EARTH — A volcano is an opening in the earth's surface where molten rock can escape. The earth's crust is made up of tectonic plates that shift and move. Volcanoes are often located at the fault lines between these plates.

Volcanoes can be active, dormant or extinct, according to Universe Today.

There are four major types of volcanoes: cinder cone, composite, shield and lava domes.

Cinder cone volcanoes occur when lava is ejected from a volcanic vent. Lava is shot into the air, and pieces accumulate around the vent. Over time, this creates a circular or oval-shaped cone, with a crater at the top.

Composite volcanoes contain a conduit system that channels magma to the surface. These volcanoes can have clusters of vents along the sides of the mountain where lava flows out.

Shield volcanoes are large, broad volcanoes where lava pours out in thin layers, allowing it to travel farther down the shallow slopes of the volcano. Shield volcanoes build up slowly with hundreds of eruptions creating several layers.

Lava domes are created when small masses of thick lava that can't flow far from the source, so domes pile up around the vent. The dome grows by expansion of lava from within, and the mountain forms from material spilling off the sides of the dome.