OF VOLCANOES
Volcano is a mountain or a hill formed by the accumulation of materials erupted through one or more openings (called volcanic vents) in the earths surface. The term volcano can also refer to the vents themselves. Most volcanoes have steep sides, but some can be gently sloping mountains or even flat tablelands, plateaus, or plains. The volcanoes above sea level are the best known, but the vast majority of the worlds volcanoes lie beneath the sea, formed along the global oceanic ridge systems that crisscross the deep ocean floor. 1511 above-sea volcanoes have been active during the past 10,000 years, 539 of them erupting one or more times during written history. On average, 50 to 60 above-sea volcanoes worldwide are active in any given year; about half of these are continuations of eruptions from previous years, and the rest are new.
Volcanic eruptions in populated regions are a significant threat to people, property, and agriculture. The danger is mostly from fast-moving, hot flows of explosively erupted materials, falling ash, and highly destructive lava flows and volcanic debris flows. In addition, explosive eruptions, even from volcanoes in unpopulated regions, can eject ash high into the atmosphere, creating drifting volcanic ash clouds that pose a serious hazard to airplanes.
Volcanology is a branch of geology, the study of the earth. Volcanology emphasizes studies of the processes, products, hazards, and environmental impacts of volcanic eruptions. Volcanologists are geologists who specialize in studies of "young" volcanism. They focus on eruptions within the past 10,000 years, especially on those within recorded history. Volcanologists also study currently active or potentially active volcanoes. They use conventional geologic methods, including geologic mapping and age determination of the deposits of past eruptions. They also use field and laboratory studies of volcanic products, geophysical surveys, and drilling studies. The information they gather provides clues about the volcanos eruptive style (explosive vs. nonexplosive; eruption sizes), eruption frequency, underground structure, and magma reservoir. Volcanologists use this information to evaluate the likelihood of future eruptions (long-term forecasts) and other hazards. They also try to construct maps that show the most vulnerable areas on and around the volcano.
All eruptions are accompanied by geophysical and (or) geochemical changes, including earthquake activity, deformation of the volcano, and increased release or change in volcanic gases. To make regular measurements of such changes, scientists install sensors on active and restless volcanoes. Information from these instruments is sent to a volcano observatory for analysis and interpretation by volcanologists. There, they make short-term forecasts of possible eruptions or changes in the course of an on-going eruption. Since the advent of space technology, volcanologists have been using satellite-based systems in addition to ground-based methods to study volcanoes.
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Predicting Eruptions. A major challenge of volcanology is to predict the next eruption of an active or dormant volcano. Scientists generally consider a volcano active if it has erupted one or more times in historical time. This guideline is poor, however, because written history is much longer for volcanoes in some parts of the world, for instance in Japan and Italy, than in other parts, such as the United States and New Zealand. Dormant volcanoes are currently inactive but considered by scientists to have potential for future eruption. Long-dormant volcanoes believed to lack potential for renewed activity are defined as extinct. The distinction between dormant and extinct is based on the amount of knowledge about a given volcano and is not absolute.
Scientists try to predict eruptions by taking measurements of events leading up to possible activity, such as earthquakes, ground movement, and the release of gases. Despite several encouraging successes, including the 1991 eruption of Mount Pinatubo, Philippines, and several recent eruptions of Sakurajima Volcano, Japan, the prediction of explosive eruptions still eludes volcanologists. The success rate is better for prediction of nonexplosive eruptions at well-monitored volcanoes. For example, nearly all of the lava dome-building eruptions at Mount Saint Helens since May 1980 have been predicted successfully, days to weeks in advance. The biggest obstacle to improving eruption prediction is that only a tiny fraction of over 500 active volcanoes in the world are adequately monitored by modern instruments and well-trained volcanologists.
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NOTES:
volcanic vent ;
explosive eruption .
