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REPORT:
Postglacial Lahars From Mount Rainier Volcano, Washington


-- Dwight R. Crandell, 1971,
Postglacial Lahars From Mount Rainier Volcano, Washington U.S. Geological Survey Professional Paper 677

Abstract

More than 55 lahars (mudflows and debris flows form a volcano) originated at Mount Rainier during Holocene time. These ranged in length from a few miles to 70 miles and in volume from a few million cubic yards to more than half a cubic mile. Some lahars were created by landslides of altered volcanic rock; they contain significant amounts of clay, which consists of montmorillonite and kaolinite and lesser amounts of other clay minerals, which were formed by hydrothermal alteration of rock within the volcano. Some of these lahars have been deep enough, when flowing, to fill valleys to depths of hundreds of feet; the fluidity of the lahar permitted most of it to progress downvalley and to leave in passing only a thin deposit on valley sides and terraces. Smaller lahars of the same kind seem to have progressed downvalley in a single massive wave hundreds of feet high. Clay-rich lahars originated in massive slides of clayey rock that had a high moisture content. The slides may have been triggered either by strong earthquakes or by volcanic explosions, or possibly by swelling that accompanied the ascent of molten rock into the volcano. Other lahars were created by the eruption of hot volcanic bombs, dense rock fragments, and ash onto ice and snow at the summit of the volcano. Rapid melting of snow created floods down the volcano's flanks which carried down rock debris and also picked up loose detritus on valley floors to form lahars. Still other lahars resulted from precipitation of cloudburst proportions and by glacier outburst floods. Some outburst floods may have been caused by excessive melting of ice around subglacial steam vents.

There have been repeated episodes of volcanism at Mount Rainier within about the last 10,000 years. The last major eruptive period, between about 2,000 and 2,500 years ago, witnessed the eruption of pumice and lava flows and the formation of many lahars. During this period, erosion alternated with aggradation of lahars and fluvial gravels on the valley floors of the White and Nisqually Rivers, and possibly on those of some other rivers as well. The last pumice eruption, which occurred in the mid-1800's, apparently was on a very small scale, and there is no known record of floods or lahars at that time.

If future eruptions of Mount Rainier were to be similar in scale and type to those of the last 10,000 years, the greatest hazard would be that of lahars. Because of the restriction of lahars to the lower parts of valleys away from the immediate flanks of the volcano, valley floors would be especially hazardous. In view of the increased probability of lahar formation during an eruption, valley floors should be evacuated immediately within a radius of at least 25 miles from the volcano if an eruption should begin. It is proposed that permanent residences should not be constructed on certain valley floors near Mount Rainier, that consideration be given to the relocation of campgrounds that are now in potentially hazardous areas, and that future highways and bridges be designed and located to minimize destruction by future lahars. Likewise, the planning of all other residential, economic, and recreational developments within valleys that head on the volcano should be concerned with lahars as potential geologic hazards.

Artificial traps that might prevent large lahars from entering densely populated areas now exist in the form of hydroelectric power dams and flood-control dams in some valleys. To control a lahar, reservoirs behind these dams would have to be empty. No reservoir, however, would be of any avail in controlling or diverting a lahar comparable in size with the largest that originated at Mount Rainier in postglacial time.

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07/24/00, Lyn Topinka