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What is volcanic glass called?

Volcanic Glass

Volcanic glass is the amorphous (uncrystallized) product of rapidly cooling magma. Like all types of glass, it is a state of matter intermediate between the close-packed, highly ordered array of a crystal and the highly disordered array of gas. Volcanic glass can refer to the interstitial, or matrix, material in an aphanitic (fine grained) volcanic rock or can refer to any of several types of vitreous igneous rocks. Most commonly, it refers to obsidian, a rhyolitic glass with high silica content.

Volcanic Glass: Obsidian

Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock.

It is produced when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. Obsidian is commonly found within the margins of rhyolitic lava flows known as obsidian flows, where the chemical composition (high silica content) induces a high viscosity and polymerization degree of the lava. The inhibition of atomic diffusion through this highly viscous and polymerised lava explains the lack of crystal growth. Obsidian is hard and brittle; it, therefore, fractures with very sharp edges, which were used in the past in cutting and piercing tools, and it has been used experimentally as surgical scalpel blades.

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.

Volcanic Glass called→ Obsidian

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