Pyroclastic surge

A pyroclastic surge is a fluidised mass of turbulent gas and rock fragments that is ejected during some volcanic eruptions. It is similar to a pyroclastic flow but it has a lower density or contains a much higher ratio of gas to rock,^[1] which makes it more turbulent and allows it to rise over ridges and hills rather than always travel downhill as pyroclastic flows do.

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). Estimates of other modern eruptions are around 360 km/h, or 100 m/s (225 mph).^[2] Pyroclastic flows may generate surges. For example, the city of Saint-Pierre in Martinique in 1902 was overcome by one. Pyroclastic surge include 3 types, which are base surge, ash-cloud surge, and ground surge.