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STRUCTURE OF THE EARTH (Structure (Continental Crust (Not subducted =…

- - - - Uranium decays into Thorium + releases heat
    - - Contains liquid Fe, Ni, S
    - - Solid Fe, Ni, S
  - - - Fluid
      - Convection currents
        
        bc of heat from the Earth's formation
      - Flow + convection currents = plate movement
    - - Below lithosphere
      - Semi-molten
      - Made of Periodite
      - Deforms + flows slowly
    - - Below rigid crust
      - Solid
      - Crust + lithosphere = plates
  - - - Constructive
        
        Make new crust
      - Destructive
        
        Destroy (oceanic) crust
      - Conservative
        
        No crust destroyed/made
- - - - Together they provide the evidence for plate tectonics
  - - - 250Ma
      - S America, Africa, Antarctica, India, Australia
      - Single continent = same rocks and fossils + was glaciated at same time
      - Broke up in Jurassic period, 167 Ma
      - Evidence - fossils in S Am + Africa
    - - Not exact fit of present coastline of S Am + Africa bc:
        
        S.l. constantly changes - coastline is temporary feature
        
        Deposition + erosion
        
        Is less at continental shelf
      - Today's continents = Pangea
      - Matching sequences of glacial, marine + non-marine rocks found across several continents
      - Thick rock sequences (inc. coal) can only have been formed in tropical conds.
        
        BRITAIN
        
        Coal + evaporite deposits all suggest tropical climates during Carboniferous + Permian times
        
        Assumes that climate belts have followed same pattern always as they are created by relation of Earth to Sun
        
        So GB must have moved from tropical latitudes 250 Ma to current temperate climate belt
    - - Proof that 2 rock types on either side of ocean are from same outcrop, they must have the same:
        
        Distinctive characteristics of mineral composition + physical features
        
        Age - determined by radiometric dating
      - Precambrian cratons, carboniferous coals
    - - Fold mt. chains are linear features
      - Gondawanaland had a Precambrian fold mt. chain across Africa to S Am
      - Trend = way to match geology across continents
    - - If Africa + S Am were always separated, they should have diff. fossil records - esp. for orgs. that would not have been able to spread across a wide ocean
      - During the Carboniferous, land-based orgs. are found on both continents
    - - Cannot form over ocean and move over land
      - Leaves movement of glaciers e.g. striations in bedrock
      - Orientation of striations in bedrock in Aus.. S Am + India suggest glaciers formed in oceans and moved on land
      - Glacial evidence is present in rocks on continents that are now near equator
      - Rock sequences containing glacial deposits cannot have formed near equator
      - S Am + Africa have sedimentary deposits of angular clasts
        
        This is a fossil boulder clay or tillite deposited by an ice sheet during Carboniferous
      - Gondawanaland was probs. near the S pole bc ice sheets cannot extend to the equator
        
        Africa + S Am are now much further N - evidence of movement
    - - Iron-rich minerals in magnetic field hold a record of the E's mg. field at the time of their formation
      - Large no. of rocks collected, dated and direction of the PM measured
      - Data plotted as a polar wandering curve - the curve joins up the apparent positions of the mg. N over time
      - S Am + Afria curves suggest that the N pole was once in 2 positions at same time
        
        Mg. pole cannot sig. change position
        
        Assuming mg. pole remained fixed, it must have been the continents that moved
      - If the 2 continents are repositioned next to each other, the curves match = one position for the pole
        
        The curves diverge only after the continents started to drift apart
      - General
        
        Rapid rotation + liquid conducting interior = strong m.f.
        
        Mantle is convecting but may change dir. or slow down periodically = m.f. reversal
        
        Changes becoming mroe frequent - 200,000 years avg.
        
        Normal polarity - mg. N + geographic N match
        
        Reverse polarity - mg. N + geographic S match
  - - - Rocks can be dated isotopically
      - Changes in polarity across fold mts.: reversal + normal events
      - new rock coming thru ridge pushes old rock on either side along
      - the rocks pushed away is the same on either side
      - bc of changes in polarity, there is a pattern along the fold mts; there is a ridge in the middle e.g. MA Ridge
      - All igneous rocks formed at MA ridges contain iron
      - Fe minerals cool as lava erupts
      - Fe minerals become aligned with E's m.f.
      - Fe minerals is rocks becomes a record of magnetic polarity at the time the rock was formed
      - When polarity of rocks formed on each side of ridge was compared, they were found to match
    - - M.f. appears as a striped pattern on ocean floor
      - Igneous rocks form oceanic crust + floor
      - Lava erupts + cools and mg. orientation of Fe particles is locked in the rock, depending on polarity at that time
      - 400-500k years polarity changes orientation
      - Wegner's CD theory in '12 was opposed bc of lack of driving mechanism
        
        In 60s-70s military surveyed oceans and found magnetic field change + ridges = evidence for CD
- - - - Occurs at Constructive boundaries
      - Magma rises to surface + heats surrounding rocks
      - Crust is elevated above the sea flood at ridge
      - As new rock forms, it cools + becomes denser
      - Gravity causes it to slide down each side of ridge
    - - Ridge push = submarine ridge 3.5 km high
      - Rising magma/convection = rift in centre on slow spreading ridges
      - Magma melting = fracture zone
      - Faulting = transform faults
      - Underwater cooling = pillow lavas
      - Chemosynthesis = black smokers/hydrothermal vents
- - - - Increased pop. densities + growth in EDCs e.g. Haiti
      - Deforestation = floods + soil erosion e.g. Haiti