Volcanic Eruptions as Risk and Hazard

Volcanoes are a principal agent of degassing of the interior of the Earth and other planets. They are thought to be the main source of the Earth's atmosphere and hydrosphere, and thus they are a vital part of the sustainable Earth system. Volcanic activity has provided the air we breathe and the water that makes up most of our body mass. Volcanism also provides vital enrichment to the Earth's soils, particularly in tropical areas. In spite of the long-term benefits, the effects of eruptions can be seriously negative in the short run, so volcanoes can be considered a type of natural hazard that threatens life and property.

Among natural hazards, volcanoes have caused far fewer deaths and damage to property historically than have floods, coastal hazards, and earthquakes and so are considered hazards of local or minor rank by some people. The historic record may be misleading because it is brief compared with geologic time and because large eruptions occur too rarely to have been accurately assessed and understood. There are at least 1,500 volcanoes that are considered active in the world, and about 400 have erupted in the past century. Eruptions span at least eight orders of magnitude, ranging from erupted magma volumes of less than 10,000 m3 (cubic meters) to more than 1,000 km3 (cubic kilometers). But most eruptions are quite small in scale, and their hazards are generally quite localized to areas near the volcano. Larger eruptions are known to have global impact, but because of their statistical rarity, the scientific understanding of these events is very limited. Understanding of the largest eruptions (super-eruptions) is the most limited, but these possibly include worldwide loss of multiple growing seasons and huge global loss of life (volcanic winter).

The assessment of volcanic hazards begins with geologic maps, which record the distribution of past volcanic deposits that reflect past activity and the locations where hazardous events occurred, leading to hazard maps that cover the volcano environs, especially in areas downslope from active vents. Volcanic deposits are dated to assess the frequency and probability of hazards, and vulnerability is assessed through geographic information system population density maps and social science surveys. The topography of areas near volcanoes provides an important key to hazard zonations, because many types of volcanic hazards are governed by gravity and the damaging volcanic agents (lava flows, pyroclastic flows, mudflows, avalanches, debris flows) descend from high to low elevation following valleys. A useful tool in hazard work has been the development of numerical models of hazards, which can be applied to the individual topography to derive “objective” hazard maps. Completed hazard maps exist for only a small fraction of Earth's active volcanoes. To accompany hazard maps, hazard reports are written in plain language for the public to explain the nature of hazards that occur during eruptions. These reports are used in local communities and especially schools. Where risk is high, additional measures for public education and outreach are undertaken, and zoning measures may be implemented.

Atmospheric hazards that result from eruptions are of special interest, because they typically extend farther from the volcano and may thus affect a broader area. Explosive eruptions create finely divided magma that transfers its heat rapidly to the air and rises to stratospheric height. Since many particles are so fine that they do not fall, they can be carried hundreds to thousands of kilometers before fallout. Ice nucleates on this fine ash and helps it grow and fall out. Stratospheric ash is a severe aircraft hazard, while fine ash fallout can occur on a continental scale in super-eruptions and is a deadly hazard to lungs. Ash fallout masses and particle sizes decrease [Page 3028]with distance for the first 100 kilometers or more, but distal fallout is governed by meteorological processes and may result in heavy fallout far from the volcano. Sulfur dioxide is a second hazardous material contributed by eruptions to the stratosphere. It reacts with the atmosphere and forms sulfuric acid, which exists as a liquid aerosol particle that stays in the stratosphere much longer (a few years) than ash (a few days) and can create stratospheric warming and surface cooling.

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