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What Is Volcanic Glass?

B. Leslie Baird
B. Leslie Baird

Volcanic glass is that which is naturally formed when high viscosity molten lava cools. The glass is created because this lava contains less water and more crystal material. Its chemical composition is similar to that of granite, which is composed of alkali feldspar and quartz. The materials in the magma, the water content, and speed of cooling all contribute to a variety of natural glasses.

Magma can crystallize while cooling. The higher the viscosity of the lava, the more likely it will be to turn into glass. Igneous rock is also produced from the cooling of lava, and often contains crystal particles. Much of Earth’s crust is formed from igneous rock material.

After a volcanic eruption, some high viscosity molten lava can cool into what is known as volcanic glass.
After a volcanic eruption, some high viscosity molten lava can cool into what is known as volcanic glass.

Rapid vaporization of rhyolitic or felsic lava produces the volcanic glass called obsidian. While obsidian can be formed with a variety of crystals, it has high silica content. This glass is usually black in color, but the presence of hematite can create brown, green, and red shades as well. Mount Helka in Iceland is one of the volcanoes known for producing obsidian. While it is a glass, it is also considered a natural gemstone.

Pumice igneous rocks contain volcanic ash, which has very small amounts of volcanic glass.
Pumice igneous rocks contain volcanic ash, which has very small amounts of volcanic glass.

Tachylyte is an igneous rock that is glassy in appearance. It is low in silica and formed when non-acidic lava cools very quickly. This glassy rock is commonly black with thin brown sections. Magnetite granules can be found in tachylyte as well. This type of volcanic glass can be found in the Hawaiian Islands and the Inner Hebrides of Scotland.

Pele’s tears are small pieces of lava that have cooled quickly. These pieces become teardrop-shaped and black in color. Often found in Hawaii, the glass was named for the Hawaiian goddess of volcanoes. Apache tears are another type of black volcanic glass. These tear-shaped pieces of obsidian can be translucent when held to light, and sometimes include shades of brown or red.

Volcanic glass can form when lava cools.
Volcanic glass can form when lava cools.

Pumicite is volcanic ash and is very abrasive. It contains very small particles of volcanic glass. The ash can be used for polishing and grinding. Perlite is a volcanic rock that can be used as an insulator in building construction. It can also be found in potting soils, where it is used to improve drainage and aeration.

Spontaneous changes, devitrification, occur in volcanic glass that can make it become stone-like in appearance. These changes frequently begin in areas where larger crystals or cracks are present in the glass. Over time, the glass will become crystals of feldspar, tridymite, and quartz. This process occurs over a long period of time — up to millions of years — but most volcanic glass found is less than 65 million years old.

Frequently Asked Questions

What is volcanic glass and how does it form?

Perlite is a volcanic rock, and can be used in planting mixtures to aid in aeration.
Perlite is a volcanic rock, and can be used in planting mixtures to aid in aeration.

Volcanic glass, commonly known as obsidian, forms when lava cools rapidly without crystal growth. This rapid cooling occurs when lava is ejected from a volcano and comes into contact with air or water. The quick solidification prevents the formation of a crystalline structure, resulting in a smooth, glassy texture.

Are there different types of volcanic glass?

Yes, there are several types of volcanic glass, including obsidian, pumice, and scoria. Obsidian is typically smooth and dense, while pumice is light and porous, often able to float on water. Scoria, similar to pumice, is more mafic, containing more iron and magnesium, and is darker and denser.

What are the uses of volcanic glass in modern times?

Volcanic glass has various uses today. Obsidian, due to its sharp edges when fractured, is used in surgical scalpels and delicate cutting instruments. It's also popular in jewelry and ornamental objects. Pumice is widely used as an abrasive in polishes, erasers, and exfoliants, and for its lightweight properties in construction materials.

Can volcanic glass be found all over the world?

Volcanic glass can be found globally, particularly in areas with volcanic activity. Obsidian deposits are common in regions like the western United States, Italy, and Iceland. However, the distribution of volcanic glass types varies depending on the specific geological conditions present during volcanic eruptions.

Is volcanic glass dangerous to handle?

Handling volcanic glass, especially obsidian, can be dangerous due to its sharp edges, which can easily cut skin. It's important to handle it with care and potentially use protective gloves. Pumice and scoria are less hazardous but can still cause abrasions or dust inhalation if not handled properly.

How is volcanic glass studied to understand past volcanic activity?

Scientists study volcanic glass to gain insights into past volcanic activity. By analyzing its chemical composition and formation, they can infer eruption temperatures, magma sources, and eruption dynamics. This information helps in understanding the history of a volcano and predicting future behavior, contributing to hazard assessment and mitigation efforts.

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    • After a volcanic eruption, some high viscosity molten lava can cool into what is known as volcanic glass.
      By: R. Sueswit Apriliant
      After a volcanic eruption, some high viscosity molten lava can cool into what is known as volcanic glass.
    • Pumice igneous rocks contain volcanic ash, which has very small amounts of volcanic glass.
      By: Patricia Hofmeester
      Pumice igneous rocks contain volcanic ash, which has very small amounts of volcanic glass.
    • Volcanic glass can form when lava cools.
      By: Vibe Images
      Volcanic glass can form when lava cools.
    • Perlite is a volcanic rock, and can be used in planting mixtures to aid in aeration.
      By: monamakela.com
      Perlite is a volcanic rock, and can be used in planting mixtures to aid in aeration.