Some appeared hundreds of thousands of years ago, while others just appeared recently.

But each time, it’s the same phenomenon: magma, a mixture of molten rock and hot gas from the depths of the earth, escapes through a crack in the Earth’s crust. 

On contact with the air, magma cools down and hardens. With each eruption, the rock piles up. A volcano is born!

But why does the Earth’s crust crack?

Because tectonic plates move.