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Summary Hazardous Earth OCR A Level Geography notes full
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A* standard hazardous Earth OCR A Level Geography notes full. Covers specification fully in detail with case studies and examples.
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Hazardous Earth:Hazards occur when and where physical and human systems meet.
The basic structure of the earth-
The earth consists of four concentric layers (Inner core, outer core, mantle and crust). The
crust is made up of tectonic plates.
Inner core- Solid, made of iron and nickle with temperatures up to 5,500 degrees’ c (hottest
part of the earth)
Outer core- A liquid, made of iron and nickle with temperatures similar to the inner core
Mantle- Rich in magnesium and iron, widest section of the earth (2900km thick), made up of
semi molten rock called magma, upper parts are hard, lower parts are beginning to melt.
Convection currents take place, due to radioactive decay in the core (this causes plates to
move). Top part of the mantle= Lithosphere (molten), bottom part of the mantle from 100-
300km is the asthenosphere
Density= 3.3 kg/m cubed at Moho
5.6 at core
The boundary between the mantle and the crust is clearly marked by the mohorovicic
discontinuity
Crust- A thin layer between 0-60km thick, made up of plates (continental crust carries land,
oceanic crust carries water)
- Continental crust:
Mean depth= 35km
Density= 2.6-2.7 kg/m cubed
Composition= Granitic silicon and aluminium
- Oceanic crust:
Mean depth= 7.5km
Density= 3 kg/m cubed
Composition= Basaltic, silicon and magnesium
Continental drift and the theory of plate tectonics:
In 1912 Alfred Wegener proposed the theory of continental drift, that proposes that
continents shift position of the earths surface. He suggested that during the Carboniferous
period, 250 million yrs ago, a single continent (Pangea) existed. This slowly broke apart into
two large land masses, Laurasia to the North and Gondwanaland to the South, and these
continental movements continue to this day.
,Evidence for Wegener’s theory
Geological
1. The fit of continents (Africa and South America)
2. Evidence of contemporaneous glaciation in southern Africa, Australia, South America
and India (all hot climates), suggesting these land masses were previously in
glaciated areas and conjoined
3. Similarity of mountain ranges on either sides of oceans (eg; North East Canada and
Northern Scotland)
4. Sea floor spreading
Biological
1. Similar fossil bronciopods found in Australian and Indian limestones
2. Similar fossils reptiles found in South America and South Africa
3. Fossils from rocks younger than the carboniferous period, in places such as India and
Australia) show fewer similarities suggesting that they followed different
evolutionary paths
, Seafloor spreading= a process that occurs at mid-ocean ridges, where new oceanic crust is
formed through volcanic activity and then gradually moves away from the ridge, evidence of
sea floor spreading can be seen through;
The study of paleomagnetism
- Over time magnetic reversal takes place from normal polarity, where the north and
south magnetic poles are in the orientation that they are today and reversed
polarity, where the north and south magnetic poles are in the opposite orientation
than they are today. Eg; currently a compass needle points toward the north
magnetic pole (normal polarity), however it is known that 800,000 years ago, the
same compass needle would have pointed south instead of north (reversed polarity),
this is as the magnetic poles have reversed direction
- Evidence for magnetic reversal comes from a physical property of magnetic
materials known as the Curiee temperature. Magnetic minerals lose their magnetism
when the temperature exceeds their Curie temperature (different magnetic
materials have different Curie temperatures). Meaning when magnetic materials
cool below their Curie temperature the magnetic grains in the material align
themselves with the magnetic poles and therefore a record of their magnetic
orientation is “frozen” into them
- Therefore, studying the magnetic orientation of rocks (known as paleomagetism)
enables you to find their age
- Studies of paleomagetism in the 1960s found that the ocean floor was comprised of
parallel bands of crust having alternating magnetic polarity as well as the bands
being symmetric on either sides of ocean ridges
- British scientists Fred Vine and D. H. Matthews explained the phenomenon of
magnetic striping in 1963, they suggested the magnetic striping was as a result of
magma flowing from the mid-oceanic ridges. They explained that as the magma
reached its curie temperature it cooled, preserving that current magnetic orientation
of the earth and therefore that magnetic reversals show up as bands of alternating
polarity. This process of magma flowing out from mid-ocean ridges is now known as
sea floor spreading
The use of thermal probes
- The use of thermal probes to measure heat flow through bottom sediments is
generally consistent throughout the oceans floor and the continents
- However, over ocean ridges, the heat flow often measured 3-4 times the normal
value
- These anomalous values are thought to reflect the leaking of molten material near
the crest of the ridges
- Research also revealed that the ridge crests are characterized by anomalously low
seismic wave velocities, which can be attributed to thermal expansion and
microfacturing that takes place with the upwelling of magma
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