Summary OCR Geography A Level - HAZARDOUS EARTH notes

HAZARDOUS EARTH
1. WHAT IS THE EVIDENCE FOR CONTINENTAL DRIFT AND PLATE TECTONICS?
1.A. THERE IS A VARIETY OF EVIDENCE FOR THE THEORIES OF CONTINENTAL DRIFT AND
PLATE TECTONICS.
Structure of the Earth
1. Crust
o Oceanic
 Thinner – 5-10km
 Denser – 3.0kg/m3
 SIMA – silicon, magnesium (basaltic)
o Continental
 Thicker – 35km (mean) – 30-70km
 Less dense – 2.6-2.7kg/m3
 SIAL – silicon, aluminium (granitic)
2. Mantle (upper)
o Lithosphere
 Rigid layer – lithosphere and crust make up plates
 Varies in thickness
 Boundary with asthenosphere is hard to locate as it begins to melt
o Asthenosphere
 Semi-molten layer that flows slowly
 100-250km thick
 Convection currents here  moves the crust and lithosphere
o MOHO
 Mohorovicic Discontinuity marks the boundary between the mantle and
the core
3. Core
o Outer core
 Semi liquid
 As earth rotates outer core spins creating magnetic field
 Iron
o Inner core
 Solid
 Extreme pressure  centrifugal force  mass moves to centre
 Hottest part – 5000C
 4x as dense as crust
 Iron-nickel alloy

,Plate Movement
 Convection Currents
1. Area of intense heat in core caused by radioactive decay/primeval heat
2. This heats mantle  causes powerful thermal current of magma  rises
towards surface
3. Reaches surface and forced to diverge dragging plate with it (constructive
margin)
4. This occurs due to friction at the Moho
5. Where 2 convection currents converge, crust is dragged inside the earth –
subduction (destructive margin)
6. Magma begins to cool and sink, and the process starts again
 Ridge push
o Plates diverge  upwelling of hot magma
o This pushes upwards creating a buoyant elevated area of earths surface called
a ridge
o Gravity acts on downward slope of the ridge, pulling it away from the margin
o Rock cools as it moves away, becomes more dense
 Slab pull
o Pulling force exerted by old, cold dense oceanic plate subducting
o Cooler sinking plate pulls the rest of the young warmer plate behind it




Evaluation
 Importance varies depending on location
 Importance varies depending on plate – each plate moves at its own rate
 All are important – there is no single mechanism for plate movement

, Continental Drift
 ‘The present distribution of continents is the result of the breakup of larger
landmasses’
 Pangea (supercontinent)  Laurasia and Gondwanaland  7 continents
 Alfred Wegener
o Meteorologist not geologist
o Disproves lots of other theories  wasted research
o Rocks shouldn’t be able to move


Biological Evidence
 Fossils of the same organisms have been found in different continents:
o Cynognathus
 Reptile found in South America and Africa
o Mesosaurus
 Freshwater reptile found in South America and Africa
o Glossopteris
 Plant found on all continents
 Therefore, land masses must have once been joined  land bridge theory? (Edward
Suess)


Geological Evidence
 Continents eg South America and Africa fit together almost perfectly – Jigsaw fit
theory
 Appalachian Mountains = same rock type, structure and strata as Scottish Highlands
and Scandinavia
 Therefore, suggests land masses were once joined together


Paleoclimatic Evidence
 Ice Sheets
o Glacial moraine in Africa suggests it was once glaciated so must have been
further south
 Tropical Swamps
o Coal found it UK – comes from dead organic matter from nutrient rich tropical
trees so UK must have been further South and had a warmer climate