volcanic glass


volcanic glass

noun

Definition of VOLCANIC GLASS

:  natural glass produced by the cooling of molten lava too rapidly to permit crystallization

First Known Use of VOLCANIC GLASS

circa 1840

volcanic glass

noun    (Concise Encyclopedia)

Any glassy rock formed from lava or magma that has a chemical composition close to that of granite. Such molten material may reach very low temperatures without crystallizing, but its viscosity may become very high. Because high viscosity inhibits crystallization, the combination of sudden cooling and loss of volatiles, as when lava extrudes from a volcanic vent, tends to chill the material to a glass rather than crystallize it.

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