What hot, fast-moving mixture of gas, ash, and rock fragments flows down a volcano?

What hot, fast-moving mixture of gas, ash, and rock fragments flows down a volcano? A pyroclastic flow does.

More broadly called a pyroclastic density current, it is a ground-hugging mixture of volcanic gases and tephra—ash, pumice, and larger rock fragments. It forms when an explosive eruption column collapses, when part of a lava dome breaks apart, or when an eruption sends dense material down a slope.

These currents are among volcanoes’ deadliest hazards because they combine extreme heat, crushing force, and great speed. Typical temperatures range from about 200°C to 700°C, and speeds can reach roughly 700 km/h. The French term nuée ardente, meaning “burning cloud,” became famous after Mount Pelée’s 1902 eruption.

A pyroclastic flow is not the same as a lava flow: lava is molten rock, while a pyroclastic current is mostly fragmented material and gas. It is also different from an ash plume, which rises into the atmosphere rather than racing along the ground.