Big Glass Mountain
| Big Glass Mountain | ||
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 Big Glass Mountain as seen from the eastern rim of Medicine Lake Volcano |
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| location | Siskiyou County , California , United States | |
| Mountains | Cascade chain | |
| Coordinates | 41 ° 36 '39 " N , 121 ° 33' 13" W | |
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| Type | Obsidian flow | |
| rock | Dacite , rhyolite | |
| Age of the rock | Holocene , Subatlantic | |
| Last eruption | 1065 | |
The Big Glass Mountain is from the last eruption on Medicine Lake Volcano in Northern California emerged around 1000 years ago. It consists mainly of black obsidian rhyolite to Dazite composition.
etymology
Big Glass Mountain (often referred to simply as Glass Mountain) uses the adjective Big (large) to distinguish it from its namesake, Little Glass Mountain . Its name was derived from the English words glass (volcanic glass) and mountain (mountain), which means Great Glass Mountain in German .
description
The Big Glass Mountain (abbreviated BGM) has a three-kilometer-long, 500-meter-wide and up to 75-meter-thick rhyolite obsidian flow, which begins on a 9 ° northeast dipping surface of Medicine Lake Volcano from a central production vent ( Central Dome ) north-easterly and then in a north-northeast direction (called North Lobe ). Its surface is crossed by topographic ridges ( pressure ridges ) caused by flow pressure at a distance of 39 ± 15 meters (maximum 60 meters), the height amplitude being 8 meters. The ridges were created by shear forces as the inside of the stream was still fluid and kept moving when the surface was already beginning to crust. The obsidian flow starts from a large dome, which comes to lie on a south-south-east trending (N 155) fracture zone, on which 11 other smaller rhyolite domes are lined up. A passage-like crevice was probably opened along this direction. The leaked volume on Big Glass Mountain is approximately one cubic kilometer.
Petrology
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The effusive phase was preceded by an explosive phase that covered around 320 square kilometers with rhyolite tephra (with an SiO 2 content of 73.8 to 74.2 percent by weight).
The chemical composition of the early lava of the main stream varies from dacite to rhyodacite to rhyolite (63.8 to 74.2 percent by weight SiO 2 ). This rather wide range is explained by the mixture of a rhyolite magma with an andesite component. The latter, however, is not uniform; its composition was changed before and during the outbreak. The mixing of andesite and rhyolite can be observed in different dimensions - from the meter range in the exposure to the centimeter range of the handpiece to the millimeter range of the thin section. Banding of rhyolite, rhyodacite and dacite with andesite can be clearly seen in handpieces, and large andesite bubbles are enclosed by rhyolite in thin sections.
As inclusions in the lava act quenched Andesitmagma, grinding stones of kumulatischem gabbro and granitic Xenoliths, originate from either the upper crust or represent subvolcanic equivalents of silicon-rich lava.
Finally, the central rhyolite dome and the externally located eleven smaller domes with rhyolite chemistry were built above the crevice.
Age
The obsidian of Big Glass Mountain has been dated to 885 years BP by Donnelly-Nolan and colleagues (1991) , which corresponds to AD 1065. In the period from 1250 to 850 BP (700 to 1100 AD), six eruptions of Dacite-rhyolite composition occurred at Medicine Lake Volcano, the most recent being the obsidian flow of Big Glass Mountain. Its predecessors are the Medicine Dacite lava flow , the Mount Hoffman lava flow , and the Little Glass Mountain lava flow .
Cultural use
The obsidian of Big Glass Mountain was mined intensively by the Indians of Northern California and Southern Oregon for the production of stone tools (blades etc.), which, in addition to their purely practical use, also gave their owners a high status. The obsidian found widespread distribution in western North America through trade. The mountain was also very important as a place of worship among the indigenous people.
Individual evidence
- ^ Fink, JH and Pollard, DD: Structural evidence for shallow intrusion under Medicine Lake volcano, California . In: Geology . tape 10 , 1983, p. 503-507 .
- ↑ Heiken, G .: Plinian-type eruptions in the Medicine Lake Highland, California, and the nature of the underlying magma . In: J. Volcanol. Geothermal. Res. Band 4 , 1978, p. 375-402 .
- ↑ Donnelly-Nolan, JM, Champion, DE, Miller, SD, Grove, TL and Trimble, DA: Post-11,000-tear volcanism at Medicine Lake Volcano, Cascade Range, northern California . In: Journal of Geophysical Research . tape 95 , 1991, pp. 19693-19704 .
- ^ Dillian, CD: More than toolstone: differential utilization of Glass Mountain obsidian (doctoral thesis) . University of California, Berkeley 2002, pp. 1-332 .