Earth's crust, or outermost rocky layer, sits on top of a deeper layer called the mantle, which stores heat from two sources: the formation of the Earth 4.65 billion years ago and the radioactive decay of uranium, thorium, and potassium. When cracks between huge crustal plates open up, the gap causes the underlying mantle to rise up. The upwelling partially melts that region of the mantle; scientists call that decompression melting. The molten rock, or magma, is less dense than solid rock, so it moves upward, the way a cork bobs to the surface of water. As the magma reaches the upper layers of the crust or even Earth's surface, it cools and hardens into a solid known as igneous rock. Scientists categorize igneous rocks according to their chemical composition, the method of their formation, and their degree of crystallization.

Inside some volcanoes, gases such as water vapor, carbon dioxide, and sulfur dioxide are dissolved within the melted rock (magma), lowering the density of the magma and increasing its buoyancy. If the magma contains a large fraction of trapped gases when it reaches the open air at the mouth of the volcano, the rapid decompression fragments the magma into pumice and ash (pieces of foamed-up magma) that explode from the volcano and rapidly harden in the air. (Think of the spray from bottles of soda or champagne if you shake them hard and open their tops.) Sometimes explosive eruptions release dangerous currents of hot gas, ash, and rock down the sides of volcanoes. These fast-moving currents, called pyroclastic flows, killed 16,000 people in the Roman towns of Pompeii and Herculaneum when Mount Vesuvius erupted in 79 C.E.