Plate Tectonics/Formation/Case Studies

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  • Plate tectonics
    • Constructive
      • Plates moving in the opposite directions
      • RIFT VALLEY
        • most commonly formed on oceanic crust
        • due to the land being pulled apart, the tension results in part of the land dropping causing faults/fractures resulting in grabens(valley floor). the upright land which is left standing is called a horst.(valley wall)
        • volcanoes form here due to the uprising magma in the asthenosphere due to convection currents which then cause eruptions. the lava solidifies on the floor creating a new very thing/brittle crust.
        • East African Rift Valley
          • extends Northwards 3000 miles into the red sea, eastwards 37 miles into the Indian ocean and its deepest part is 6,000ft deep compared to the surrounding area
          • African catergorised into 2 plates, Somalian moving SE and the Nubian moving NW
          • Lava erupts from the fractures along the system, = flood basalts as it acts like a sheet of lava
      • MID OCEAN RIDGE
        • 2 oceanic plates diverge and form an underwater mountain range
        • Magma rise from the magma chamber as the plates are pulled apart. however the magma cools as it reaches the ocean water surface forming part of the new crust
        • can create volcanic islands as a continual build up of crust may emerge out of the water. such as surtsey in Iceland.
        • Transform faults occur due to the varying speed of different parts of the crust, due to the varying speed it can cause friction resulting in earthquakes as the tension is released on either side of the spreading plates faults. often shallow focus and near the surface
        • Mid Atlantic Ridge
          • 10,000 miles long but the ridges along the Atlantic ocean connect to make a 40,000 mile long system.
          • slow separation rate of 10-15mm/pa and has a wide ridge axis (30-50km)
          • Eurasian and North American plate boundary
          • Shield volcanoes and basaltic lava
    • Conservative
      • 2 plates slide past each other in opposite or similar directions
      • crust is not being destroyed by subduction so no new crust is being formed and there is no volcanic activity
      • San Andreas Fault
        • North American plate (1cm pa) and Pacific Plate (6cm pa) but are both moving NW at different speeds. the earthquake occurs at the strike-slip fault
        • 1906 San Francisco =, over 700 people died..  Northridge fault is a secondary fault north of LA
      • friction between the plates build up stresses and sticking occurs. the plates will slip and will be suddenly released and powerful shallow focus earthquakes
    • Destructive
      • Oceanic/ Oceanic
        • faster/denser plate will subduct under the other forming a deep ocean trench and melts due to extreme mantle heat
        • the resulting rising magma from the benioff zone forms crescents of submarine volcanoes which may grow into island arcs
          • Marianas Trench
            • Both the Philippine plate and the Pacific plate move NW but at different rates (Pacific is faster) trench is about 2,550km long and average width of 69km
            • Challenger Deep is the deepest known point in the oceans. 36,070ft below sea level
            • The plates do not move smoothly, they stick and causes a build up of pressure (stress) and it will eventually slip causing an earthquake
          • ISLAND ARCS
            • Caribbean Islands
              • South America/ Caribbean plate is subducting
              • accretionary wedges which get pushed onto the Caribbean islands. point of subduction is marked by the Puerto Rico Trench.
              • andesitic magma due to ocean water being subducted too causing more violent eruptions . ie Montserrat 1990's
                • shallow+deep focus earthquakes
      • Oceanic/ Continental
        • oceanic plate is denser so will subduct. the exact point of collision is marked by a deep ocean trench
          • Peru-Chile trench
          • The descending plate melts beyond 100km and is gone by 700km (Benioff zone - where melting occurs and shallow focus earthquakes are often a result of this and friction from the descending plate)
        • as the 2 plates converge, the continental land mass is uplifted, compressed and will buckle forming fold mountains
          • Andes
            • Nazca(subducts) /South American plate, 7000km long. highest peak is the south face of Aconcagua in Argentina (6,962m above sea level)
            • friction between the 2 plates causes shallow-focus earthquakes
            • all of the seafloor sediments from the Nazca plate build up at the point of subduction and form an accretionary wedge which also buckles up
              • the subducting plate can also bring ocean water into the mantle which can rise and cause an explosion of andesitic magma
                • less dense than the surrounding material, temps of 1000 degrees c, traps gases as it rises and causes explosive activity in volcanoes which have laid dormant for 100years+
        • the melted plate creates magma which is less dense (than surrounding asthenosphere) and therefore rises in great plumes. magma may pass through crack on the surface and cause a volcanic explosion
      • Continental/ Continental
        • subduction does not occur as both plates are less dense than those underneath
          • they plates collide and buckle to form fold mountains. as there is no subduction, there is no volcanic activity. however shallow-focus earthquakes may be triggered here
            • Himalayas
              • Indo-Australian/ Eurasian plate boundary
              • still rising by 1cm a year as india is still drifting into Asia (shallow-focus earthquakes) (continental drift theory link)
              • Himalayas and Tibetan plateau(large flat plane of land over 1000km long and a height of 4,500m)extends over 2,900km long, reaching a max height of 8,848m- Mount Everest
              • COLLISION ZONE
                • Fold Mountain Shapes
                  • anticline - like a hill shape
                  • syncline - depression
                  • overfolds /nappes -anticlines but at an angle, lean to one side
                  • geosyncline-  fold mountains can also occur here. sediments undergo compaction and cementation and sedimentation to form sedimentary rock
                    • as the plates move towards each other, the sediments begin to buckle
                    • geosyncline- rivers carry sediment to the large depression
      • plates moving towards each other
      • slab pull

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