Mazama Ash

The Mazama Ash (formally named the Mazama Member in some areas)[1] is an extensive, geologically recent deposit of volcanic ash that is present throughout much of northern North America. The ash was ejected from Mount Mazama, a volcano in south-central Oregon, during its climactic eruption about 7600 years ago when Crater Lake was formed by caldera collapse. The ash spread primarily to the north and east due to the prevailing winds, and remnants of the ash have been identified as far northeast as the Greenland ice sheet.[4]

Mazama Ash
Stratigraphic range: Holocene
TypeMember[1]
Unit ofMount Mazama Formation[2]
Sub-unitsTsoyawata Bed, Mazama Bed[2]
Location
RegionNorthern North America
CountryUnited States, Canada, Greenland
Type section
Named forMount Mazama
Named byB.N. Moore, 1934.[3]

Because it was deposited throughout a wide area at a known time, the Mazama Ash is an important marker bed for paleoclimatology, paleoecology, and archeology, as well as for Quaternary geology and stratigraphic correlation.[5][6][7]

The ash particles and gasses from the Mazama eruption would have caused climate cooling for a period of several years after the eruption.[4] Throughout the northern Great Plains, the ash would have darkened the sky and a layer of ash at least several centimetres thick would have blanketed much of the landscape, causing severe disruptions for the native people and wildlife.[6][8]

Age

The climactic eruption of Mount Mazama during which the Mazama Ash was ejected occurred 7627 ± 150 calendrical years Before Present (5677 ± 150 B.C.E.), based on its identification in an ice core from the Greenland Ice Sheet Project.[4]

Distribution
Mount Mazama's eruption timeline, showing ejection of ash and caldera collapse.

The Mazama ash spread over an area of at least 900,000 km2 (350,000 sq mi) in the northern Great Plains, where it is most commonly preserved within peat, alluvial, lacustrine, and aeolian sediments.[9] It is present in portions of the U.S. states of California, Oregon, Washington, Idaho, Montana, Nevada, Wyoming and Utah,[7] as well as in the Greenland ice sheet,[4] and in marine sediments off the coast of Oregon, Washington,[10] and southernmost British Columbia.[11]

In Canada, deposits of Mazama Ash several centimeters thick are commonly present in southern areas of British Columbia, Alberta, and Saskatchewan.[9] In southern Alberta, about 1,000 kilometres (620 mi) northeast of the eruption site, the Mazama Ash is typically found as a white band located several metres below the present ground surface.[8] Shards of volcanic glass from the Mazama Ash have also been identified in the sediments of Lake Superior and in a bog in Newfoundland.[5]

Composition and identification

The Mazama ash includes plagioclase, hypersthene, magnetite, hornblende, clinopyroxene, and volcanic glass.[12] It can be distinguished from other volcanic ash deposits, such as those from eruptions of Glacier Peak, Mount St. Helens and Mount Rainier, by the unique chemistry of those constituents.[13] This can be determined by electron microprobe analysis, by the refractive index of the volcanic glass,[9] and by neutron activation analysis[13] and similar techniques. Radiocarbon dating of associated carbon-bearing material may also aid identification of the Mazama Ash.[9]

Impact

Comparison with the effects of the Mount St. Helens eruption of 1980 indicates that the Mazama Ash would have covered the landscape in a blanket up to 15 cm (6 in) thick, coating vegetation and clogging watercourses throughout the ashfall area. This would have caused an immediate scarcity of resources for the native people and wildlife, necessitating the movement of people out of the main ashfall area. Available archeological evidence from a site in the Cypress Hills of southern Alberta suggests a hiatus in human occupation of the ash-affected area there of perhaps 200 years.[6][8]

The particles and gasses released during the Mazama eruption caused climate cooling. Studies of the Greenland ice core suggest that the eruption produced a substantial stratospheric aerosol loading spread over a period about 6 years. This may have produced a temperature depression of about 0.6 to 0.7 °C at mid to high northern latitudes for 1 to 3 years. The release of chlorine during the eruption may also have led to substantial depletion of stratospheric ozone.[4]

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References
  1. National Geological Map Database. "Geologic Unit: Mazama". Retrieved 2 March 2020.
  2. Jonathan O. Davis (1978), Quaternary tephrochronology of the Lake Lahontan area, Nevada and California, Wikidata Q63856078
  3. Moore, B.N. 1934. Deposits of possible Nuee ardente origin in the Crater Lake region, Oregon. Journal of Geology, vol. 42, p. 353-375.
  4. Zdanowicz, C.M., Zielinski, G.A. and Germani, M.S. 1999. Mount Mazama eruption: Calendrical age verified and atmospheric impact assessed. Geology, vol. 27, no. 7, p. 621-624.
  5. Spano, N.G., Lane, C.S., Francis, S.W. and Johnson, T.C. 2017. Discovery of Mount Mazama cryptotephra in Lake Superior (North America): Implications and potential applications. Geology, vol. 45, p. 1071-1074.
  6. Oetelaar, G.A. and Beaudoin, A. 2005. Darkened skies and sparkling grasses: The potential impact of the Mazama ash fall on the northwestern Plains. Plains Anthropologist, vol. 50, no. 195, p. 285-305.
  7. White, J.M. and Osborn, G. 1992. Evidence for a Mazama-like tephra deposited ca. 10 000 B.P. at Copper Lake, Banff National Park, Alberta; Fig. 1 (inset), p. 53. Canadian Journal of Earth Sciences, vol. 29, p. 52-62.
  8. Beaudoin, A. and Oetelaar, G.A. 2014. Investigating the environmental impacts and cultural responses to the Mazama ashfall on the northern Plains. Geological Society of America, Abstracts with Programs, vol. 46, no. 6, p. 460.
  9. David, P.P. 1970. Discovery of Mazama Ash in Saskatchewan, Canada. Canadian Journal of Earth Sciences, vol. 7, p. 1579-1583.
  10. Nelson CH; Kulm LD; Carlson PR; Duncan JR (1 July 1968), "Mazama ash in the northeastern pacific.", Science, 161 (3836), doi:10.1126/SCIENCE.161.3836.47, PMID 17756513, Wikidata Q42101098
  11. Debret, M., Desmet, M., Balsam, W., Copard, Y., Francus, P. and Laj, C. 2006. Spectrophotometer analysis of Holocene sediments from an anoxic fjord: Saanich Inlet, British Columbia, Canada. Marine Geology, vol. 229, p. 15-28.
  12. Kittleman, L.R. 1973. Mineralogy, correlation, and grain-size distributions of Mazama tephra and other post-glacial pyroclastic layers, Pacific Northwest. GSA Bulletin, vol. 84, no. 9, p. 2957-2980.
  13. Theisen, A.A., Borchardt, G.A., Harward, M.E. and Schmitt, R.A. 1968. Neutron activation for distinguishing Cascade Range pyroclastics. Science, vol. 161, p. 1009-1011.
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