The term "pyroclastic" is derived from the Greek words pyro (fire) and klastos (broken) and describes materials formed by the fragmentation of magma and rock by explosive volcanic activity. During the 1902 eruption of Mount Pelée in Martinique, a nuee ardente demolished the coastal city of St. Pierre, killing nearly 30,000 inhabitants. Pyroclastic flows and surges have caused most of the destruction in Montserrat during the Soufrière Hills Volcano eruption.

Pyroclastic flows, sometimes called "nuees ardentes" (French for "glowing clouds") are hot, rapidly-moving mixtures of ash, rock fragments and gas flowing from a collapsed eruption column or lava dome. Pyroclastic flows travel down valleys and completely devastate the area over which they flow. They can travel at speeds at speeds of 100 mph or more and can be extremely destructive because of their high temperature and mobility.

They are both used to describe the molten rock in a volcano. "Magma" is used for molten rock below the ground and "lava" for molten rock above the ground.