PLANET EARTH — The most explosive volcanic eruptions spit out a fast-moving and incredibly destructive mass of material known as a pyroclastic flow. <br /> <br />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. <br /> <br />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. <br /> <br />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. <br /> <br />These dense pyroclastic surges can even move over water. <br /> <br />Pyroclastic flows generally destroy everything in their path including vegetation, buildings and people. <br /> <br />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. <br /> <br />The second type is rarer and occurs when so much pressure builds up inside a volcano that it erupts laterally and boils over. <br /> <br />The last known example of this is when Mount St. Helens in Washington state erupted in 1980.
