By Grace Watson-Martin

Volcanoes begin deep within the mantle where magma forms. Magma can gush out of weak spots in the crust. A volcano is an opening out of which this hot material escapes.

Eruptions happen often in volcanoes. Bubbles of gas separate from the magma and force the hot material out of the volcano. This occurs when pressure builds underneath Earth's crust and the pressure needs to be released. Materials that erupt from a volcano contain ash, cinders, and lava.

As lava cools and hardens, it becomes rock. That's brand new crust! If the rock builds to great heights, a mountain or island forms. 

Magma collects in large pockets called magma chambers. As magma leaves the chamber, it moves up a tunnel or central vent. Sometimes, magma escapes from the central vent and erupts from side vents. Most magma, however, erupts at the top of the volcano through a bowl-shaped center.