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TitlePage   Preface   Contents    

Volcanic Events, pg. 2
Mount St. Helens History, pg. 3-15
Eyewitnesses, pg. 53-67
Absolute Times, pg. 81-82, 86
Activity Sequence, pg. 127-134
Gas Studies, pg. 190-191
Chemical Compositions, pg. 233-250
Ash Clouds, pg. 323-333
Blast Dynamics, pg. 379-400
Rapid Deposition, pg. 466-478
Phreatic Explosions, pg. 509-511
New Lava Dome, pg. 540-544
Ash-Fall Deposits, pg. 568-584
Water Chemistries, pg. 659-664
River Water Quality, pg. 719-731



PREFACE

The 1980 eruptions of Mount St. Helens, although neither the most voluminous nor the greatest in released energy among recent eruptions, are unusual among historic worldwide volcanic activity, in having occurred close to a major industrialized urban area. Mount St. Helens in 1980 was therefore remarkable for a high degree of national attention, public interest, and social and physical disruption. In addition, exceptional opportunities were presented for scientific observation of infrequently occurring volcanic processes.

When renewed activity was heralded on March 20, 1980, by the first strong earthquake at shallow depth beneath the volcano, it was felt on. site and also recorded instrumentally by a joint University of Washington-USGS group in Seattle who had seismometers in operation near the volcano as a part of studies of potential geothermal resources. Within the next 2 days, the unusual character of the seismic activity became clear, and more instruments were set out. A striking increase in earthquakes emphasized the likelihood of an eruption, and warning was given to Federal and State authorities responsible for the land surrounding the volcano.

With the advice of USGS scientists who had been evaluating volcanic hazards of the Cascade Range, access to the volcano and adjacent areas was restricted on March 26. A center was established at the U.S. Forest Service headquarters in Vancouver, Wash., for hazards evaluation, monitoring activity, coordination of activities of the agencies involved, and press briefings and public information.

On March 27, hydrothermal explosive activity began at the summit of Mount St. Helens, accompanied by formation of a small crater, ground fracturing, and beginning of a topographic bulge on the upper north flank of the volcano. Strong seismic activity and relatively mild steam-blast eruptions continued intermittently into mid-May. During that time, the new crater gradually enlarged and the north-flank bulge became visually conspicuous. At the same time, the USGS was developing an extensive program of geophysical monitoring and volcanic hazards analysis; especially helpful was the ability to bring in equipment, staff, and experienced former staff from the Survey's Hawaiian Volcano Observatory on short notice. The continued high rate of seismic energy release, intermittent steam-blast eruptions, and remarkable ground deformation all pointed toward the possibility of a significant volcanic event in the near future; these factors also helped provide justification for Federal and State authorities to continue to limit access to areas near the volcano, despite heavy pressure to relax restrictions. Monitoring techniques were developed that, it was hoped, could provide a short-term warning of any major impending eruption, but the catastrophic eruption on the morning of May 18 commenced without any additional precursory signs that were recognized either in advance or by hindsight.

After May 18, studies began on the deposits and effects of the eruption: the debris avalanche that formed by landsliding of the bulge on the north flank, the northward-directed volcanic blast triggered by the massive landslide, mudflows generated by melting of snow and ice, and the pumiceous pyroclastic flows and extensive ash falls. Updated assessments of the status of the volcano and potential hazards received constant attention. Continued monitoring generally provided warnings a few days to a few hours before additional pyroclastic eruptions on May 25, June 12, July 22, August 7, and October 16-18, and before lava domes were emplaced or enlarged in June, August, October, and December.

This report summarizes early results of wide-ranging studies of the volcanic activity and eruptive products at Mount St. Helens in 1980 by the USGS, including some contributions by other participating government, university, and industry scientists. The papers collected here were mostly written in October and November, 1980, with some revisions and updating as late as January 1981. They accordingly constitute initial, in part preliminary, reports and interpretations of the 1980 activity; time constraints on publication have precluded full analysis and interpretation of some data, especially those related to the later events of 1980. More detailed discussions of some aspects of the volcanic activity summarized here can be expected in the future.

Peter W. Lipman & Donal R. Mullineaux, Editors


 



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