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Cinder cone
A cinder cone or scoria cone is a steep conical hill of loose pyroclastic fragments, such as either volcanic clinkers, cinders, volcanic ash, or scoria that has been built around a volcanic vent. They consist of loose pyroclastic debris formed by explosive eruptions or lava fountains from a single, typically cylindrical, vent. As the gas-charged lava is blown violently into the air, it breaks into small fragments that solidify and fall as either cinders, clinkers, or scoria around the vent to form a cone that often is beautifully symmetrical; with slopes between 30-40°; and a nearly circular ground plan. Most cinder cones have a bowl-shaped crater at the summit.
== Mechanics of eruption ==
The rock fragments, often called cinders or scoria, are glassy and contain numerous gas bubbles "frozen" into place as magma exploded into the air and then cooled quickly.〔 Cinder cones range in size from tens to hundreds of meters tall.〔 Cinder cones are made of pyroclastic material. Many cinder cones have a bowl-shaped crater at the summit.
During the waning stage of a cinder-cone eruption, the magma has lost most of its gas content. This gas-depleted magma does not fountain but oozes quietly into the crater or beneath the base of the cone as lava. Lava rarely issues from the top (except as a fountain) because the loose, uncemented cinders are too weak to support the pressure exerted by molten rock as it rises toward the surface through the central vent.〔 Because it contains so few gas bubbles, the molten lava is denser than the bubble-rich cinders.〔 Thus, it often burrows out along the bottom of the cinder cone, lifting the less-dense cinders like a cork on water, and advances outward, creating a lava flow around the cone's base.〔 When the eruption ends, a symmetrical cone of cinders sits at the center of a surrounding pad of lava.〔 If the crater is fully breached, the remaining walls form an amphitheatre or horseshoe shape around the vent.
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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