Explosive eruption

This cloud may subsequently collapse, creating a fast-moving pyroclastic flow of hot volcanic matter.

The pressure of the magma builds until the blockage is blasted out in an explosive eruption through the weakest point in the cone, usually the crater.

(However, in the case of the eruption of Mount St. Helens, the pressure was released on the side of the volcano, rather than the crater.[3]).

The cooling of the gas in the ash as it expands chills the magma fragments, often forming tiny glass shards recognisable as portions of the walls of former liquid bubbles.

[2] When an emulsion of volcanic gas and magma falls back to the ground, it can create a density current called a pyroclastic flow.

[2] Earthly pyroclastic flows can travel at up to 80 km (50 mi) per hour and reach temperatures of 200 to 700 °C (392 to 1,292 °F).

The high temperatures can burn flammable materials in the flow's path, including wood, vegetation, and buildings.

Alternately, when an eruption has contact with snow, crater lakes, or wet soil in large amounts, water mixing into the flow can create lahars,[4] which pose significant known risks worldwide.