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Lava is molten rock or magma which has flowed from the interior of the earth onto its surface, through the crater of a volcano or through fissures on its side. On the surface the molten material solidifies more quickly than in the interior of the earth. Some lavas contain much silica and are said to be acid, others have little silica and are termed basic lavas. The acid lavas are very viscous, flow slowly, and do not travel far. While the basic lavas are very fluid, move rapidly, and may flow for several miles before solidifying. Acid lavas thus lose their gases less readily, and cause volcanos to erupt explosively, the whole mass bursting into fragments or even dust; basic lavas produce comparatively quiet eruptions, with a widespread flow of lava. Most volcanoes are of an intermediate kind. The surface of solidified lava is usually very rough. In both acid and basic lavas, escaping gases cause the upper layers of the lava to be vesicular. Smaller and more numerous vesicles occur in acid lavas, while larger and less numerous vesicles occur in basic lavas.

Tongue of pahoehoe lava in dead forest
Tongue of pahoehoe lava in dead forest
Earth Science Slides by John S. Shelton
Tongue of pahoehoe lava in dead forest  Basalt in Grand Coulee  Lava Flow  500 foot lava fountain at Kilauea Iki, Hawaii, November 1959. Slopes of Mauna Loa in the background  Pahoehoe lava flows, Hawaii  Pahoehoe lava oozing out at edge of a lava lake in Kilauea Iki, Hawaii 

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