Geographical Debates is the third exam paper in OCR A Level Geography. Hazardous Earth is one of the five topics you can choose from. Includes case study examples.
Hazardous earth
Risk = the probability of a hazard event occurring and creating loss of lives and
livelihood.
Hazard = a perceived natural event which has the potential to threaten both life and
property.
Vulnerability = a high risk combined with an inability of individuals and communities
to cope.
Capacity to cope = the ability of affected communities to cope with a given hazard.
Risk is affected by the hazard, vulnerability and capacity to cope: R = H x V / C
Structure of the earth
Lithosphere = 0-100km deep is a cool, rigid and brittle layer sandwiched
between the crust and the asthenosphere. Lithosphere varies in thickness.
Together the lithosphere and the crust make up the oceanic and continental
plates.
Asthenosphere = 100-300km deep is a hot, weak, plastic layer which is semi-
molten. Within the asthenosphere, there are convection currents caused by
heat generated from the inner core of the earth.
The semi-molten asthenosphere flows, carrying with it the solid lithosphere
and crust.
Continental crust = approximately 35km thick and oceanic = 5-10km.
Mantle = to a depth of 2900km
Continental drift
Continental drift theory = in the Carboniferous period, a large single continent
existed called Pangea. This slowly broke apart and the movement continued
to present day as the continents separated and spread across the globe.
Geological evidence:
Continents can be fitted together to make the supercontinent Pangea.
A good example is South America and Africa.
Glaciations leaves behind the evidence of the movement of glaciers
such as scratch marks in the bedrock called striations. The orientation
of striations in the bedrock of Australia, South America and India
suggest that these land masses were joined at the time.
Matching rock sequences can be found on opposite sides of the globe.
Thick rock sequences like coal can only form at the equator but are
found at the poles.
Biological evidence:
, Freshwater reptiles such as the Mesosaurus couldn’t swim across the
Atlantic Ocean as they’re freshwater reptiles but fossils have been
found in both South America and South Africa only suggested they
were once joined.
1
Paleomagnetism
This is the remnant magnetism is ancient rocks recording the direction
and the intensity of earth’s magnetic field at the time of the rocks
formation.
As magma cools, magnetic minerals (those that contain iron), will
become aligned with the earth’s magnetic field. Recent rocks formed
show alignment with today’s magnetic field.
The magnetic field is strongest at the poles and weakest at the
equator.
Paleomagnetism of the sea floor shows a stripy pattern at each side of
the ocean ridges (normal and reversed polarity).
This shows evidence of sea floor spreading2 - the width of each strip of
rock with the same magnetic orientation was found to correspond with
the time scale of each magnetic reversal. Symmetrical pattern of
geomagnetic reversals on either side of mid-ocean ridges indicated
that as fresh molten rock from the asthenosphere reached the seabed,
older rock was ‘pushed away’ from the ridge. Eventually the sea floor
reaches an ocean trench3 where material is subducted into the
asthenosphere and becomes semi-molten again.
Tectonic plate boundaries
Divergent/constructive plate boundaries
Locations where plates are moving apart and magma is rising up from the
asthenosphere and forcing its way to the surface.
Mostly takes place at mid-ocean ridges4. Mid-ocean ridges vary in shape
depending on the rate of sea-floor spreading and this is determined by the
amount and rate of magma brought to the surface by convection currents.
The eruption of magma along divergent boundaries occurs mostly
underwater. Magma erupting directly onto the sea bed is cooled rapidly,
forming rounded mounds called pillow lavas.
As magma rises to the surface, the overlying rocks can be forced up into a
dome. The rigid lithosphere is placed under great stress and eventually
1
Traces of changes in the earth’s magnetic field in the alignment of magnetic minerals in sedimentary and
igneous rocks.
2
Lateral movement of new oceanic crust away from a mid-ocean ridge (constructive plate boundary). Key
process in the theory of continental drift.
3
Narrow, deep depression on the ocean floor adjacent to a subduction zone.
4
The boundary between 2 diverging oceanic plates. Consists of 2 parallel chains of submarine mountains
separated by a graben, and offset in places by transform faults.
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