Volcanic glass is theamorphous (uncrystallized) product of rapidly coolingmagma. Like all types ofglass, it is astate of matter intermediate between the closely packed, highly ordered array of acrystal and the highly disordered array ofliquid.[1] Volcanic glass may refer to the interstitial material, ormatrix, in anaphanitic (fine-grained)volcanic rock, or to any of several types of vitreousigneous rocks.
Volcanic glass is formed whenmagma is rapidly cooled. Magma rapidly cooled to below its normal crystallization temperature becomes asupercooled 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 producetephra composed entirely of volcanic glass and may also form glassylava flows.[2]Ash-flowtuffs 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,tachylite andsideromelane. Tachylite is opaque to transmitted light because of the abundance of tinyoxide 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 asphreatomagmatic eruptions.[5] Basaltic volcanic glass is also present inpillow 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 aneruption column. The least effective mechanism is cooling at the bottom of a flow in contact with the ground.[4]
Most commonly,volcanic glass refers toobsidian, arhyolitic glass with highsilica (SiO2) content.[7]
Other types of volcanic glass include the following:
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 (authigenic) minerals. As a result,lithification of volcanic ash is one of the fastest low-temperature lithification processes. Alteration of volcanic glass atmid-ocean ridges may have contributed significantly to the formation ofmassive sulfide deposits, and alteration of volcanic ash beds formed economically importantzeolite andbentonite deposits.[9]
Authority control databases: National |
|---|