Abstract

Glacier Peak Volcano erupted many times during the period from ~11 000 to ~13 000 years ago and produced at least Three widespread tephra layers that serve as valuable stratigraphic markers in northwestern United States and the adjacent plains of southwestern Canada. Each of these units can be recognized by their stratigraphic, petrographic, and chemical attributes.Tephra was shed over eastern Washington, northern Idaho, western Montana, and southernmost Alberta during each of the eruptions responsible for the older two units; the other layer was deposited during the youngest and most violent event that directed tephra to the southeast as far as Yellowstone National Park, ~950 km from the volcano.Chemical data on glass, hornblende, hypersthene, feldspar, magnetite, and ilmenite show a systematic and unidirectional trend in tephra composition with age; earlier eruptions produced slightly more acidic tephra. This age–composition relationship is confirmed by palaeomagnetic data obtained from 116 specimens collected from two 5 m thick sections. It should therefore be possible to determine the relative age of Glacier Peak tephra samples by their composition alone.The chronology of the Glacier Peak eruptive sequence is still poorly understood. This is especially true of the oldest unit which has so far only been found in southeastern Alberta. Proximal occurrences should be sought in the bogs of eastern Washington where prospects of preservation and age definition are optimised.

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