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.
The distance that ejecta travels away from a volcano depends on the height of the eruption column, temperature of the air, wind direction and wind speed. An erupting column that reaches into the stratosphere will be sheared by strong winds in this region and cause the eruption cloud to spread out over a larger area. The temperature of the air during an eruption will increase due to the hot material ejected into the atmosphere. This produces a bouyant force that carries tephra higher into the atmosphere which allows it to be deposited over a larger area. Wind direction and wind speed are very important in determining where and how large an area will be covered by ash.
Most deaths resulting from the eruption of Mount Pinatubo in 1991 were due to roofs collapsing under the weight of volcanic ash. In 1815, after the eruption of Tambora which ejected 151 cubic kilometers of ash into the atmosphere, 80,000 people died due to famine caused by the destruction of vegetation. The eruption of Krakatau deposited ash over an area of 800,000 square kilometers. People 70-80 km away from the volcano had their clothes burnt by ash. Fallout from the eruption of Tambora in 1815, which is the largest eruption in recent history, was deposited as far away as 1300 km. More than 10 cm of ash fell in India 3000 km away from the Toba Caldera when it erupted 75,000 years ago.
Ash from the 1982 eruption of Galunggung Volcano in West Java, Indonesia caused engines in two jet airplanes to fail. Both aircraft dropped 25,000 feet before they could get their engines to start again.