AQA A2 Geography Plate Tectonics: Plate Margins and Associated Landforms with Case Studies

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Preview of AQA A2 Geography Plate Tectonics: Plate Margins and Associated Landforms with Case Studies

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Constructive Plate Margins
Where convection currents rise and then diverge, they can create high temperatures that cause updoming
of the crust and tensional forces that pull it apart.
There are two types of divergence:
In oceanic areas, seafloor spreading occurs on either side of mid-ocean ridges such as the Mid-
Atlantic Ridge
In continental areas, stretching and collapsing of the crust creates rift valleys ­ such as the Great
African Rift Valley
It is as constructive plate margins that some of the youngest rocks on the Earth's surface are to be found.
This is because new crust is being formed as the gap created by the spreading plates is filled with magma
rising from the asthenosphere.
As the magma cools, it solidifies to form dense new basaltic rock.
Oceanic Ridges
Where two plates pull apart there is a weaker zone in the crust and an increase in the heat near the
surface. The hotter, expanded crust forms a ridge. The central part of the ridge may feature a central
valley where a section of crust has subsided into the magma below.
The split in the crust provides a lower pressure zone where the more liquid lavas can erupt to form
submarine volcanoes. If these eruptions persist, volcanoes may develop until they reach the surface.
As crust is pushed away from the heat source at the mid ocean ridge, it cools, contracts and sinks towards
the deeper regions, where it becomes covered in fine sediments. Occasionally fragments of ocean floor
(ophiolites) are left at the surface during subduction, and the layers of pillow basalts and later deep ocean
sediments are left exposed.
Mid-ocean ridges are irregular, curving around the planet. Regular breaks called transform faults cut
across the ridges which allow the crust created at the ridges to move outwards at different rates.
Rift Valleys
Continental divergence forms massive rift valleys. These valleys are formed when the lithosphere
stretches, causing it to fracture.
The land between these faults then collapse into deep, wide valleys which are separated by upright blocks
of land called horsts.
Volcanoes are found in rift valleys ­ Mount Kilimanjaro and Mount Kenya are volcanoes in the East African
Rift System.

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Case Study: The Great African Rift Valley
A rift is taking place right now and it is tearing the eastern side of Africa apart.
Uncertainty remains as to the exact causes of the Great African Rift Valley, but it is widely accepted that it
is a result of plumes of magma rising from the asthenosphere. This magma heats the overlying plates,
causing them to expand and bulge to create horsts, such as the Ethiopian Highlands.…read more

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Destructive Plate Margins
Destructive plate margins are found where plates converge.
There are three types of converge margins:
Oceanic plate meeting continental plate
Oceanic plate meeting oceanic plate
Continental plate meeting continental plate
Oceanic Plate Meets Continental
Because oceanic crust is denser than continental crust, when plates collide the oceanic crust is
subducted, or taken down, into the upper mantle.
The exact point of collision is marked by the bending of the oceanic plate to form a deep ocean trench.…read more

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Case Study: Peru-Chile Trench and the Andes
The Peru­Chile Trench is the longest ocean trench, marking the point at which the Nazca Plate meets
the South American Plate.
The Nazca Plate is primarily dense ocean crust and so is being subducted beneath the more buoyant
South American continental plate.
The South American crust has been deformed and thickened by the convergence, creating the Andes
Mountains.…read more

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Oceanic Plate Meets Oceanic
When two oceanic plates collide, one plate subducts beneath the other.
The one that is subducted may be marginally denser or moving quicker.
This leads to the formation of a deep ocean trench and melting into the upper mantle.
The resulting rising magma from the Benioff Zone forms crescents of submarine volcanoes along the
plate margins which may grow to form island arcs.…read more

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Case Study: Marianas Trench
The Marianas Trench and the associated Marianas Islands in the Western Pacific are an example of where
oceanic plate meets oceanic plate.
The Marianas Trench is located in the Pacific Ocean, East of the 14 Marianas Islands, near Japan.
It is the deepest part of the earth's oceans, and the deepest location of the earth itself.
Here, the Pacific plate is being subducted beneath the smaller Philippine plate.…read more

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Continental Plate Meets Continental
When two plates of continental crust move towards each other; neither is subducted due to the fact they
have similar densities. They have a lower density than the asthenosphere beneath them.
The colliding plates, and any sediments deposited between them, simply become uplifted and buckle to
form high fold mountains such as the Himalayas.
Volcanic activity does not occur at these margins because there is no subduction; however; shallow-focus
earthquakes can be triggered.…read more

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Conservative Plate Margins
When two plates slide past each other they form a conservative plate margin.
Along these margins crust is not being destroyed by subduction.
There is no melting of rock and therefore, no volcanic activity or formation of new crust.
Despite the absence of volcanic activity, these margins are tectonically extremely active and are
associated with powerful earthquakes.
Friction between the two moving plates leads to stresses building up whenever any `sticking' occurs.…read more

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Hot Spots
While most of the world's volcanoes are located along plate margins, there are notable exceptions.
Radioactive decay within the Earth's core generates very hot temperatures. If the decay is
concentrated, then hot spots will form around the core. These hot spots heat the lower mantle, creating
localised thermal currents where plumes of magma rise vertically.
Whilst these plumes are usually found close to plate margins, they may occasionally rise within the centre
of plates.…read more

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Case Study: Hawaii Hotspot
The current hotspot is underneath the southern end of the island of Hawaii.
As the Pacific plate moves slowly north-west at a rate of 5-10 cm per year, the next volcano in the chain is
forming on the sea floor just to the South of Hawaii, it is called Loihi. It should appear above the sea in
around another 200 000 years.
To the north-west of Hawaii, extinct volcanoes, battered by erosion, cool and slowly sink into the
ocean.…read more

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