Glass: The Geologic Connection

Abstract

Silicate glasses can be formed by several geological processes all of which involve melting of rock and rapid cooling to prevent crystallization. The most common and widespread natural glasses are volcanic glasses produced when magma is extruded onto the Earth's surface. The type of volcanic glass is determined by the composition of the magma and type of eruption, which in turn are related to the geological setting. During high velocity impacts of large extraterrestrial bodies, melted rock can be ejected into the atmosphere — and sometimes into space — where it cools to form large glass bodies centimeters in size or larger, called tektites, and smaller glass bodies, called impact spherules. Tektites and spherules can be thrown over large areas of the Earth's surface. Impact spherules can also form by condensation from an impact plume of vaporized surface rocks and impactor. Lightning strikes generate high enough temperatures to melt and vaporize rock. When they hit sand, they can produce tube‐like structures called fulgurites. Burning of organic material, like oil or coal, underground can melt rock and produce combustion‐metamorphic glasses. Glasses called frictionites can be produced by frictional heating at the base of massive landslides. Friction glasses can also be generated during faulting.

Determining the Volcanic Eruption Style of Tephra Deposits From Infrared Spectroscopy

“…Hydration may be one good indicator of volcanic glass. On Earth, volcanic glasses tend to have higher water contents than glasses formed by impacts due to differences in temperature of formation, and the difference in water content can constrain formation processes (Glass, 2016; Zhang, 1999). However, some impact melts formed underwater may also be hydrated (Osinski et al., 2019).…”

“…Glass forms by numerous natural processes, including impacts, tectonics, and volcanic eruptions (Glass, 2016). Glasses form via quenching of molten material, where rapid cooling prevents the crystallization of minerals.…”

Magnetic Properties and Redox State of Impact Glasses: A Review and New Case Studies from Siberia

“…One may question the possible use of magnetic measurements to distinguish impact glasses from other glasses produced by volcanism, lightning, pyrometamorphism (coal, hydrocarbon, and vegetation fire; see [1]) and human activity. The presence of metal for example cannot be taken as a proof for interaction with extraterrestrial matter (as described in [48,49,56,57]).…”

“…Various types of silicate glass occur in nature [1] and may show exotic iron-bearing inclusions representative of a wide range of redox state [2][3][4]. Impact generated glasses, having often recorded particularly high pressures and temperatures (>10 GPa and >1700 • C; see [5]) on a short time scale (seconds) and endured equally fast quenching, may yield original magnetic properties that we will describe in the present review.…”

“…This differentiates them from other impact glasses that are found near or inside their source crater. Only five tektite (and three microtektite) strewn fields have been identified on Earth (Figure 1), the largest field by far being the Australasian one [1,8]. The latter is the only confirmed strewn field for which a source crater has not been identified.…”