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The Earth's crust is made up of seven
principal tectonic plates and numerous other
smaller plates. The plates are sections of the
crust that "float" on the mantle, which is
made up of molten rock. Where the plate's
meet, huge forces mean that they can form
features such as volcanoes, fold mountains,
deep-sea trenches and earthquakes.
There are two main types of tectonic plate.
Oceanic crust is often only about 5km thick,
but is very dense. Continental crust is
considerably thicker, often being
approximately 30km deep, but is less dense.…read more

Slide 2

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The Earth ­ cross section…read more

Slide 3

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Convection Currents
· The Earth's Tectonic Plates all move very
slowly on the mantle.
· The plates move due to convection
currents in the mantle. These are hot
currents of molten rock that slowly move
within the mantle and cause the plates
above them to move, usually by as little as
one or two centimetres each year.…read more

Slide 4

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Compressional boundaries.
· These cause violent volcanoes and earthquakes, as well as deep-
ocean trenches and fold mountains.
· An oceanic plate and continental plate move towards each other.
· The denser oceanic plate dives under the lighter continental one,
creating a deep ocean trench.
· As the oceanic plate goes deeper into mantle it melts in the
subduction zone, due to friction and the increased temperature.
· The newly molten rock is lighter that that which surrounds it, so it
will rise towards the surface and cause volcanoes on the earth's
· The continental crust is crumpled by the collision of the two plates
creating Fold Mountains.
· If the magma rises offshore it will form an Island Arc, like the West
Indies and Japan.
· Example ­ Nazca plate dives under South American plate…read more

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Compressional plate boundary…read more

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Tensional boundaries
· Although often not as violent as those on destructive
plate boundaries, volcanoes and earthquakes do occur
on constructive plate boundaries. They also cause mid-
ocean ridges to form.
· Two plates move away from each other.
· Molten rock (magma) rises from the mantle to fill the
gap between the two plates. This forms a mid-ocean
· Volcanoes can also form here, along the edges of the
plate boundary, due to the rising magma. These
volcanoes are called shield volcanoes.
· Example ­ Mid Atlantic ridge…read more

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