But why did they erupt?

More than 80% of the Earth's surface above and below sea level is volcanic and deep below.

The Earth's tectonic plates are always moving.

Most volcanic activity occurs where these plates collide deep within the Earth.

It is so hot that rock slowly melt and become magma.

Because this flowing substance is lighter than the rocks around it, it rises.

When some of these tectonic plates shift, the magma rises, even hire.

Some of the magma pushes through the cracks in the earth's crust advance and at Fisher's, reaches the surface where it is that called lava.

What kind of eruption the volcano will have depends on the properties of the magma.

Thin and running Madre means gas, trying to escape conduce so easily.

When this happens, the lava will flow out of the volcano.

We've seen this with Hawaii's volcanoes because the lava flows slowly, people can get away quickly, and it rarely results in victims.

But thick and sticky magma is a different story.

There's less room for gases to escape on the pressure mounts, gas gets trapped in the magma, which then explodes at the surface.

This'd called an explosive eruption, and we've seen it with Mount Etna in Sicily and Mount ST Helens in the U.

S.

About 500 volcanoes have erupted since we started keeping records.

Many around the Ring of Fire, an area from New Zealand's to the coast of South America, where there are the most subduction zones.

Plate movements, all volcanic activity is closely monitored by scientists around the world.

By tracking active volcanoes, experts can see the warning signs and alert the public before an eruption.