Abstract
The most extensive volcanic ash units of western United States that are important for tephrochronologic studies include the Pearlette-type ash layers (0.6, 1.2, and 1.9 x 106 years old), the Bishop ash (0.7 x 106 years old), Glacier Peak tephra (ca. 12,500–11,250 years old), Mazama ash (6700 years old), several layers from Mt. St. Helens (3400, 450, and 180 years old), two lobes of White River ash in eastern Alaska and Yukon Territory (1890 and 1250 years old), and the Katmai tephra of southern Alaska (A.D. 1912). These and other tephra layers of western United States have been used to assess the age of Pleistocene and Holocene glacial, pluvial, and alluvial records, to estimate the relative magnitude and character of eruptive events, and to infer prevailing wind direction(s) at times of major eruptions. Pollen influx and pollen types have been employed to determine the duration and season(s) of deposition of certain ancient ash layers. Potential applications include use of tephra layers to evaluate rates of geologic processes, to correlate marine and terrestrial stratigraphic sequences, to determine rates of reforestation at the end of the last glaciation, and to assess the extent to which volcanic ash in the atmosphere can contribute to climatic change.
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