From The Art and Popular Culture Encyclopedia
A pyroclastic flow (also known scientifically as a pyroclastic density current) is a common and devastating result of certain explosive volcanic eruptions. The flows are fast-moving currents of hot gas and rock (collectively known as tephra), which travel away from the volcano at speeds generally as great as 700 km/h (450 mph). The gas can reach temperatures of about Template:Convert. The flows normally hug the ground and travel downhill, or spread laterally under gravity. Their speed depends upon the density of the current, the volcanic output rate, and the gradient of the slope.
The word pyroclast is derived from the Greek πυρ, meaning fire, and κλαστός, meaning broken. A name for some pyroclastic flows is nuée ardente (French for "glowing cloud"); this was first used to describe the disastrous 1902 eruption of Mount Pelée on Martinique. These pyroclastic flows glowed red in the dark.
Pyroclastic flows that contain a much higher proportion of gas to rock are known as "fully dilute pyroclastic density currents" or pyroclastic surges. The lower density sometimes allows them to flow over higher topographic features such as ridges and hills. They may also contain steam, water and rock at less than Template:Convert these are called "cold" compared with other flows, although the temperature is still lethally high. Cold pyroclastic surges can occur when the eruption is from a vent under a shallow lake or the sea. Fronts of some pyroclastic density currents are fully dilute, for example during the eruption of Montagne Pelée in 1902 a fully dilute current overwhelmed the city of Saint-Pierre and killed nearly 30,000 people.
A pyroclastic flow is a type of gravity current; in scientific literature they are sometimes abbreviated to PDC (pyroclastic density current).