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Amy has taught university-level earth science courses and has a PhD in Geology.
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.
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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.
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!
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.
One of the most familiar associations with obsidian is its use in arrowheads by Native Americans. Because the natural glass does not have an orderly internal structure, obsidian will break into conchoidal fracture. Conchoidal fracture is a smooth scallop-shaped surface formed when a glassy material like obsidian is broken. The tendency of obsidian to form conchoidal fracture is what allows it to form such sharp surfaces. People learned to skillfully chip away and sculpt obsidian to form extremely sharp and effective cutting tools.
We know that obsidian was highly valued by Native Americans as a trading object. Obsidian from any particular eruption has a unique chemical fingerprint that distinguishes it from that of other sources. Archaeologists have been able to trace the trade of obsidian for thousands of miles across the Americas by identifying these unique chemical signatures. Some scientists even suggest that obsidian can be used as a modern medical tool. Some veterinary researchers have observed that using an obsidian scalpel rather than a metal one produces smaller scars that heal more quickly. While a small number of surgeons also use these scalpels, they have yet to be approved by the FDA. The most common modern use of obsidian is decorative, both as jewelry and as art. Artists sculpt the natural glass into any number of decorative shapes and appearances.
Obsidian is a natural volcanic glass that forms from cooled felsic lava. Felsic is a type of lava that has high concentrations of the chemical element silica. This is opposed to mafic lava, which is a silica-poor type of lava that typically forms into basalt. These lavas form glass rather than crystallizing minerals due to their high viscosity which is due to the presence of polymers, which are when silica forms bonds with oxygen in lava creating linked molecule chains. The high viscosity results from this polymerization (which is the process of forming polymers) of the abundant silica atoms in the lava, which prevent the lava from flowing easily or producing crystals. The dark color of obsidian is due to minor impurities in the lava.
Obsidian has been valued for several millennia due to its beauty and its ability to form sharp edges. The pattern, known as a conchoidal fracture, which is a smooth scallop-shaped surface formed when a glassy material like obsidian is broken, allows workers to chip sharp surfaces, which were used as cutting surfaces by early people and are also used today by researchers. However, you are much more likely to see obsidian as a necklace these days than as a knife.
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