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Obsidian: Definition, Properties & Uses

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Instructor: Amy Lange

Amy has taught university-level earth science courses and has a PhD in Geology.

Obsidian is a hard, glassy volcanic rock that is absent crystalline structures. Learn how to define obsidian and explore the properties, appearance, and uses of the naturally occurring volcanic glass. Updated: 11/02/2021

Definition and Origin of Obsidian

If you've ever held a black glassy arrowhead, you've held obsidian. Before it was expertly shaped into a sharp point, obsidian was formed through a complex and rare earth process. Obsidian is a naturally occurring volcanic glass. It is formed during the eruption of felsic lavas, which are distinguished by having high concentrations of the chemical element silica. Because of their high silica content, felsic lavas do not behave like the mafic, or silica-poor, lavas we see on the island of Hawaii. Silica forms bonds with oxygen in lava creating linked molecule chains. These linked molecule chains are called polymers, and the process of forming them is called polymerization. Polymers increase the viscosity of the lava. A more familiar example of substances with differing viscosities is the difference between honey and water. Water has very low viscosity, so it flows very easily, whereas honey has a high viscosity and flows much more slowly. The silica-rich lavas that form obsidian flow extremely slowly due to the effects of polymerization of the silica atoms.

Take a look at the image onscreen for an example of what we're talking about in this lesson.

obsidian lava flow

This is a rare obsidian lava flow in Oregon. You can tell that the lava had an extremely high viscosity because of the steep edges of the solidified flow. If it were a low-viscosity lava, it would have flowed more freely across the landscape in a thin layer.

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  • 0:00 Definition and Origin…
  • 1:30 Properties of Obsidian
  • 2:07 Appearance of Obsidian
  • 2:45 Uses of Obsidian
  • 4:09 Lesson Summary
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Properties of Obsidian

Obsidian is marked by its absence of crystals. To understand the formation of obsidian, we must first review how crystals form. You can think of the components in minerals as building blocks. In order for minerals to grow, the correct blocks must be present, and they must be able to connect in the lava. The chains of polymers in the felsic lava get in the way of mineral components connecting with each other to form crystals. Also, the overall high viscosity of the lava prevents much movement from occurring. Because crystals cannot form in this situation, the lava cools into a volcanic glass containing no crystals!

Appearance of Obsidian

A common misconception about obsidian is that its dark color is due to having crystallized from mafic lava. Mafic lavas crystallize to form basalt, which is also dark in color. Obsidian is commonly a translucent dark brown or black. Unlike basalt, obsidian's dark color is due to high amounts of impurities rather than the presence of dark-colored minerals. The color of obsidian depends on the chemical composition of the impurities. A unique type of obsidian is called snowflake obsidian due to the presence of radiating cristobalite, a light colored material. This obsidian variant is commonly used for jewelry.

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