...rough experimental studies ith artificial melts geologists seek to determine the physical conditions and appearance, chemical compositions, paths of evolution, loci of origin, and causes of formation of magmas.VolcanEconomic Resources------------------Volcanic activity has yielded economic resources such as lighteight and heavy aggregates, pumice for abrasives and polishing compounds, and elements such as sulfur, zinc, copper, and lead in the massive sulfide deposits associated ith undersea volcanic activity. Recently, volcanic areas have become important sources of geothermal energy, as in Ne Zealand, Iceland, and California. Research into applications for volcanic products is being pursued by various agencies, including NASA.James A. hitneyEruption Prediction-------------------Research continues to find methods to predict volcanic eruptions accurately. Indications that an eruption may be imminent include sarms of small earthquakes as the magma rises up through the volcano, increases in sulfur dioxide emissions, and physical selling of mountain slopes. hile prediction has not been perfected, these factors ere able to predict the 1991 eruptions of Mount Pinatubo in the Philippines and Mount Unzen in Japan.Volcanic Origins----------------Most of the orlds volcanoes are found along the margins of huge plates into hich the Earths crust is divided see PLATE TECTONICS. The largest volume of volcanic material is formed at SEAFLOOR SPREADING centers beteen 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 ithin a plate, as in the cases of the Haaiian Islands and the Yellostone volcanic field.Volcanic lava derives from deep-lying molten material see MAGMA that tends to rise and infiltrate the Earths crust. The ultimate source of the magma-producing heat is commonly thought to be the decay of radioactive isotopes ithin the Earth, perhaps combined ith residual heat from the time hen the planet as formed see EARTH, HEAT FLO IN.Types of Eruptions------------------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 Haaiian and is characterized by extensive fluid lava flos from central vents or fissures and occasionally accompanied by lava fountains. Strombolian eruptions are characterized by moderately fluid lava flos, 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 flos 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 flos commonly accompany the dome formation.The most violent eruptions, such as that of ashingtons Mount St. Helens in 1980, are termed Plinian after Pliny the Elder, ho died in the Vesuvius eruption of AD 79 . They include the violent ejection of large volumes of volcanic ash, folloed by collapse of the central part of the volcano caldera collapse.Overvie--------A volcano is a vent in the Earth from hich 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 flo out as a viscous liquid, or it may explode from the vent as solid or liquid particles.Types of Volcanoes------------------The most fluid magmas are erupted quietly and flo from the vent to form gently sloping shield volcanoes, a name derived from their resemblance to the shields of early Germanic arriors. The lava flos from shield volcanoes are usually only 1 to 10 m 3.3 to 33 ft thick, but they may extend for great distances aay from the vent. The volcanoes of Haaii and Iceland are typical shield volcanoes.Magmas ith high gas contents and high viscosities are usually more explosive than those floing from shield volcanoes. These gas-rich magmas in many instances are blon high into the air during an eruption. The magma falls as volcanic bombs, hich 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 anyhere 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 folloed 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 ashington 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, hoever, are currently being formed at the mid-ocean ridges.-- Das ar mein Stichortzettel zu diesem Thema --active erupt regularlydormant sleepingextinct not activeERUPTIONerurtion character -- viscousity ,gas, ....different eruptions shield volcanoeshot spots quiet eruption - fluid lava1 - 10 m thick - big distancesHaaii-20 volcanoesvolcanoes ith gas rich magmabig viscousityexplosive, lava high into air, volcanic bombs fine ash--housesize blocksStrombolian moderately lava fluid, violent fountaining, a lot of volcanic bombsmost violent eruptions termed PilanMount St. Helens, erupted in Mai 1980, the top of the2950m high mountain as forced aai, after pilny the elder --died in pompeiiviolent ejection of ash, collaps of central part of volcano-- caldera collapsmagma--mixture of crystals, rock fragments, liquid,gases --deep in earth---------------------------------------------
--------------------------------------tna most active volcano in sicilia,3323 m high,90 eruption since BC 1800--Der Referatstext fr ein oberflchliches Referat aus Englisch. --VolcanoesA volcano is a hole in the Earth from ich molten rock magma and gas erupt. The molten rock that erupts from the volcano lava forms a hill or mountain around the hole. The lava may flo out as a viscous liquid, or it may explode from the hole as solid or liquid particles.Volcanoes can be o...
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