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Economic Resources
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Volcanic activity has yielded economic resources such as lightweight and heavy aggregates, pumice for abrasives and polishing compounds, and elements such as sulfur, zinc, copper, and lead in the massive sulfide deposits associated with undersea volcanic activity. Recently, volcanic areas have become important sources of geothermal energy, as in New Zealand, Iceland, and California. Research into applications for volcanic products is being pursued by various agencies, including NASA.
James A. Whitney
Eruption Prediction
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Research continues to find methods to predict volcanic eruptions accurately. Indications that an eruption may be imminent include swarms of small earthquakes as the magma rises up through the volcano, increases in sulfur dioxide emissions, and physical swelling of mountain slopes. While prediction has not been perfected, these factors were able to predict the 1991 eruptions of Mount Pinatubo in the Philippines and Mount Unzen in Japan.
Volcanic Origins
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Most of the world\'s volcanoes are found along the margins of huge plates into which the Earth\'s crust is divided (see PLATE TECTONICS). The largest volume of volcanic material is formed at SEAFLOOR SPREADING centers between plates, such as the Mid-Atlantic Ridge. These mid-ocean ridges are usually basaltic in composition and erupt quietly onto the ocean floor. In contrast, the volcanoes formed over subduction zones, in regions of plate convergence, may be among the most violent.
Along continental margins, chains of volcanoes such as the Andes have formed. Collisions of oceanic crust produce island arcs such as the Antilles, the Aleutians, and the Japanese Islands. In addition to these plate margin areas, volcanoes also form over \"hot spots\" within a plate, as in the cases of the Hawaiian Islands and the Yellowstone volcanic field.
Volcanic lava derives from deep-lying molten material (see MAGMA) that tends to rise and infiltrate the Earth\'s crust. The ultimate source of the magma-producing heat is commonly thought to be the decay of radioactive isotopes within the Earth, perhaps combined with residual heat from the time when the planet was formed (see EARTH, HEAT FLOW IN).
Types of Eruptions
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A volcano may exhibit different styles of eruption at different times, and eruptions may change from one type to another as the eruption progresses. The least violent type of eruption is termed Hawaiian and is characterized by extensive fluid lava flows from central vents or fissures and occasionally accompanied by lava fountains. Strombolian eruptions are characterized by moderately fluid lava flows, usually accompanied by a violent lava-fountaining that produces an abundance of volcanic bombs and cinders. Vulcanian eruptions are characterized by viscous magmas that form short, thick flows around vents; very viscous or solid fragments of lava are violently ejected from these vents. Pelean eruptions are similar to Vulcanian eruptions but have even more viscous lava; domes form over the vents, and ash flows commonly accompany the dome formation.The most violent eruptions, such as that of Washington\'s Mount St. Helens in 1980, are termed Plinian after Pliny the Elder, who died in the Vesuvius eruption of AD 79 . They include the violent ejection of large volumes of volcanic ash, followed by collapse of the central part of the volcano (caldera collapse).
Overview
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A volcano is a vent in the Earth from which molten rock (magma) and gas erupt. The molten rock that erupts from the volcano (lava) forms a hill or mountain around the vent. The lava may flow out as a viscous liquid, or it may explode from the vent as solid or liquid particles.
Types of Volcanoes
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The most fluid magmas are erupted quietly and flow from the vent to form gently sloping shield volcanoes, a name derived from their resemblance to the shields of early Germanic warriors. The lava flows from shield volcanoes are usually only 1 to 10 m (3.3 to 33 ft) thick, but they may extend for great distances away from the vent. The volcanoes of Hawaii and Iceland are typical shield volcanoes.
Magmas with high gas contents and high viscosities are usually more explosive than those flowing from shield volcanoes. These gas-rich magmas in many instances are blown high into the air during an eruption. The magma falls as volcanic bombs, which accumulate around the vent and form steep-sided but relatively small cinder cones. Volcanic bombs range in size from fine-grained ash to house-size blocks. Cinder cones most commonly consist of volcanic fragments anywhere from ash to small-pebble size, less than 3 cm (1.2 in) in diameter.
Most of the tallest volcanoes are composite volcanoes (stratovolcanoes). These form from a cycle of quiet eruptions of fluid lava followed by explosive eruptions of viscous lava. The fluid lava creates an erosion-resistant shell over the explosive debris, forming strong, steep-sided volcanic cones.
In the past, great eruptions of extremely fluid basaltic lava from extensive systems of fissures in the Earth have occurred. These series of eruptions formed extensive plateaus of basaltic lava. In India the Deccan basalts cover 260,000 sq km (100,000 sq mi), and in Oregon and Washington the Columbia Plateau basalts cover approximately 130,000 sq km (50,000 sq mi). No eruptions of this extent have been observed during historic times. Even more voluminous accumulations of basaltic lava, however, are currently being formed at the mid-ocean ridges.
active : erupt regularly
dormant :sleeping
extinct :not active
ERUPTION
erurtion character --> viscousity ,gas, ....
different eruptions : shield volcanoes(hot spots): quiet eruption - fluid lava(1 - 10 m thick) - big distances(Hawaii-20 volcanoes)
volcanoes with gas rich magma(big viscousity):explosive, lava high into air, volcanic bombs( fine ash-->housesize blocks)
Strombolian: moderately lava fluid, violent fountaining, a lot of volcanic bombs
most violent eruptions termed Pilan(Mount St. Helens, erupted in Mai 1980, the top of the2950m high mountain was forced awai), after pilny the elder -->died in pompeii
violent ejection of ash, collaps of central part of volcano--> caldera collaps
magma-->mixture of crystals, rock fragments, liquid,gases -->deep in earth
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Ätna: most active volcano in sicilia,3323 m high,90 eruption since BC 1800
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