Geology And Geophysics

How Obsidian is Formed



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Obsidian is a volcanic glass. It forms when lava that contains large amounts of silica cools without forming crystals. This generally happens when a volcano extrudes lava, rather than deep within the magma chamber. Obsidian is usually black, but can be dark green, red, or even clear.

Composition

Obsidian has a chemical composition that is very similar to other kinds of rock, such as rhyolite and granite, but its physical structure is very different. Because it has no crystalline structure, amorphous obsidian is not classed as a true mineral, but as a mineraloid.

Why does obsidian cool without forming crystals? Partly because the lava that will become obsidian is very sticky. This high viscosity, combined with a lack of included seed crystals to form patterns for crystal formation may be part of the reason why this glass hardens without crystallizing. In addition, the temperature difference between liquid obsidian and solid obsidian is very slight. Therefore, obsidian is solid before crystals can form.

Snowflake obsidian, however, is a type of obsidian that includes crystals of cristobalite. Cristobalite is a white polymorph (form) of quartz created at high temperatures. It can form delicate patterns across the surface of some obsidian specimens. Sheen and rainbow obsidian are two other varieties of volcanic glass with inclusions, but in this case, the inclusions are gas bubbles that were mixed into the stone during its formation.

Uses

Many cultures have made knives, spear points, and arrowheads from obsidian. The lack of crystalline structure allowed each stone to be shaped to an edge of molecular thinness. Obsidian scalpels have actually found some uses in modern society, because of their sharp edges and lack of reaction to magnetism.

Jewelers have long made use of this glistening material. Obsidian talismans and amulets exist from earliest history. New age believers in crystal healing label obsidian the "Stone of Truth," and promote its use in personal growth and healing.

History

Obsidian degrades. Water penetrates the outside of a piece of shiny obsidian and produces a "rind." From the depth and quality of this rind, archaeologists can make good estimates of the age of a volcanic eruption, or of the time that a particular tool was chipped from a chunk of obsidian.

Obsidian was useful. Highly valued in earlier cultures, it was heavily traded. By mapping the locations of obsidian tools, and matching them to their source material, archaeologists can trace out trade routes from long ago.

Obsidian is a glassy igneous mineraloid. Old lava flows are the best source for new specimens. Obsidian's lack of crystalline structure makes it unusual, useful, and beautiful.

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