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Summary OCR Alevel Geography- Hazardous Earth EXTREMELY INDEPTH NOTES
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Hazards
Continental Fit: The theory that the continents have moved across the Earth’s surface over time.
Evidence for continental drift.
Alfred Wegner’s theory of continental drift.
Continental fit: the apparent fit or the continents. "All continents were once connected as a super
continent called, Pangea”
Fossil distribution/Fossil correlation: Evidence that shows how animals and vegetation found
on continents such as Africa can be found in south America. For example, the Cynognathus, which
was a land mammal. Fossils were found in central and southern Africa and south America showing
how the two continents were once connected, as a land mammal would have been unable to cross
the Atlantic Ocean.
Paleoclimate data also shows that coal deposits are found in similar areas. For example, Antarctica
and the UK, which host high concentrations of Bituminous coal. This tells us that the land was once
closer to the equator as Bituminous coal only forms in rainforest regions, which means that the
areas have shifted away from these regions.
Geology: Wegener discovered that mountains and rock types were the same. For example, the
UK, Scandinavia and the US all have the same rock types and structure. This is seen at the
Pennines and Appellations.
Palaeomagnetism: Whilst this piece of evidence is linked with sea floor spreading, it still shows
how the continents have moved apart allowing for the magnetic history of the Earth to be
preserved. Palaeomagnetism shows how the magnetic alignment of the Earth has changed over
the years. The magnetic field often reverses as it wobbles along its axis, this is shown in magnetic
alignment of bands of rock which have solidified along the faults of a divergent plate as it spreads
apart.
Sea Floor spreading: Sea floor spreading shows how plates have shifted apart, causing an
increase in size of ocean plates and the basin. This is due to ridge push, which causes a divergent
plate boundary, in which magma flows up and solidifies along the ocean floor which pushes the two
plates apart. This is sea at the Mid-Atlantic ridge.
Evidence that shows this:
Youngest rock nearest to the fault, with oldest rock closest to the continental plate shows how
Why do plates move?
Convection Currents
Def: A current in a fluid that results from convection. Currents of hot magma rise from the lower
mantel, then cool as they reach the lithosphere and sink back down due to gravity.
How do they cause plate motion?
Plate motion is largely due to convections currents. Hot mantel material rises upwards and makes
contact with the lithosphere. The lithosphere then cools the magma and it begins to sink. These
convection currents cause two different processes: Slab Pull and Ridge Push. Ridge push occurs
between 2 convection currents, whereby rising magma lifts the plates and causes them to slide
apart or split. This leads to a separation in the 2 plates which allows for magma to rebuild the gap.
This is known as a Divergent or Constructive plate boundary and is the leading cause of sea floor
spreading. Ridge Push often causes landforms such as Mid Ocean Ridges and Rift Valleys for
example the East African Rift Valley or the Mid- Atlantic Ocean Ridge.
Slab Pull occurs at the edge of convection currents, whereby the magma that has been cooled by
the lithosphere drags the plate into the hot mantel. This is known as subduction. As the plate is
pulled under, it aids ridge push, creating more divergent boundaries. Slab Pull is the driving force
of plate motion. Slab Pull often leads to tectonic activity such as Earthquakes and Volcanoes.
Earthquakes are often observed here due to large build ups of friction as the plate is pulled under,
grinding against the less dense plate. Volcanoes are also observed here due to the melting of the
Plate Boundaries Summary:
Convergent Boundaries: Continental- Continental
When 2 continental plates collide, very little subduction occurs. This is due to them having
relatively the same densities and being the same thickness. When 2 continental plates collide,
nothing is destroyed however, fold mountains can be often observed where the two plates have
been crushed together. Examples of this are the Himalayas, where the indo-australian plate has
, Eurasian and African Plate have collided together. At continental-continental boundaries, huge
pressure builds up caused by friction often lead to deep and powerful earthquakes.
Convergent Boundaries: Continental- Oceanic
Here, the denser oceanic plate is subducted under the continental plate. This is usually due to Slab
Pull. Many different tectonic landforms can be observed at these boundaries such as, Marine
Trenches, Mountain, Volcano chains. Marine trenches are due to the subduction pulling the plate
under the continental plate. Mountain Chains are due to smaller scale rock folding. Again, powerful
earthquakes are observed here due to build ups of friction.
Volcanoes form largely due to the subducting oceanic plate melting and causing magma to rise up.
These form “Plutons” which lead to large magma chambers forming and allowing for eruption. The
Mariana Trench (Ring of Fire) is an example of this type of convergent boundary.
A of convergent boundary of this type is known as destructive.
Convergent Boundaries: Oceanic- Oceanic
Here, the less dense oceanic plate is subducted under the other oceanic plate. This is usually due
to Slab Pull. Many different tectonic landforms can be observed at these boundaries such as,
Marine Trenches, Mountain, Volcano chains and Island Arcs. An example of this is the Aleutian
Islands, Alaska.
Conservative/ Transform Boundaries:
At conservative margins, plates slide past each other, so that the relative movement is horizontal.
Lithosphere is neither created nor subducted, and whilst conservative plate margins do not result
in volcanic activity, they are the sites of extensive shallow focus earthquakes, occasionally of
considerable magnitude. Here, distinctive folds and rock strata can be viewed. Earthquakes are
usually of enormous energy as friction causes pressure build ups. Examples of this can be seen at
the San Andreas Fault and the Turkish Fault.
Divergent Plate Boundaries:
Here, Ridge Push causes 2 oceanic plates to be pushed apart, allowing for magma to flow up. There
Magma and it’s Silica Continent:
45% Silica. This is known as Mafic Magma and is usually found at oceanic plate boundaries (Divergent). The
magma here is usually low viscosity and runny which allows for easy and fast eruption.
60% Silica. This usually occurs when continental and oceanic plate rock mixes.
75% Silica. This is known as Felsic Magma and is found at continental plate boundaries. The magma here is
high viscosity and thick.
How do landforms and rock types allow reveal the origins of the Earth’s plate tectonics:
At the start of Earth’s tectonic history, the majority of crust would have been oceanic; thin and
dense. As this type of rock cooled quicker and relatively no subduction had begun. Oceanic crust
has relatively low silica content. Rock types such as Basalt can typically reveal how the crust was
formed (on the surface of oceanic crust). Landforms also show what crust it is, for example, shield
volcanoes are typically found on oceanic plate boundaries as the basaltic lava that flows are of low
viscosity and can travel further.
Continental plate boundaries formed via the mixing of different plates at subduction zones. Rock
types such as rhyolite and granite typically found here and depend on whether the rock was cooled
on the Earth’s surface or whether underneath. Rock gradients and crystal size also reveal where
the plate originated from.
Amongst the rocks, igneous rocks and metamorphic rocks make up 95% of the rocks, with granite
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