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Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Mark Stelten, research geologist with the U.S. Geological Survey.

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Obsidian is a naturally occurring volcanic glass that forms when viscous rhyolite magma, which is rich in silica (typically >74% SiO₂ by weight), erupts as a lava flow then cools rapidly. The rapid cooling and high viscosity of the magma prevents crystals from forming and instead leads to the formation volcanic glass (obsidian). Like most other volcanic eruptions at Yellowstone, obsidian is chemically classified as rhyolite, but it has a distinct physical appearance characterized by a glassy texture and lack of crystals. In contrast, the infamous explosive, caldera-forming rhyolite eruptions at Yellowstone produce tephra fall or pyroclastic flow deposits that contain fragments of pumice, crystals, and foreign rock material in a matrix of fine ash.

Obsidian rocks have been used by humans to make tools for thousands of years. The volcanic rock breaks with what is called “conchoidal” fracture, which results in a smooth, curved type of fracture surface that has characteristically sharp edges. Using a process called knapping, which involves shaping a piece of obsidian by striking it to flake off material, humans have long used obsidian to make knives and spear points that can be sharper than a modern razor. In fact, obsidian surgical scalpels for use in delicate operations such as eye surgery are still hand-made today using similar flaking techniques.

One of the best examples of obsidian in Yellowstone is the aptly named Obsidian Cliff flow, which erupted around 106,000 years ago just north of Yellowstone caldera. The lava flow’s primary exposure is at Obsidian Cliff, located approximately 13 miles south of Mammoth Hot Springs along the east side of Grand Loop Road. The exposure at Obsidian Cliff stands 150 to 200 ft above the surrounding ground surface and extends for approximately a half-mile. The Obsidian Cliff flow and a few other rhyolite flows located in the corridor between Norris and Mammoth Hot Springs are unique among rhyolites at Yellowstone in that they are crystal-poor, or almost completely lack crystals, whereas most other rhyolites are crystal-bearing with 5% to 15% crystals by volume. The crystal-poor nature and large exposure of Obsidian Cliff made it an ideal source of obsidian. Obsidian derived from this site was traded as far away as Ohio and Canada. Due to its historical significance, Obsidian Cliff was designated a National Historic Landmark 1996.

The secret to Obsidian Cliff’s glassy texture is that it was heated to higher temperatures relative to other Yellowstone rhyolites before being erupted, thereby preventing the formation of crystals. Whereas crystal-bearing rhyolite lava flows erupted within Yellowstone caldera likely originated from a large, crystal-rich magma reservoir. Crystal-poor rhyolites, like Obsidian Cliff, that were erupted outside the caldera boundary formed in a different way. Extremely hot mafic magma called basalt (which is very similar to the type of magma that erupts in Hawaiʻi) rose from a deep source and was injected into the crust along faults between Norris and Mammoth. These mafic magmas heated and melted the surrounding crust, making a small and very hot batch of rhyolite magma that subsequently erupted to form features like the Obsidian Cliff flow. Next time you visit the park, take the time to look at this geological and historical wonder!