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. Blocks and bombs are normally shot ballistically from the volcano (refer to the gas thrust zone described in the direct blast section). Because these fragments are so large they fall out near their source. Blocks and bombs as large as 8-30 tons have fallen as far away as 1 km from their source (Bryant, 1991). Small blocks and bombs have been known to travel as far away as 20-80 km (Scott, 1989)! Some of these blocks and bombs can have velocities of 75-200 m/s (Bryant, 1991). 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. Most particles greater than a millimeter in size will fall out within 30 minutes of the time they are erupted (W.I. Rose personal communication). The smallest  particles which are less then .01 mm 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. Some scientists believe that these particles may contribute to global warming.

The size of particles that fall out is largest near the volcano and gets progressively smaller further from the volcano. The thickness of material usually decreases the further away from the volcano. Occasionally, as occurred in Ritzville, Washington when Mount St. Helens erupted in 1980, secondary thickening will occur. Secondary thickening means that ash deposits are thicker in a particular area than in surrounding areas. This results when ash particles and water form clumps which produce larger particles that have higher terminal velocities and so fall out of the ash cloud. When these particles hit the ground they break apart and produce a thicker deposit of ash than in surrounding areas.

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. Ash erupting from Mount St. Helens on May 18, 1980 covered the town of Yakima, which is approximately 80 miles to the east of the volcano, with 10 mm of ash (Foxworthy and Hill, 1982). This caused the sky to become as dark as night during the middle of the day. The town of Vancouver, approxiamately 50 miles south of the volcano, had no ash deposited from the eruption because the wind direction was blowing away from it toward the northeast.

So how far away can ash be deposited? 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 (Bryant, 1991). Fallout from the eruption of Tambora in 1815, which is the largest eruption in recent history, was deposited as far away as 1300 km (Francis, 1993)! More than 10 cm of ash fell in India 3000 km away from the Toba Caldera when it erupted 75,000 years ago.

Tephra produces a wide range of hazards. When the ejected material is in the atmosphere it is electrically charged and often produces lightning. Several people have been killed by lightning from volcanic eruption clouds. 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. Most deaths resulting from the eruption of Mount Pinatubo in 1991 were due to collapsing roofs (Wolfe, 1992).

Ash is also a great hazard to airplanes. 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. Ash end St. Elmo's fire entered the cabin and caused damage to the aircraft. Greater than sixty airplanes have been damaged by ash from various volcanic eruptions. Damage can include pitting of wind sheilds which causes them to look foggy. The fuselage, wings, engines, and light covers can also be damaged by ash and are very expensive for the airlines to repair. Volcanic eruption clouds cause rerouting, cancellation, and delays in flights which is also expensive for the airlines. The accumulation of ash can also load down a plane which may cause the plane to rest on its tail. Ash on runways is another problem because anytime a plane lands or takes off the ash is resuspended. Special techniques have been developed to remove ash from aircraft and runways (Casadevall, 1993). Currently much research is being done in using satellites to track volcanic clouds. For more information about tracking ash clouds using remote sensing devices click here.

Tephra can also destroy vegetation which can result in famine. Famines are the largest indirect hazard produced by volcanic eruptions. In 1815, after the eruption of Tambora which ejected 151 cubic kilometers of ash into the atmosphere, 80,000 people died due to famine (Bryant, 1991 and Francis, 1993).

Residents in areas that may be affected by volcanic ash during an eruption, need to be prepared. They should stock up on water and food supplies, stay indoors to avoid having clothing burnt or skin irritated by acids carried by volcanic ash, avoid going outside without a wet cloth or some sort of filter over their mouth and nose, stay in areas that are unlikely to receive large amounts of tephra or large sized tephra, avoid staying in buildings that are flat-roofed, and be prepared to be without telephones, electricity, and radio communication. Residents should also be prepared to do a lot of cleanup, and when doing this be careful not to resuspend the ash.

Tephra produces many hazards but is also very beneficial. Its greatest benefit is that it produces fertile soil for crops. This is the biggest reason why so many people live on or near active volcanoes.

Text by C.M. Riley