Plate Tectonics

Plate Tectonics

The crust is broken up in pieces called plates
1. Plates are driven by convection currents
  1. Heat rising and falling inside the mantle creates convection currents
    1. Convection currents are created by radioactive decay in the core
2. The point where two plates meet is called a plate boundary
  1. Volcanos and earthquakes are most likely to occur on or near boundaries
  2. Plate tectonics cause earthquake and volcanoes
3. They move in different directions
History
1. Continental drift
  1. Concept attributed to Alfred Wegener
    1. F.B Taylor independently came to the same idea
      1. Francis Bacon in 1620 first saw that the continents fit like a jigsaw
    2. Supercontinent - Pangea
      1. In Quaternary split to Laurasia in the north and Gondwanaland in the south
    3. Theory rejected at first as could not describe drifting the continents
  2. Evidence
    1. Biology
      1. Mesosaurus, a small Permian reptile, found only in SA and Brazil
      2. A plant only existed in coal-forming times found only in India and Antarctica
    2. Geology
      1. Rocks of a similar age and type found only in SA and Brazil
        Like Mesosaurus
      2. Mountain ranges and fold belts all become consistent if fitted back into Pangean landmass
        E.g. NW European mountains and the Appalachians of the USA
    3. Climatology
      1. Evidence of glaciation found in tropical Brazil and central India
      2. Coal forms under warm, humid conditions, found in Antarctica
      3. Limestone found in England could not have formed under current climate
2. Later evidence for sea spreading
  1. Paleomagnetism
    1. 1950's
    2. As molten lava cool on surface the material, iron, within align themselves with the magnetic pole
      1. creating a record of the Earth's magnetic polarity
        Symmetrical either side of the Mid Atlantic Ridge
  2. 1962 Maurice Ewing discovered mountain range extending the entire length of the Atlantic sea floor
    1. Rocks volcanic in origin and young
    2. He had discovered the Mid Atlantic Ridge
  3. Harry Hess discovered rocks became increasingly younger towards the Ridge and the sea floor was spreading
    1. He confirmed the newest rocks were still being formed in Iceland
    2. However if new crust is being created it had to be being destroyed elsewhere as the world was not expanding in size
      1. Evidence found around the Pacific
        Plate tectonics theory was universally accepted
        Theory of the plates constantly moving, driven by convection currents
Plate boundaries
1. Constuctive
  1. plates move away, new crust formed
    1. transform faults perpendicular
  2. mainly between oceanic so underwater
    1. rarely on land E.g. East Africa, Great African Rift Valley
      1. It is said the Red Sea is the beginning of a new ocean, associated volcanoes include Mount Kilimanjaro and Mount Kenya
  3. rift valleys initially, marked by ocean ridges
  4. submarine volcanoes
    1. may grow above sea level e.g. Ascension Island on the Mid-Atlantic RIdge
    2. 73% of lava (magma after it reaches the surface) is found at mid-ocean ridges.
2. Destructive
  1. 2 plates move towards each other and one is subducted (forced below into the mantle)
  2. Pacific ocean surrounded by them, perimeter is known as Pacific Ring of Fire
  3. different plates converging means different features
  4. OC vs CO = Oc dips, deep-sea trench forms, sediments pushed up to form fold mountains, heat generated by friction helps to melt platemagma less dense then mantle so rises and forms vol, des margins magma acidic so may cool before surface and form a Batholith at base of mountains
    1. shallow Eqs occurs as plate initially stretched and dipped. Deeper eqs arise from increased friction.
    2. At subduction zome where more eqs take place is known as the Benoiff zone
      1. greater subduction angles gives shallower eqs
    3. If occurs off shore Island Arcs may form E.g. Japan and WI
  5. Oc vs Oc = either subducted
  6. CO vs CO = very rare, Oc cannot sink so crumple to form deep-rooted fold mountains
    1. mark site of the Earth's thickest crust E.g. the Indo-Australian Plate is moving NE to the Eurasian Plate creating the Himalayas. Uplift is a continuous process as is weathering and erosion.
3. Passive
  1. slide past eachother
    1. boundary between two plates pronounced by transform faults
    2. plates stick, friction builds, releases eqs
  2. E.g. San Andreas Fault
    1. North America and Pacific plates, both moves NW But pacific moves faster (6CM TO 1CM)
4. Mark the site of major landforms.
5. mountain-building, volcanoes and earthquakes are found here

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