Geography Plate Tectonics

Tectonic Plates

Oceanic Crust

•Is located under the oceans.

•It’s made of basalt.

•It’s between 5 and 10 km thick.

•It’s dense so it sinks into the mantle where oceanic and continental crusts meet.

Continental Crust

Makes the continents.

•It’s between 25 and 100 km thick.

•Not very dense.

•It’s lighter than oceanic crust.

Constructive plate boundary

At a constructive plate boundary, two plates move apart. As the two plates move apart, magma rises up to fill the gap, constructing new crust.

This causes volcanoes. However, since the magma can escape easily at the surface the volcano does not erupt with much force.

Earthquakes are also found at constructive boundaries, caused by the friction of the plates as they move over the mantle.

You get lots of eruptions at these margins – from volcanoes, like the Azores, and from cracks in the ground called fissures, like in Iceland.

The crust on either side of the margin is often faulted. This means it has big cracks in it.

An example of a constructive boundary is the Mid-Atlantic Ridge.

Destructive plate boundary

A destructive plate boundary is found where a continental plate meets an oceanic plate.

The oceanic plate descends (subducts) under the continental plate because it is denser.

The friction between the two plates causes earthquakes.

The land on the edge of the continental crust gets squashed up to make fold mountains, like the Andes.

As the plate descends it starts to melt due to the friction caused by the movement between the plates. Because it takes some sea water with it, it’s less dense than the mantle around it. This means it rises.

This melted plate is now hot, liquid rock (magma). The magma rises through the gaps in the continental plate. If it reaches the surface, the liquid rock forms a volcano.

The trapped sea water turns into steam and this makes the volcanoes very explosive. There isn’t much lava but there is a lot of ash, steam and gas.

Conservative plate boundary

Conservative plate boundaries exist where two plates do not directly collide but slide past each other along a fault (weakness).

No volcanoes are found along these plate boundaries, but earthquakes do occur.

An example of such a boundary is the San Andreas Fault in California.

Collision plate boundary

Collision boundaries occur when two plates of similar densities move together (i.e. a continental plate and a continental plate). Because they are the same density, neither of them can sink. This causes the material between them to buckle and rise up, forming fold mountains.

The Himalayas are an example of a chain of fold mountains. They have been formed by the African plate colliding into the Eurasian plate.

Because of the massive pressure caused by two plates crashing into each other, there are strong earthquakes at these margins.

	Tectonic Plates

	Oceanic Crust
	•Is located under the oceans. •It’s made of basalt. •It’s between 5 and 10 km thick. •It’s dense so it sinks into the mantle where oceanic and continental crusts meet.

	Continental Crust
	Makes the continents. •It’s between 25 and 100 km thick. •Not very dense. •It’s lighter than oceanic crust.

	Constructive plate boundary
	At a constructive plate boundary, two plates move apart. As the two plates move apart, magma rises up to fill the gap, constructing new crust. This causes volcanoes. However, since the magma can escape easily at the surface the volcano does not erupt with much force. Earthquakes are also found at constructive boundaries, caused by the friction of the plates as they move over the mantle. You get lots of eruptions at these margins – from volcanoes, like the Azores, and from cracks in the ground called fissures, like in Iceland. The crust on either side of the margin is often faulted. This means it has big cracks in it. An example of a constructive boundary is the Mid-Atlantic Ridge.

	Destructive plate boundary
	A destructive plate boundary is found where a continental plate meets an oceanic plate. The oceanic plate descends (subducts) under the continental plate because it is denser. The friction between the two plates causes earthquakes. The land on the edge of the continental crust gets squashed up to make fold mountains, like the Andes. As the plate descends it starts to melt due to the friction caused by the movement between the plates. Because it takes some sea water with it, it’s less dense than the mantle around it. This means it rises. This melted plate is now hot, liquid rock (magma). The magma rises through the gaps in the continental plate. If it reaches the surface, the liquid rock forms a volcano. The trapped sea water turns into steam and this makes the volcanoes very explosive. There isn’t much lava but there is a lot of ash, steam and gas.

	Conservative plate boundary
	Conservative plate boundaries exist where two plates do not directly collide but slide past each other along a fault (weakness). No volcanoes are found along these plate boundaries, but earthquakes do occur. An example of such a boundary is the San Andreas Fault in California.

	Collision plate boundary
	Collision boundaries occur when two plates of similar densities move together (i.e. a continental plate and a continental plate). Because they are the same density, neither of them can sink. This causes the material between them to buckle and rise up, forming fold mountains. The Himalayas are an example of a chain of fold mountains. They have been formed by the African plate colliding into the Eurasian plate. Because of the massive pressure caused by two plates crashing into each other, there are strong earthquakes at these margins.