PLANET EARTH — The most explosive volcanic eruptions spit out a fast-moving and incredibly destructive mass of material known as a pyroclastic flow.
Should you ever find yourself in the path of one, you should run in the opposite direction as quickly as possible, according to the United States Geological Survey.
Pyroclastic flows are made up of a basal flow of volcanic ash, lava, rock and gases, which move beneath a cloud of ash. Their temperatures can exceed 1,000 degrees Celsius, and they can move at 700 kilometers per hour.
Typically, pyroclastic flows move downslope but they can go uphill when the ratio of gas to ash is higher, according to Forbes. This is known as a pyroclastic surge.
These dense pyroclastic surges can even move over water.
Pyroclastic flows generally destroy everything in their path including vegetation, buildings and people.
There are generally two kinds of pyroclastic flow. The first type forms when an eruption column cools and the ash becomes too dense to maintain an upward thrust.
The second type is rarer and occurs when so much pressure builds up inside a volcano that it erupts laterally and boils over.
The last known example of this is when Mount St. Helens in Washington state erupted in 1980.
