack of subduction
• Transform plate boundary


  Two plates are sliding alongside each other, no subduction or creation of magma
• Transform plate boundary


  • Earthquakes are very common as the plates move past each other
• The Caribbean plate is being squeezed by converging plates on either side
• Boundaries of the Caribbean plate

  • Northern edge is a transform boundary with the North American plate
  • Western boundary has oceanic crust of the Cocos plate subducting beneath the Caribbean plate
  • Southern boundary is part convergent and part transform
  • Eastern boundary has an active convergent boundary between the South American plate and the Caribbean plate
• Caribbean island arc


  Chain of mostly volcanic islands formed at a convergent plate boundary where the South American plate is subducting beneath the Caribbean plate
• Caribbean island arc


  • Lines of weakness develop in the overlying continental crust and magma rising from deep underground reaches the surface to form volcanoes
  • Many of the volcanoes have erupted violently in the past, such as Mt Pelee on Martinique and Soufriere Hills on Montserrat
• Kingston, Jamaica has been struck by a number of powerful earthquakes in the past, including in 1692 and 1907
• The old city of Port Royal, south of Kingston harbour, was destroyed by an earthquake and 2,000 persons were killed

  1692
• An earthquake and fire killed over 800 persons


  1907
• Earthquake


  A sudden violent shaking of the ground
• Earthquake


  • The point within the crust where an earthquake originates is called the focus
  • The point on the ground surface immediately above the focus is called the epicentre
  • The shockwaves that radiate out from the focus are seismic waves
  • An earthquake usually triggers movement along a crack called a fault
• Each year there are thousands of earthquakes across the world, the majority of which are not strong enough to be felt
• Earthquakes tend to be concentrated in clear linear belts, for example around the edges of the Pacific Ocean and through the middle of the Atlantic Ocean
• The majority of earthquakes are found at the plate boundaries