Nuee ardent

A nuee ardent, or "glowing cloud," is a type of explosive volcanic eruption characterized by a dense, very hot mass of ash, gasses, and volcanic material traveling down a volcanic slope at high velocity. A nuee ardent, also called a pyroclastic flow, can reach speeds of 450 mi/hr (720 km/hr) and temperatures of 1,500°F (830°C). They can travel as much as 124 mi (200 km) from the source and cover areas as large as 12,000 mi² (20,000 km²). The volume of transported material can be as large as 250 mi³ (1,000 km³) or more.

Nuees ardentes can be destructive and deadly. An eruption of Mount Pelee, Martinique, in 1920 produced a nuee ardent that, within minutes, killed 30,000 inhabitants of a nearby town. In 1982, eruptions and pyroclastic flows from El Chichòn Volcano , southeastern Mexico, killed 2000 people, in villages as far as 5 mi (8 km) from the source.

Nuees ardentes are typically composed of two parts; a basal or lower part that hugs the ground and contains the larger volcanic fragments, and an upper part composed of a turbulent mix of hot ash and gas. When cooled, the gas and ash can form pumice , a very porous and lightweight volcanic rock .

Pyroclastic flows commonly are produced either by the downslope movement of fragments ejected upwards as they fall back, or by explosive eruptions that pulverize existing rock and throw the rock, ash, and gas in a more horizontal direction. In many cases, pyroclastic flows result from a combination of mechanisms. The sequence of events leading up to and during the 1980 eruption of Mount St. Helens illustrates some of the complexity of the process. An increase in seismic activity beneath the volcano in March marked the beginning phase of the eruption. Magma injected into the cone of the volcano at high pressure created a bulge on the north flank that grew outward at rates as high as 8.2 ft (2.5 m) per day. For the next two months only minor eruptions occurred. On May 18, a 5.1 magnitude earthquake triggered a series of massive landslides of material over the bulge. Remove of this material destabilized the north slope, which failed explosively releasing a pyroclastic flow horizontally. The eruptions flattened trees and killed wildlife in a 212 mi² (550 km²) area . The eruption removed the upper 1,300 ft (400 m) of the cone and left a crater 2,000 ft (625 m) deep, 1.7 mi (2.7 km) long, and 1.3 mi (2.0 km) wide.