Pyroclastic Falls

Pyroclastic falls, also known as volcanic fallout, occur when tephra – fragmented rock ranging in size from mm to tens of cm is ejected from a volcanic vent during an eruption and falls to the ground some distance away from the vent. Falls are usually associated with Plinian eruptive columns, ash clouds or volcanic plumes. Tephra in pyroclastic fall deposits may have been transported only a short distance from the vent or if it is injected into the upper atmosphere, may circle the globe. Any kind of pyroclastic fall deposit will mantle or drape itself over the landscape, and will decrease in both size and thickness the farther away it is from its source. Tephra falls are usually not directly dangerous unless a person is close enough to an eruption to be struck by larger fragments. The effects of falls can be, however. Ash can smother vegetation, destroy moving parts in motors and engines, and scratch surfaces. Scoria and small bombs can break delicate objects, dent metals and become embedded in wood. Some pyroclastic falls contain toxic chemicals that can be absorbed into plants and local water supplies, which can be dangerous for both people and livestock. The main danger of pyroclastic falls is their weight: tephra of any size is made up of pulverized rock, and can be extremely heavy, especially if it gets wet. Most of the damage caused by falls occurs when wet ash and scoria on the roofs of buildings causes them to collapse. Pyroclastic material injected into the atmosphere may have global as well as local consequences. When the volume of an eruption cloud is large enough, and the cloud is spread far enough by wind, pyroclastic material may actually block sunlight and cause temporary cooling of the Earth’s surface.

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