Plate Tectonics - Volcanoes

Volcanoes

What is a volcano?

A volcano is an opening or vent in the earth’s surface through which molten material erupts and solidifies as lava.

Cross section of a volcano

Why do they happen at destructive plate boundaries ?

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

The oceanic plate descends under the continental plate because it is denser. As the plate descends it starts to melt due to the friction caused by the movement between the plates. 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.

Why do they happen at constructive plate boundaries ?

At a constructive plate boundary, two plates move apart.

As the two plates move apart, magma rises up to fill the gap. This causes volcanoes. However, since the magma can escape easily at the surface, the volcano does not erupt with much force.

A Shield Volcano

They occur at constructive plate boundaries.

At constructive plate boundaries the lava is runny and it can flow far away from the volcano's vent.

This makes the volcano gentle-sided.

Shield volcanoes do not erupt violently as the lava flows freely.

A Composite Volcano.

They occur at destructive plate boundaries.

At destructive plate boundaries the lava is viscous (thick like treacle) and it cannot flow very far from the volcano's vent.

This makes the volcano steep-sided.

Such volcanoes erupt violently because the rapidly hardened rock blocks the vents and allows pressure to build up under the blockage.

A Dome Volcano.

Very viscous lavas.

Very explosive with pyroclastic flow.

Large debris emitted from a central vent.

A Cinder Volcano.

Explosive liquid lava.

Small debris emitted from a central vent.

If continued long enough may build up a shield volcano.

A Caldera Volcano

Very large composite volcano.

Collapsed after an explosive period.

Frequently associated with plug domes.

A Fissure Volcano.

Very liquid lava.

Flows very widespread.

Emitted from fractures.

Predicting Volcanic Eruptions.

Volcanologists (people who study volcanoes) are skilled at predicting the likelihood of an eruption.

However, it's very difficult to pinpoint exactly when an eruption will happen. Often, moving magma doesn't result in an eruption, but instead cools below the surface. Monitoring potential eruptions is expensive. With many volcanoes erupting only every few hundred years, it's not possible to monitor every site.