Volcanic glass

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A sand grain of volcanic glass under the petrographic microscope. Its amorphous nature makes it disappear in cross-polarized light (bottom frame). The scale box is in millimeters.

Volcanic glass is the

aphanitic (fine-grained) volcanic rock, or to any of several types of vitreous igneous rocks
.

Origin

Volcanic glass is formed when

lava flows.[2] Ash-flow tuffs typically consist of countless microscopic shards of volcanic glass.[3] Basalt, which is low in silica, forms glass only with difficulty, so that basalt tephra almost always contains at least some crystalline material (quench crystals).[2] The glass transition temperature of basalt is about 700 °C (1,292 °F).[4]

The mechanisms controlling formation of volcanic glass are further illustrated by the two forms of basaltic glass,

phreatomagmatic eruptions.[5] Basaltic volcanic glass is also present in pillow lavas.[6]

Of the cooling mechanisms responsible for forming volcanic glass, the most effective is quenching by water, followed by cooling by entrained air in an eruption column. The least effective mechanism is cooling at the bottom of a flow in contact with the ground.[4]

Types

Most commonly, volcanic glass refers to

silica (SiO2) content.[7]

Other types of volcanic glass include the following:

  • 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 of tachylyte.
  • Palagonite, an alteration product of basaltic glass.[8]
  • 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.

Alteration

Volcanic glass is chemically unstable and readily decomposes. Water molecules readily react with the open, disordered structure of volcanic glass, removing soluble cations from the glass and precipitating secondary (

mid-ocean ridges may have contributed significantly to the formation of massive sulfide deposits, and alteration of volcanic ash beds formed economically important zeolite and bentonite deposits.[9]

References