volcanic glass

n.
Natural glass produced by the cooling of molten lava too quickly to permit crystallization.

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A natural glass formed by rapid cooling of magma. Magmas typically comprise crystals and bubbles of gas within a silicate liquid. On slow cooling, the liquid portion of the magma usually crystallizes, but if cooling is sufficiently rapid, it may convert to glass—an amorphous, metastable solid that lacks the long-range microscopic order characteristic of crystalline solids. See also Lava; Magma.

Silica-rich, rhyolitic magmas frequently quench to glass during explosive eruptions and make up the bulk of the solid material in many pyroclastic deposits (usually as shards, pumice lumps, and other fragments); but they also can erupt quiescently to form massive glassy rocks (known as obsidian, the most common source of volcanic glass on land) even in the slowly cooled interiors of flows tens of meters thick. In contrast, more basic, basaltic glasses (sometimes known as tachylite) are less common and rarely form in more than small quantities unless rapidly cooled in a volcanic eruption. Pele's hair is an example of basaltic glass formed in this way. See also Basalt; Obsidian; Rhyolite.

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The noun has one meaning:

Meaning #1: a kind of natural glass produced when molten lava cools very rapidly

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