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Hawaiian eruption
A Hawaiian eruption is a type of volcanic eruption where lava flows from the vent in a relatively gentle, low level eruption; it is so named because it is characteristic of Hawaiian volcanoes. Typically they are effusive eruptions, with basaltic magmas of low viscosity, low content of gases, and high temperature at the vent. Very little amounts of volcanic ash are produced. This type of eruption occurs most often at hotspot volcanoes such as Kīlauea and Iceland, though it can occur near subduction zones (e.g. Medicine Lake Volcano in California, United States) and rift zones. Another example of Hawaiian eruptions occurred on Surtsey from 1964 to 1967, when molten lava flowed from the crater to the sea.
Hawaiian eruptions may occur along fissure vents, such as during the eruption of Mauna Loa Volcano in 1950, or at a central vent, such as during the 1959 eruption in Kīlauea Iki Crater, which created a lava fountain 580 meters (1,900 ft) high and formed a 38-meter cone named Puu Puai. In fissure-type eruptions, lava spurts from a fissure on the volcano's rift zone and feeds lava streams that flow downslope. In central-vent eruptions, a fountain of lava can spurt to a height of 300 meters or more (heights of 1600 meters were reported for the 1986 eruption of Mount Mihara on Izu Ōshima, Japan).
Hawaiian eruptions usually start by the formation of a crack in the ground from which a curtain of incandescent magma or several closely spaced magma fountains appear. The lava can overflow the fissure and form aā or pāhoehoe style of flows. When such an eruption from a central cone is protracted, it can form lightly sloped shield volcanoes, for example Mauna Loa or Skjaldbreiður in Iceland.
== Petrology of Hawaiian Basalts ==
The key factors in generating a Hawaiian eruption are basaltic magma and a low percentage of dissolved water (less than one percent). The lower the water content, the more peaceful is the resulting flow. Almost all lava that comes from Hawaiian volcanoes is basalt in composition. Hawaiian basalts that make up almost all of the islands are tholeiite. These rocks are similar but not identical to those that are produced at ocean ridges. Basalt relatively richer in sodium and potassium (more alkaline) has erupted at the undersea volcano of Lōihi at the extreme southeastern end of the volcanic chain, and these rocks may be typical of early stages in the "evolution" of all Hawaiian islands. In the late stages of eruption of individual volcanoes, more alkaline basalt also was erupted, and in the very late stages after a period of erosion, rocks of unusual composition such as nephelinite were produced in very small amounts. These variations in magma composition have been investigated in great detail, in part to try to understand how mantle plumes may work.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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