The May 18, 1980, eruption of Mount St. Helens was one of the most important geologic events of the century. The eruption produced the largest mass movement in recorded history. The explosion that resulted from the depressurization of the volcano devastated the surrounding landscape and killed 53 people, and the subsequent Plinian eruption produced tephra that spread around the world.
The eruption provided an unprecedented opportunity to understand the processes occurring during a major volcanic eruption. This report is the summary of one of the studies that capitalized on that opportunity. The study involved detailed field and laboratory work on the 2.5 cubic kilometer deposit of the rockslide-debris avalanche that was one of the initial events of May 18. Combining this work with studies of the geology of the old mountain (C.A. Hopson, written communication, 1980) and studies of eyewitness photographs of the first moments of the eruption (Voight, 1981; Foxworthy and Hill, 1982; Moore and Rice, 1984) this report tells much of the story of how the volcano fell and blasted apart. It builds upon preliminary work by Voight and others (1981, 1983).
Large volcanic debris avalanches are not uncommon around volcanoes (Siebert, 1984) but they are not well understood. The 1980 debris avalanche at Mount St. Helens is the best exposed of these deposits. This detailed study of the geology of the 1980 Mount St. Helens deposit should provide information that will help interpret old, poorly-exposed deposits at volcanoes around the world.
The work also provides information that will help in understanding the transport of large (>1 cubic km), non-volcanic mass movements. Although many of these mass movements have occurred in historic and prehistoric time (Voight, 1978) little detailed work has been done on the resulting deposits.
I attempt to answer specific questions that are important to the general problem of how the volcano collapsed and blasted apart. Those questions include:
How did the material break up from its source on the mountain to its place of deposition? How did the material transform from a slide to a flow? How important were grain-grain interactions in the movement? What accounts for the production of mixed "matrix facies?" What accounts for the relatively long travel distance of the material?
Many different data sets are used to answer these questions. Some rely on traditional methods. The geology of the source area was compiled, primarily from the work of C.A. Hopson (University of California, written communication, 1980). The deposit was mapped at a scale of 1:24,000, and six morphologic units were identified. A detailed lithologic map of facies and rock types in the deposit at a scale of 1:12,000 was compiled in order to determine the resting places of various pieces of the old mountain and to determine their sequence of deposition. Stratigraphic relationships helped in the understanding of the relationship of the rockslide-debris avalanche to the lateral blast and the other events of May 18.
Other data sets were compiled by studying individual exposures in much greater detail than is common in geologic field work. Forty-four 1-square-meter exposures of the debris-avalanche deposit were cleared of slope wash and examined in detail. The facies and rock types in the exposures were mapped and the large clasts were measured and identified. The field density of the exposures was measured by the sand-cone technique. laboratory grain-size analyses were run on samples from the exposures. The data on exposures provided information on the breakup of the material at the mountain and during transport.
Aerial photographs and topographic maps of the uneroded deposit are important data sets, and they were also studied in detail. The post-eruption maps were compared to the pre-eruption maps in order to construct an isopach map. Various morphologic parameters were measured from the maps in order to quantify the description of the morphology of the deposit.
Although selected problems are emphasized in this work, the detailed description of the deposit is a major contribution of the study. There is no large volcanic debris avalanche deposit in the world that has been studied in as much detail as this one.
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