Download Explain with reference to examples you have studied how plate

Explain with reference to examples you have studied how plate tectonics helps
us understand the forces at work along plate boundaries. (30 marks)
The main forces at work along plate boundaries are collision and separation. Plate tectonics
tells us that the earth’s crust is split into sections called plates. The heat from the earth’s core
generates slow moving convection current in the mantle (asthenosphere). The convection
currents drag the plates along with them, causing them to collide and seperate.
Collision (Destructive Plate boundaries)
Collision happens in three different situations; 1. oceanic and continental collission (Nazca plate
colliding with the South American plate), 2. Where two oceanic plates collide (pacific plate and
phillipine plate) and continental - continental collision (Indian plate colliding with the Eurasian
plate).
At oceanic - continental collision, the heavier oceanic plate slides down into the mantle in a
process known as subduction. As the oceanic plate gets subducted it is melted in the mantle.
This causes the formation of explosive volcanoes on the land, e.g. the Andes in Chile.
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The Japan tsunami of 2011 was caused by the collision of the pacific plate (oceanic plate)
and the North American plate (continental). The pacific plate was subducted under the North
American plate causing the north American plate to rise and produce the gigatic waves. This
was a 8.9 magnitude earthquake and quite shallow (the focus was 17 km down in the crust).
This resulted in a huge tsunami and many aftershocks in mainland Japan. Follow this link to the
bbc website to see some footage.
http://www.bbc.co.uk/news/world-asia-pacific-12719788
At oceanic - oceanic collision, subduction also occurs and leads to the formation of volcanic
island arcs, e.g. Japan and the Phillipines. In Japan, the collision of the pacific plate and the
phillipines plates causes subduction and volcanic mountains, e.g. Mt Fuji (Japan). In time, the
volcanic eruptions form volcanic islands. Mount Pinatubo in the Phillipines erupted in 1995.
In the case of continental - continental collision, subduction does not happen as they are
less dense than the mantle below. The continental plates will both be destroyed by uplifting to
form fold mountains, e.g. the Himalayas form from the collision of Indian and Eurasian plates.
Shallow earthquakes may happen at these boundaries, e.g. the Pakistan earthquake in 2008.
See the textbook (Planet & People) for details of this quake.
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Transverse Plate boundaries
These are plate boundaries that do not fall into the constructive or destructive category. Instead,
the plates move side by side. In some cases, one plate moves slower than the other creating
friction and occasionally earthquakes, e.g. San Andreas Fault in California. In other cases, the
plates are moving side by side but in opposite directions. In Haiti in 2010, a transverse plate
boundary where the North American plate and the Carribbean plate are moving in opposite
directions, caused a strong earthquake, which killed hundreds of thousands.
Separation (Constructive Plate boundary)
Constructive plate boundaries are places where new crust is formed. This occurs at midocean ridges, e.g. the Mid-Atlantic ridge where the Eurasian plate is separating from the
North American plate. At seperating plate boundaries, the convection currents in the mantle
are flowing away from each other and drags the plates apart. This splits the crust and allows
magma to rise to the surface, where it creates new ocean floor. This cooled lava is an igneous
rock known as basalt. The island of Iceland is made of basalt and exists as a result of sea floorspreading. In other parts of the mid-atlantic ridge, large underwater volcanoes are found.These
volcanoes are less explosive than those found at subduction zones, as they contain less silica
and this means that the gases can escape. As a result, the lava is runny.
Note: I have included more information here than is required for a 30 mark answer. In plate
tectonics (and physical geography in general), actual examples from the real world are important.
Know the details of one recent earthquake, know the location of one or two volcanoes, know the
names of one or two fold mountain ranges. Be able to draw a diagram of a subduction zone (it does
not have to be a work of art). You don’t need to say much about continental drift, except to say
that this is evidence of plate movement. The plate boundaries and the resulting landforms are what
is important.
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