Volcano Hazards: | Types and Effects | Location | | Gas | Lahars | Landslides | Lava Flows | Pyroclastic Flows | Tephra |

Scientists use a wide variety of names to describe specific types of hot, dry flows of rock fragments and gas produced by erupting volcanoes. The terms below are used to describe either (1) the way in which a pyroclastic flow originates and moves; or (2) a predominant characteristic of the resulting deposit.

Ash Flow or Ash Cloud
A pyroclastic flow consisting primarily of ash-sized particles, including glass shards and mineral fragments

Block and Ash Flow
A pyroclastic flow consisting of ash and large lava fragments with few gas bubbles, which typically forms as a consequence of a collapsing lava flow or dome.

Base Surge
A turbulent, low-density flow of rock debris and water and (or) steam that moves at high speeds. Base surges may occur when an explosive eruption occurs from within a crater lake or an ocean.

Directed Blast
A volcanic explosion of rocks and magma (or both) with a low-angle component. When the rock debris from a directed blast falls to the ground, it behaves like a pyroclastic flow or surge and moves rapidly away from the volcano.

Nuée Ardente (glowing cloud)
A French term introduced in 1904 to describe pyroclastic flows erupted on Mount Pelée on the Island of Marginique. When viewed at night or in low light, pyroclastic flows may appear to glow red. The term is widely used to describe pyroclastic flows, but not to imply the way a flow is generated.

Pumice Flow
A pyroclastic flow consisting predominantly of pumice fragments, which contain many gas bubbles.

Pyroclastic Surge
A turbulent, low-density cloud of hot rock debris and gases that moves at extremely high speeds. Because surges are low density, they tend to spread over large areas and move up and over ridge crests easily. By contrast, pyroclastic flows are high-density masses of hot rock debris and gases that tend to be confined in valleys.