Volcanic glass is an amorphous or uncrystallized extrusive igneous rock. It forms from the rapid cooling or quenching of magma that doesn’t allow crystallization. Such a rock is said to have a glassy or vitreous texture.

In this discussion, we will give you the definition or meaning of volcanic glasses, how they form, their composition, and their alternation. We will also look at the types of volcanic and mention a thing on uses.

A volcanic glass refers to any amorphous igneous rock formed when lava or magma is quenched or rapidly cooled. The rapid cooling and sometimes high viscosity magma inhibit the nucleation and growth of crystals. So, you end up with an amorphous or uncrystallized solid with randomly arranged atoms, molecules, or ions.

The state of matter of volcanic glass is intermediate, i.e., between solids with highly ordered, closely packed assemblage and highly disordered arrays seen in liquids. Besides the geometric shapes, solids may have crystal faces if crystal growth is not hindered, unlike glass.

In terms of appearance, most volcanic glasses are dark or black, with some being tan, green, blue, red, orange, brown, yellow, etc., depending on their inclusions. Also, glasses may be homogenous (pure glass), banded (laminated), or damascened (contorted streaks). Some may be spherulitic (with small spherical bodies) or axiolitic (with elongated spherulite).

Note that volcanic glass is a rock and not a mineral. Minerals are naturally occurring inorganic solids with unique physical and chemical properties, crystal forms, and ordered internal structures, while glass is an amorphous solid without organized internal structure and crystals.

Lastly, volcanic glass may also mean some igneous rocks’ matrix or interstitial glassy material. A good example is porphyritic textured rocks with volcanic glass groundmass.

Volcanic glass forms when magma is quenched or rapidly cooled. Rapid cooling doesn’t allow atoms, molecules, or ions in magma to reorganize and form defined crystalline structures characteristic of certain minerals. Instead, the molten magma will transform into an amorphous solid, i.e., glass with disordered atoms, ions, or molecules.  

Besides rapid cooling, high viscosity may inhibit crystallization. How? Because it will not allow atoms, molecules, or ions to diffuse to crystal nucleation and crystal growth sites.

The process of volcanic glass formation is exactly like how artificial glass forms. When rapidly cooling magma temperature goes below temperatures at which crystallization occurs, such magma is said to be a supercooled liquid. Any further cooling will transform it into an amorphous solid, i.e., glass.

The temperature at which supercooled magma transitions to glass is called glass transition temperature. This temperature depends on how much water is in the magma and the cooling rate.

Lastly, during the formation, depending on the magma involved, you may have glassy lava flows or volcanic glass shards where fragments of molten magma solidify without cooling during an eruption. Also, volcanic glass ash can occur.

Most naturally occurring volcanic glass forms from felsic magma, especially rhyolitic or granitic magma. However, it is possible to get volcanic glasses with mafic composition, but they are uncommon. An example is interstitial basaltic glasses, which are common in places with basaltic lava flows.

In a strict sense, since volcanic glass doesn’t form mineral crystals, we infer its composition by looking at the pristine magma.

However, a thin volcanic glass section may reveal a small percentage of crystallite, incipient crystals, or microlites (minute crystals). Some may also have empty or glass-filled cavities or even larger grains visible to the naked eye.

Typical compositions are:

Most terrestrial volcanic glasses are highly silicic in composition, i.e., they form from silica-rich rhyolitic or granitic magmas. These thicker magmas can reach low temperatures without crystallizing, making them easily form glass.

For instance, a drop in pressure during an explosive eruption of silicic magma will allow water vapor to escape increasing viscosity. In some cases, very viscous rhyolitic magma may produce glass bodies tens of meters thick. Such tend to be high in silica (71-77%). So, you can have tephra and magma flows made entirely of glass.

Basaltic magmas are low in silica (40-55 wt.%). These magmas rarely form glass because their structure allows the fast growth of crystals. So, you have glasses only a few inches thick, even in a rapidly cooled ocean or deep-sea extrusions. Also, most will have some crystalline material.

Like all other natural glass, volcanic glasses are unstable thermodynamically. Why? Because of the poorly ordered internal structure with poorly linked silicon-oxygen tetrahedra whose intermolecular spaces are considerably large.

So, volcanic glasses will readily decompose, i.e., undergo alternation (change in mineralogic composition) or devitrification, forming secondary or authigenic crystalline minerals.

For instance, water will readily react with this glass, removing soluble cations into interstitial pores containing water, i.e., dissolution. Then, there will be precipitation, forming a secondary microcrystalline mineral and filling the pores created by the dissolution of some glass particles.

Glass alteration is the most rapid lithification process that occurs at low temperatures. It is why you cannot find very geologically ancient volcanic glasses. Most are less than 65.5 million years old, i.e., of Paleogene age.

Some volcanic glass types include obsidian, pumice, perlite, sideromelane, pitchstone, and tachylite. Others are Pele’s hair, Pele’s tears, Limu O Pele, hyaloclastite, palagonite, and apache tears.

Here is a brief description of each of these naturally occurring volcanic glasses:

Obsidian rock is a hard (Mohs hardness of 5.5), smooth, shiny, black to dark brown natural volcanic glass formed by rapidly cooling silica-rich (felsic) viscous lava, especially rhyolitic or granitic from volcanoes.

However, depending on the impurities present and trapped fluid bubbles, you may find a yellow, brown, gray, red, green, or dark green obsidian rock. For instance, the presence of crystals of some rare minerals may result in a rainbow obsidian that is stripped and colorful like a rainbow. It happens because of oriented nanorods of hedenbergite impurities.  

Lastly, contrary to what many assume, obsidian is not another name for volcanic glass but an example. Also, it is a mineraloid (amorphous rock that resembles minerals) and not a mineral. For instance, it has a conchoidal fracture like quartz and has a vitreous luster. But still, it is a rock.

Pumice (pumicite) is a lightweight, light-colored, froth-like, or porous glassy rock. The bubbly, vesicular, or sponge-like appearance forms when steam rapidly escapes from fast-cooling lava or molten glass during an explosive volcanic eruption.

Most of the pumice rocks form from rhyolitic or andesitic magmas. However, although rarer, finding a basaltic pumice rock is possible.

Lastly, pumice is mostly light-colored volcanic glass, i.e., – whitish, cream, gray, or bluish – but may be greenish-brown to black.

Pitchstone is another dull black (sometimes red, green, gray, or black) volcanic rock with a glassy texture. It forms from the rapid cooling of felsic lavas or magma and may appear streaked, mottled, or uniform.

This rock has a resinous luster, unlike obsidian, with a vitreous luster. Also, it has an irregular, jagged fracture and doesn’t have a conchoidal fracture seen in obsidian.

Tachylite is black to brown glassy basaltic volcanic rock formed from the rapid natural cooling of basaltic lava or magma. This opaque mafic rock is brittle, fusible, and has a resinous, pitchlike, or greasy-looking luster.

Tachylites are common on the edges of quenched or chilled dikes, veins, sills, and extrusive masses. Did you know that acid decomposes this rock?

Sideromelane is a less-common kind of tachylite that forms from faster quenching of lava and in a wet place (submarine and subglacial volcanoes). Also, its presence indicates higher magma temperatures than tachylites.

This glassy rock occurs as palagonite tuff and alongside tachylites. However, unlike tachylite, sideromelane isn’t opaque. Instead, it is yellow-brown, transparent, and pure since it doesn’t have the dispersed iron oxide that tachylites have.

Perlite is a silica-rich natural volcanic glassy rock with a relatively high quantity of water, i.e., 2-5%. It forms mainly by hydration of obsidian rock – rapid cooling of high viscosity rhyolitic lava followed by hydration – and has a spherical or concentric fracture habit that forms small rounded rocks.

Perlite rocks have a pearly to waxy luster, mainly gray or greenish but may be brownish, blue, or reddish. However, on heating at appropriate temperatures, it will expand 7-16 times its original size and will be grayish white to snowy white.

Pele’s hair (Pele is the Hawaiian goddess of volcanoes) refers to thin volcanic glass strands formed by blowing out or stretching molten lava. It occurs on lava fountains, cascades, or rapidly flowing lava, but wind can blow or carry these glassy rocks several kilometers.

Warning: Don’t touch Pele’s hair with bare hands since they are brittle and sharp. They can easily break into small pieces that will prick into your skin. Instead, use gloves.

Pele’s tears refer to teardrop-shaped volcanic glass formed when airborne molten magma fuses. These jet-black rocks resemble obsidian; you may find them attached to Pele’s hair.

Limu o Pele or Pele’s seaweed is a geological term for thin sheets and subsequently shattered flakes of brownish-green to near-colorless volcanic glass lava spatter (“Limu o Pele,” 2022). Their name is Hawaiian and refers to the seaweed-like appearance.

These rocks form when water is entrapped in flowing lava or molten rock. This entrapped water will instantly vaporize into steam, expanding to create a large bubble. As pressure builds up, the large volcanic glass bubble will burst and blow thin fragments or flakes that look like a shattered light bulb to coastal benches.

Hyaloclastites refer to breccia or fragmented volcanic rocks (volcaniclastic) accumulation comprising glass shards, fragments, or clasts. These rocks form from quenching fragmented lava in volcanic eruptions under ice or water. However, they can also form from subaerial eruptions reaching water bodies.

Hyaloclastites can occur as thin margins between pillow lavas and lava

flows or in thicker deposits. These rocks are also associated with explosive eruptions rich in volatile and steeper topography.

Palagonite forms when water alters glassy volcanic rocks with basaltic chemical composition or if water interacts with basalt lava or magma. In this case of water quenching basalt melt, water will vaporize to steam when it encounters hot basalt melt. The steam will then react with the magma melt to form palagonite tuff cones that are light in color.

However, palagonite may also refer to the yellow-brown powder formed when basaltic glass undergoes weathering.

Apache tears or marekanite refer to small (less than 2 inches or 5.1cm) rounded, surrounded, or subangular black to dark-colored obsidian volcanic rock. These natural volcanic glasses have rhyolitic composition and show a conchoidal fracture. Also, they may be internally banded or have microlites.

Apache tears form in siliceous lava flows, domes, or ash-flow tuffs and are associated with gray perlite. These obsidianites or spherules form cores of perlite mass with onion-skin concentric fractures.

The uses of volcanic glass depend on the exact rock type. For instance, obsidian historically made scalpels, arrowheads, blades, daggers, etc. Today, it is a gemstone (cut into cabochons and beads) and is used in jewelry.

On the other hand, once heat treated, perlite helps improve drainage and aeration in potting media (soil and soilless).

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