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 closely packed, highly ordered array of a crystal and the highly disordered array of liquid. Volcanic glass may refer to the interstitial material, or matrix, in an aphanitic (fine-grained) volcanic rock, or to any of several types of vitreous igneous rocks. Volcanic glass is formed when magma is rapidly cooled. Magma rapidly cooled to below its normal crystallization temperature becomes a supercooled liquid, and, with further rapid cooling, this becomes an amorphous solid. The change from supercooled liquid to glass occurs at a temperature called the glass transition temperature, which depends on both cooling rate and the amount of water dissolved in the magma. Magma rich in silica and poor in dissolved water is most easily cooled rapidly enough to form volcanic glass. As a result, rhyolite magmas, which are high in silica, can produce tephra composed entirely of volcanic glass and may also form glassy lava flows. Ash-flow tuffs typically consist of countless microscopic shards of volcanic glass. 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). The glass transition temperature of basalt is about . The mechanisms controlling formation of volcanic glass are further illustrated by the two forms of basaltic glass, tachylite and sideromelane. Tachylite is opaque to transmitted light because of the abundance of tiny oxide mineral crystals suspended in the glass. Sideromelane is partially transparent because it contains much fewer crystals. Sideromelane is abundant only in eruptions where basalt magma has been very rapidly cooled by contact with water, such as phreatomagmatic eruptions. Basaltic volcanic glass is also present in pillow lavas. 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.

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Related concepts (20)

Igneous rock

Igneous rock (igneous ), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rocks are formed through the cooling and solidification of magma or lava. The magma can be derived from partial melts of existing rocks in either a planet's mantle or crust. Typically, the melting is caused by one or more of three processes: an increase in temperature, a decrease in pressure, or a change in composition. Solidification into rock occurs either below the surface as intrusive rocks or on the surface as extrusive rocks.

Volcanic glass

Volcanic rock

Volcanic rock (often shortened to volcanics in scientific contexts) is a rock formed from lava erupted from a volcano. Like all rock types, the concept of volcanic rock is artificial, and in nature volcanic rocks grade into hypabyssal and metamorphic rocks and constitute an important element of some sediments and sedimentary rocks. For these reasons, in geology, volcanics and shallow hypabyssal rocks are not always treated as distinct. In the context of Precambrian shield geology, the term "volcanic" is often applied to what are strictly metavolcanic rocks.