![]() Continued ebullition of gas results in a mass of small solid or quasisolid fragments, each surrounded by an envelope of still-expanding gas that pushes against all adjacent expanding envelopes. The sudden expansion of the gas may tear the froth into countless tiny shreds, each of which chills virtually instantaneously to form fragments of volcanic glass. Thus, the emergence at the surface of highly gas-charged, viscous silicic lava may be attended by a sudden frothing as the contained gas rapidly exsolves and vesiculates. The gas generally escapes readily from fluid lavas, with little or no explosion in more viscous liquids, however, the gas may acquire considerable pressure before it escapes and bursts forth in strong explosions. Magma, at depth and under great pressure, contains gas in solution, but, as the magma rises into regions of lower pressure near the surface of Earth, the gas starts to exsolve and escape from the liquid. Basaltic pillow lavas form an overwhelming bulk of the ocean floor. Where pahoehoe flows enter bodies of water or wet ground, they may form heaps of irregular ellipsoids, in cross section somewhat resembling sacks of grain or pillows. Over geologic time, vesicles can be filled by minerals precipitated from circulating mineralizing fluids or ground water. All lava flows that have poured out on land contain various amounts of open cavities (vesicles), which mark the sites of gas bubbles formed as the rising magma reached regions of increasingly lower pressure before breaching the surface to erupt. In contrast, more viscous lavas such as andesites more commonly form block lava flows. Fluid basaltic and related mafic lavas characteristically form pahoehoe or aa flows, or flows intermediate in character between these two end members. Flows in which the fragments that constitute the upper part of the flow are fairly smooth-sided polygons called block lava. The terms "pahoehoe" and "aa" (pronounced "ah-ah") are of Hawaiian origin. Flows showing very rough, irregular surfaces covered by jagged spinose fragments resembling furnace clinker are called aa ( Fig. Flows showing smooth or hummocky, gently undulating surfaces and crusts locally wrinkled into ropelike forms are called pahoehoe. Lava flows are classified in terms of their surface characteristics. ![]() Eruptions of mafic lavas commonly are nonexplosive or only weakly explosive and produce mostly lava flows eruptions of silicic lavas are typically explosive and yield predominantly pyroclastic material. In general, mafic (basaltic and similar) lavas are less viscous (more fluid) than more silicic (dacitic and similar) lavas. The character of a volcanic eruption depends largely on the viscosity of the liquid lava viscosity, for the purposes of this discussion, can be considered as a stickiness coefficient of the magma or lava. See also: Crystallization Lava Magma Volcanic glass The general term "tephra" is applied to poorly consolidated pyroclastic debris regardless of fragment or particle size. During more violent eruption, lava torn into fragments and hurled into the air is called pyroclastic (fire-broken materials). Lava may accrete near the vent to form various minor structures or may pour out in streams called lava flows, which may travel many tens of kilometers from the vents. ![]() If lava chills and solidifies quickly, it forms volcanic glass slower rates of chilling result in greater crystallization before complete solidification. Magma erupted onto Earth's surface is called lava. More broadly, volcanoes are breaks in the crusts of rocky objects, including planets, moons, and comets, though which materials such as lava, gases, and ash erupt from underground reservoirs ( Fig. Most of Earth's active volcanoes are found along tectonic plate boundaries as the product of plate recycling and creation processes that occur at these locations.Ī mountain or hill, generally steep-sided, formed by accumulation of magma (molten rock with associated gas and crystals) erupted through openings or volcanic vents in Earth's crust the term "volcano" also refers to the vent itself. ![]() Much of Earth's surface, on land and below the sea, has been shaped by volcanic activity in fact, more than 80% of Earth's crust is of volcanic origin. Lava viscosity, or stickiness coefficient, determines the type of eruption (explosive or non-explosive), the rate at which lava flows over the surface, and the physical (topographic) appearance of the volcano. In addition to lava flows, other products from volcanoes include pyroclastic materials (such as ash and debris), volcanic gases, aerosols, and mudflows. Two general types of volcanic vents are fissure vents and central (pipelike) vents. A volcano is a mountain or hill formed by the accumulation of magma from Earth's interior that has erupted through vents in Earth's crust.
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