volcanic glass

volcanic glass

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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Volcanic glass can refer to any of several types of vitreous igneous rocks. Most commonly, it refers to:

Other types of volcanic glass include:

  • Pumice, which is considered a glass because it has no crystal structure.
  • Apache tears, a kind of nodular obsidian.
  • Tachylite (also spelled tachylyte), a basaltic glass with relatively low silica content.
  • Sideromelane, a less common form tachylyte.
  • Palagonite, a basaltic glass with relatively low silica content.
  • Hyaloclastite, a hydrated tuff-like breccia of sideromelane and palagonite.
  • Pele's hair, threads or fibers of volcanic glass, usually basaltic.
  • Pele's tears, tear-like drops of volcanic glass, usually basaltic.
  • Limu o Pele (Pele's seaweed), thin sheets and flakes of brownish-green to near-clear volcanic glass, usually basaltic.

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