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The Pleistocene eruptive history of Mount St. Helens, Washington, from 300,000 to 12,800 years before present: Chapter 28 in A volcano rekindled: the renewed eruption of Mount St. Helens, 2004-2006

Professional Paper 1750-28

This report is Chapter 28 in A volcano rekindled: the renewed eruption of Mount St. Helens, 2004-2006. For more information, see: Professional Paper 1750
By:
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Edited by:
David R. Sherrod, William E. Scott, and Peter H. Stauffer

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Abstract

We report the results of recent geologic mapping and radiometric dating that add considerable detail to our understanding of the eruptive history of Mount St. Helens before its latest, or Spirit Lake, stage. New data and reevaluation of earlier work indicate at least two eruptive periods during the earliest, or Ape Canyon, stage, possibly separated by a long hiatus: one about 300-250 ka and a second about 160–35 ka. Volcanism during this stage included eruption of biotite- and quartz-bearing dacite domes and pyroclastic flows in the area west of and beneath the present-day edifice, accompanied by the deposition of set C tephras. Ape Canyon-stage rocks are compositionally similar to younger Mount St. Helens dacite. The Cougar stage, about 28-18 ka, was probably the most active eruptive stage in Mount St. Helens’ history before the Spirit Lake stage. During the Cougar stage, a debris avalanche buried the area south of the present-day edifice, and voluminous pyroclastic flows, dacite domes, tephra, and a largevolume pyroxene andesite lava flow were erupted. Two tephra sets, M and K, were deposited midway through this stage. Swift Creek-stage deposits were emplaced in two phases, beginning about 16 ka and ending about 12.8 ka. During the first phase, set S tephras and three large fans and at least one smaller fan of dacitic fragmental material were deposited on the northwest, west, south, and southeast flanks of Mount St. Helens. The fans are dominated by lithic pyroclastic-flow deposits associated with dome building but include both primary and reworked material from pumiceous pyroclastic flows and lahars. One Swift Creek-age dome on the west flank of the volcano has been located, and others must have been nearby. During the second phase, set J tephras were deposited, but no pyroclastic flows or domes are known to be associated with the andesitic set J tephras. Preliminary petrographic analysis of these older rocks suggests that the volcano’s magmatic system was simpler during the Ape Canyon stage than during subsequent stages and that the magmatic system has evolved from relatively simple to more complex as the volcano matured. Compositional cycles as envisioned by C.A. Hopson and W.G. Melson for the Spirit Lake stage probably did not occur during the Ape Canyon stage but developed later during the Cougar and Swift Creek stages.

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Additional Publication Details

Publication type:
Report
Publication Subtype:
USGS Numbered Series
Title:
The Pleistocene eruptive history of Mount St. Helens, Washington, from 300,000 to 12,800 years before present: Chapter 28 in A volcano rekindled: the renewed eruption of Mount St. Helens, 2004-2006
Series title:
Professional Paper
Series number:
1750-28
Year Published:
2008
Language:
English
Publisher:
U.S. Geological Survey
Publisher location:
Reston, VA
Contributing office(s):
Volcano Hazards Program
Description:
35 p.
Larger Work Type:
Report
Larger Work Subtype:
USGS Numbered Series
Larger Work Title:
A volcano rekindled: the renewed eruption of Mount St. Helens, 2004-2006 (Professional Paper 1750)
First page:
593
Last page:
627
Number of Pages:
35
Country:
United States
State:
Washington
Other Geospatial:
Mount St. Helens