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An ancient volcanic eruption in the upper Yukon Basin

1916; United States Government Publishing Office; Linguagem: Inglês

10.3133/pp95d

2330-7102

Stephen Reid Capps,

Climate change and permafrost

It has long been known that a large area in Alaska and Yukon Territory is covered by a layer of volcanic ash.The ash lies near the surface, beneath a thin covering of soil or silt, and gives evidence of an explosive volcanic eruption that in terms of geologic history is very recent, though antedating historic record in this part of the world.The first published description of this material is that given by Schwatka,t who observed the ash layer along the banks of Lewes River and its headward tributaries in 1883.In 1887 Dawson 2 extended the known area of the ash and estimated its total area, the direction from which it came, and the approximate length of time since it was deposited.In 1891 Hayes, 3 in company with Schwatka, conducted an exploration from Fort Selkirk, on the Yukon, to the Copper River basin.They traveled westward into an area in which the ash gradually increased in thickness to its maximum near the international boundary and rapidly-thinned west of that line.In 1898 and 1899 Brooks 4 explored the headwaters . of White and Tanana basins and made n1any observations on the areal distribution and thickness of the ash.The present article is based largely upon information personally obtained or collected by Mr. Brooks.The writer first became interested in this occurrence in 1908, when working in the N abesna and White River district, 5 and a second expedition into the same general region in 1914 gave opportunity for a more extended study of the ash fall.Records of the outer limits of the ash-covered area have been taken from many sources, especially from the published and unpublished notes of the members of the Geological Survey of Canada and of the United States Geological Survey.The general outlines of the ash fall are shown on figure 23, the relative thickness of the deposit being indicated by contours.The outer limits of the area are drawn to include all points at which the ash has been observed and at which it is still recognizable as a distinct layer.Without question a thin film of dust could at the time of the eruption have been observed over an enormously greater area than that here outlined, but, as will be shown, the ejection of the ash antedates recorded history in America, and the area affected can now be determined only by the presence of the ash that has been preserved.Less than one-fourth inch of ash falling at the time of this eruption over a vegetation-covered upland would probably be insufficient to form a layer that would now be generally recognizable.The outermost observations recorded include, on the west, observations on N abesna, Tanana, and Yukon rivers, by Brooks and others; on the northeast and east, on Gravel, Macmillan, and Pelly rivers, by Keele, .Dawson, and McConnell; on the southeast, on Teslin River and at Lakes Marsh and Bennett, by Schwatka, Dawson, and others; and on the south and southwest, along the southeast flank of the St. Elias Range, by Hayes, Brooks, and the writer.. The ash usually appears along the cut banks of the rivers as a thin white band near the top of the bank, covered by only a few inches or a foot or two of soil, silt, or vegetable humus.It is remarkably persistent and is in places continuously exposed for miles.Over any given