Origin of term
The word pyroclast is derived from the
πῦρ, meaning "fire", and
κλαστός, meaning "broken in pieces". A name for pyroclastic flows which glow red in the dark is nuée ardente (French, "burning cloud"); this was first used to describe the disastrous 1902 eruption of
Mount Pelée on
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 or water such as ridges, hills, rivers and seas. They may also contain steam, water and rock at less than 250 °C (482 °F); 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
Mount 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).