plate tectonics theory and plate margins and magma plumes

plate tectonics theory and plate margins and magma plumes

1. plate tectonic theory of crustal evolution and sea floor spreading

   • added evidence to support alfred wegener
   • updates in 1962 by Harry Hess
   • studied rocks and found that the youngest rocks were in the middle of the oldest near USA and the carribbean
     • theory that it was spreading outwards from the centre
       • the rate of spreading was estimated to be 5cm a year
   • added evidence to support the theory of continental drift

2. tectonic plates and plate movements

   • made up of 7 major and several minor tectonic plates
   • continental - over 1500 million years old
   • oceanic - less than 200 million years old
   • continental plates extend far beyond the margins of current land masses
     • and do not sink into the aethenosphere because of their low density
   • oceanic plates are constantly being formed at mid ocean ridges
     • and destroyed at deep ocean trenches, explaining their young age
       • the plates move between 2cm-16cm a year
     • radioactive deacy
       • generates high temperatures from the core and so hot sports around the core heat the lower mantle and make convention currents
         • rise towards the surface
           • cool and then sink

3. gravitational sliding, ridge push and slab pull

   • tectonic plates are driven by convection currents
     • convention currents are thousands of kilometers wide but sink to really shallow depths
       • convection cells move in different directions
   • gravitational sliding explains the tectonic plate movement
     • This push is caused by gravitational force, and it exists because the ridge occurs at a higher elevation than the rest of the ocean floor.
       • magma rises as the plates move apart
         • this magma cools to form new plate material
           • as it cools it becomes denser and slides down away from the ridge under the force of gravity
             • this causes other plates to move away from eachother (ridge push)
               • the denser plate sinks back into the mantle under the force of gravity
                 • pulls the rest of the plate along behind it ( slab pull)

4. plate margins

   • in oceanic areas sea-floor spreading occurs on either side of the mid-ocean ridges
     • e.g. mid atlantic ridge
   • in continental areas, stretching and collapsing of the crust can creates rift valleys
     • e.g. the great african rift valley

5. mid-ocean ridges

   • ocean divergence form chains of submarine mountains
     • regular breaks called transform faults are across the the ridges
       • these are at right angles to the ridge and separate sections of the boundary
   • they can rise 4000m above the ocean floor
   • rift valleys are widened by magma rising from the asthenosphere

6. rift valleys

   • continental divergence form rift valleys
   • the land between these faults then collapses into deep, wide valley that are separated by upright blocks of land
     • these are called horsts

7. oceanic plate meets continental plate

   • oceanic plate is denser than the lighter continental plate so subducts underneath it
     • as two plates converge the continetal plate is uplifted, compressed, buckled and folded into chains of fold mountains

8. oceanic plate meets oceanic plate

   • the denser one subducts underneath the other
     • this leads to the formation of a deep ocean trench
   • rising magma from the benioff zone forms crescents of submarine volcanos
     • these grow to form island arcs

9. continetal meets continetal

   • they have a lower density that the asthenosphere below them
     • so no subduction
       • no volcanic activity
   • the sediment deposits between them become uplifted and buckle to form fold mountains
   • young fold mountains are continually compressing and therefore growing higher
     • e.g. himalayas and everest

10. magma plumes

    • radioactive decay within the earths core generates very hot temperatures
      • if this is concentrated hot spots will form around the core and heat the lower mantle and created localised thermal currents where magma plumes can rise vertically
    • usually found close to plate margins but occasionally rise within the centre of plates and burn through the lithosphere
      • creating volcanic activity on the surface
        • potentially forming a chain of active and subsequently extinct volcanos as the plate moves away from the hot spot

11. conservative plate margins

    • two plates sliding past each other where no crust is destroyed by subduction
    • there is no melting of rock and so no volcanic activity
    • the two plates moving side by side creates friction
      • these stresses are built up and eventually released as powerful shallow focus earthquakes