What does the passage mainly explain?

A volcano is an opening in the surface of a planet or moon that lets material hotter than its surroundings escape from inside. Some eruptions are explosive, throwing rock and ash high into the sky; others are calm, with molten rock flowing gently out. Over many eruptions, layers of rock, ash and other material can pile up into a mountain.

Molten rock is called magma while it is underground and lava once it reaches the surface. On Earth, magma rises in three main kinds of place. Where the plates of the Earth's crust move apart, it rises to fill the space. Where plates push together, one is forced down into the hot interior, where heat and pressure melt the rock. And at hot spots, areas of great heat deep inside the Earth, magma rises because it becomes lighter than the rock around it.

Volcanoes are sorted by how recently they have erupted. An active volcano has erupted recently or is expected to erupt soon. A dormant volcano is quiet now but may erupt in the future. An extinct volcano will probably never erupt again.

Earth is not the only world with volcanoes. Venus and Mars are covered with extinct ones, and NASA spacecraft have photographed eruptions taking place today on Io, a moon of Jupiter, and on moons of Saturn and Neptune.