Pyroclastic flow

A pyroclastic flow, more broadly known as a pyroclastic density current, is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that flows along the ground away from a volcano. Pyroclastic currents travel at extremely high speeds and have extremely high temperatures.

Pyroclastic flows are the deadliest of all volcanic hazards and are produced as a result of certain explosive eruptions. They normally touch the ground and hurtle 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.

Upon the eruption of volcanic matter, much of the material sinks to the ground and flows along it, forming a pyroclastic current. Currents are composed of a continuum of materials, spanning from denser, comparatively cooler rocks to warmer, lighter ashes and gasses. Denser rocks are more likely to be influenced by gravity, becoming separated from lighter ashes and gasses that continue flowing across the surrounding ground and air. The denser components of pyroclastic currents are what is specifically meant when referring to pyroclastic flow, whereas the lighter components of pyroclastic currents are known as pyroclastic surges.

In certain cases, pyroclastic density currents can flow over water, with its denser constituents sinking into and mixing with it. Waters can even recede as a result of land production from the mass and volume flux of pyroclastic currents. Additionally, pyroclastic currents can be observed on other astronomical bodies, such as the moon.