A pyroclastic flow is a ground-hugging avalanche of hot ash, pumice, rock fragments, and volcanic gas that rushes down the side
of a volcano as fast as 100 km/hour or more. The temperature within a pyroclastic flow may be greater than 500°C, sufficient to
burn wood. Once deposited, the ash, pumice, and rock fragments may deform (flatten) and weld together because of the intense
heat and the weight of the overlying material.
Facts and figures: On 8. May 1902 a pyroclastic flow from Montagne Pelée, Martinique, caused the worst volcanic disaster of the 20th century. 28'000 people perished in the city of St. Pierre, located 7 km from the volcano's summit.
Small pyroclastic flow on Mt. Etna. Sicily, 25.10.1999. Photo: R. CarnielDeadly pyroclastic flow descending towards St. Pierre