Pyroclastic surge

A pyroclastic surge is a fluidised mass of turbulent gas and rock fragments that is ejected during some volcanic eruptions.

The speed of pyroclastic density currents has been measured directly via photography only in the case of Mount St. Helens, where they reached 320-470 km/h, or 90–130 m/s (200–290 mph).

[4] They develop from the interaction of magma (often basaltic) and water to form thin wedge-shaped deposits characteristic of maars.

They form thin deposits, but travel at great speed (10–100 m/s) carrying abundant debris such as trees, rocks, bricks, tiles, etc.

[6] Typically they are about 1 m. thick and consist mostly of lithic and crystal fragments (fine ash elutriated away).

Dune bedform formed by the pyroclastic currents related to the 2006 eruption of Tungurahua (Ecuador). A. Outer shape of a lunate dune bedform and B. internal lamination. Note preferential aggradation on upstream face (backset laminations). [ 6 ]