Obsidian Cliff: A guest post
by Anastashia Cuddihy*
Eleven miles south of Mammoth Hot Springs, and at the northern end of Beaver Lake in Yellowstone National Park, lies a geological wonder1: Obsidian Cliff. Although it is only half a mile long, it has been a source of confusion and contentious debate for years.
Map of the Obsidian Cliff area (Google Maps):
Obsidian Cliff was formed when a rhyolitic lava flow traveled down the plateau and formed the 200 foot cliff. What is so unusual about the cliff is that obsidian is characterized by being a quickly cooled glass; the quick cooling prevents any crystals from forming, leading to a glassy rock. However, the bulk of the obsidian found at Obsidian Cliff is just that - a tall cliff made of obsidian - and it is far too massive to have cooled quickly enough to form obsidian in the 'traditional' way. Since there is no possibility that this amount of obsidian was cooled quickly, geologists theorize that the rhyolitic magma that formed the cliff had an extremely low water content, which would have hindered the formation of crystals. A feature found at the cliff that is associated with relatively slow cooling is columnar jointing. Columnar jointing occurs when lava shrinks during the cooling process, forming cracks, and it contracts to form 6-sided columns1. Without the formation of crystals, the magma would have cooled to become the obsidian found in the cliff.
Columnar jointing at Obsidian Cliff:4
Another solution offered is that the lava could have come in contact with a large body of ice, such as a glacier1. Upon contact with the glacier, the lava would have been able to cool rapidly and form the obsidian, although probably only at the contact margin. However the low-water-content explanation is the more widely accepted. Underlying the obsidian is a purplish-gray rhyolite, which is visible along the cliff face2. Upon close examination of the obsidian, one can see the swirls left in it by the lava before it cooled1.
Flow-banding in Obsidian Cliff obsidian:4
(also note the spherulites)
While Obsidian Cliff is not the only place in the park to find obsidian, it is most abundant at that site. Obsidian can also be found at locations called Tanker Curve and Cougar Creek.2
Obsidian Cliff is known for more than just being an anomaly in the formation of obsidian. Obsidian was prized by Native Americans for making tools, and there was no place better to find obsidian for these tools than Obsidian Cliff. Since obsidian fractures conchoidally and has sharp edges when fractured, it was best used in arrowheads and spears and blades for hunting1. Scientists can use the chemical composition of obsidian (such as the concentrations of trace elements like rubidium and zirconium; See figure) to trace it to its source.
Scatterplot of zirconium (Zr) plotted versus rubidium (Rb) for 143 samples of artifact-quality obsidian collected in Yellowstone National Park.2
Archaeologists have concluded that obsidian from the cliff was being used as far back as 10,000 years ago, up until the arrival of Europeans in the area in the 1800's, when explorers found the Shoshone tribe using obsidian-tipped arrows3. Obsidan from the cliff has been found as far away as the midwestern United States, and it is inferred to have arrived there through the extensive Native American trade routes, where the high-quality obsidian would have been highly valued.
Obsidian Cliff is obviously an important site, both for American heritage and science. It gives us a wonderful idea of how the volcanic structures of the park work and how varied volcanic effects across the park can be. It is not suprising that a park with such a varied and explosive geological history would be home to such an intriguing structure.
__________________________________________________
* Rockies course 2009 student
1. http://www.nps.gov/yell/planyourvisit/upload/Yell250.pdf
2. http://www.obsidianlab.com/research/research_yellowstone.html
3. http://wyoshpo.state.wy.us/aamonth/2000.asp
4. Photo by Charlie Corrick.
Eleven miles south of Mammoth Hot Springs, and at the northern end of Beaver Lake in Yellowstone National Park, lies a geological wonder1: Obsidian Cliff. Although it is only half a mile long, it has been a source of confusion and contentious debate for years.
Map of the Obsidian Cliff area (Google Maps):
Obsidian Cliff was formed when a rhyolitic lava flow traveled down the plateau and formed the 200 foot cliff. What is so unusual about the cliff is that obsidian is characterized by being a quickly cooled glass; the quick cooling prevents any crystals from forming, leading to a glassy rock. However, the bulk of the obsidian found at Obsidian Cliff is just that - a tall cliff made of obsidian - and it is far too massive to have cooled quickly enough to form obsidian in the 'traditional' way. Since there is no possibility that this amount of obsidian was cooled quickly, geologists theorize that the rhyolitic magma that formed the cliff had an extremely low water content, which would have hindered the formation of crystals. A feature found at the cliff that is associated with relatively slow cooling is columnar jointing. Columnar jointing occurs when lava shrinks during the cooling process, forming cracks, and it contracts to form 6-sided columns1. Without the formation of crystals, the magma would have cooled to become the obsidian found in the cliff.
Columnar jointing at Obsidian Cliff:4
Another solution offered is that the lava could have come in contact with a large body of ice, such as a glacier1. Upon contact with the glacier, the lava would have been able to cool rapidly and form the obsidian, although probably only at the contact margin. However the low-water-content explanation is the more widely accepted. Underlying the obsidian is a purplish-gray rhyolite, which is visible along the cliff face2. Upon close examination of the obsidian, one can see the swirls left in it by the lava before it cooled1.
Flow-banding in Obsidian Cliff obsidian:4
(also note the spherulites)
While Obsidian Cliff is not the only place in the park to find obsidian, it is most abundant at that site. Obsidian can also be found at locations called Tanker Curve and Cougar Creek.2
Obsidian Cliff is known for more than just being an anomaly in the formation of obsidian. Obsidian was prized by Native Americans for making tools, and there was no place better to find obsidian for these tools than Obsidian Cliff. Since obsidian fractures conchoidally and has sharp edges when fractured, it was best used in arrowheads and spears and blades for hunting1. Scientists can use the chemical composition of obsidian (such as the concentrations of trace elements like rubidium and zirconium; See figure) to trace it to its source.
Scatterplot of zirconium (Zr) plotted versus rubidium (Rb) for 143 samples of artifact-quality obsidian collected in Yellowstone National Park.2
Archaeologists have concluded that obsidian from the cliff was being used as far back as 10,000 years ago, up until the arrival of Europeans in the area in the 1800's, when explorers found the Shoshone tribe using obsidian-tipped arrows3. Obsidan from the cliff has been found as far away as the midwestern United States, and it is inferred to have arrived there through the extensive Native American trade routes, where the high-quality obsidian would have been highly valued.
Obsidian Cliff is obviously an important site, both for American heritage and science. It gives us a wonderful idea of how the volcanic structures of the park work and how varied volcanic effects across the park can be. It is not suprising that a park with such a varied and explosive geological history would be home to such an intriguing structure.
__________________________________________________
* Rockies course 2009 student
1. http://www.nps.gov/yell/planyourvisit/upload/Yell250.pdf
2. http://www.obsidianlab.com/research/research_yellowstone.html
3. http://wyoshpo.state.wy.us/aamonth/2000.asp
4. Photo by Charlie Corrick.
Labels: volcano, wyoming, yellowstone
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