Volcanic dust

When a volcano erupts it will sometimes eject material such as rock fragments into the atmosphere. This material is known as tephra. The largest pieces of tephra (greater than 64 mm) are called blocks and bombs. Smaller ejecta such as lapilli (2-64 mm) and ash (2 mm) which are convected upward by the heat of the eruption will fall out farther from the volcano. The smallest particles which are less than 0.01 mm and called volcanic dust, can stay in the atmosphere for two or three years after a volcanic eruption. Sometimes these particles produce fantastic sunsets such as was seen after the eruptions of Krakatau in 1883 and Pinatubo in 1991; they may also contribute to global warming.

Tephra produces a wide range of hazards. When the ejected material is in the atmosphere it is electrically charged and often produces lightning which has been known to kill people. Large ejecta shot ballistically from the volcano are also a hazard to those unfortunate enough to be near the volcano. Other hazards are produced when the ash is deposited on the ground. Ash can disrupt electricity, television, radio, and telephone communication lines, bury roads and other manmade structures, damage machinery, start fires, and clog drainage and sewage systems. Ash can produce poor visibility and cause respiratory problems. Often people living in areas affected by volcanic ash will wear masks with filters or wet cloth over their nose and mouth to protect themselves from breathing in volcanic ash and fumes. If ash builds up on the tops of roofs, it will often cause collapse. This is especially common on flat topped buildings.

Volcanic dust poses a number of potentially dangerous situations. Whole cities have been buried under volcanic dust. Large amounts of dust in the air can disrupt the weather. Floating clouds of volcanic ash can pose a serious hazard to international air traffic. So far it has been impossible to alert air traffic promptly enough on dangerous approaching ash clouds.