Sedimentology and clast orientations of the 18 May 1980 southwest- flank lahars, Mount St. Helens, Washington

Jon J. Major; B. Voight

doi:10.1306/212f8a1c-2b24-11d7-8648000102c1865d

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USGS · 70217595

Sedimentology and clast orientations of the 18 May 1980 southwest- flank lahars, Mount St. Helens, Washington

Jon J. Major·B. Voight

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Abstract

Three lahars that resulted from the flow transformation of an inflated pyroclastic surge caused by ejected lithic debris and hydrothermal water during the cataclysmic Mount St . Helens eruption of 18 May 1980 deposited about 1 x 10 6 m 3 of massive, poorly sorted, poorly graded volcaniclastic sediment on the SW flank (SWF). Downflow changes in mean grain size and sorting occur only in the coarse fraction of the deposits larger than a critical diameter of about 4mm, and occur only in the basal portion of the deposits; both mean grain size and sorting coefficient increase slightly with distance. The deposits show weak inverse grading with respect to mean grain size and exhibit a weak trend of upwardly poorer sorting, but lahar clast fabric may be more complex and variable than previously suggested.

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Journal of Sedimentary Petrology Article

Source-reported bounding extent: 46.1560536971598° to 46.21119992863992° latitude; -122.25860595703122° to -122.14736938476561° longitude. This indicates report coverage, not an exact sampling location. View area on OpenStreetMap.

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Jon J. Major, B. Voight. 1986. Sedimentology and clast orientations of the 18 May 1980 southwest- flank lahars, Mount St. Helens, Washington. https://doi.org/10.1306/212f8a1c-2b24-11d7-8648000102c1865d

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