Debris flows are one of the most common and devastating geologic phenomena in the postglacial history of the volcano. The largest debris flows from Mount Rainier probably originated in volcanic explosions that caused large-scale avalanching of rock debris. Other debris flows were caused by such factors as heavy rainfall and rapid snowmelt, which are unrelated to volcanism; their occurrence at Mount Rainier results from the availability there of large quantities of loose rock debris on steep slopes.
A few debris flows have constituents that clearly indicate an origin during volcanic eruptions. Such a debris flow is exposed in roadcuts in the South Puyallup River valley directly north of Round Pass (fig. 2). It contains breadcrust bombs as well as wood fragments that have been wholly converted to charcoal, the implication being that the flow was hot. One piece of charcoal has a radiocarbon age of about 2,350 years. A similar debris flow that contains unmodified breadcrust bombs as large as 10 feet in diameter underlies the Osceola Mudflow near White River campground. We found no wood, so we do not know whether the bombs were hot or cold when they were incorporated in this deposit. These deposits probably resulted from eruptions that ejected hot ash, bombs, and rock fragments onto snowfields and glaciers. Rapid melting of snow and ice could then have provided large volumes of water to flush the rock debris down the slopes of the volcano and into the stream valleys.
Even though they apparently were cold, some debris flows were of such large volume that a volcanic eruption is the most likely process by which adequate quantities of rock debris could be set in motion down the flanks of the volcano. It is possible, however, that some flows resulted from avalanches caused not by volcanism but by earthquakes or by collapse of oversteepened parts of the volcano.
The largest postglacial mudflow, which is estimated to have had a volume of nearly half a cubic mile, is the 5,000-year-old Osceola Mudflow (Crandell and Waldron, 1956). This clay-rich mudflow probably originated from the avalanching of rock, previously altered partly to clay by steam, from the summit and upper slopes of the volcano (Crandell, 1963a, b). Others (Fiske and others, 1963) have proposed, instead, that the mudflow was formed by "the collapse and flowing out of a thick fill of water-saturated sediments in the upper part of the White River valley." However, the source of the mudflow was above rather than within the valley of the White River, as is clearly shown by remnants of the Osceola deposit higher on the flanks of the volcano at Steamboat Prow, to the west of Winthrop Glacier, and on the ridge crests at the head of Inter Fork valley (Crandell and Waldron, 1956; Crandell, 1963a). Thus, the distribution of the mudflow on the north east side of Mount Rainier, inferred from detailed mapping of its remnants, indicates an origin at or near the former summit. The avalanches that caused the mudflow may have resulted from one or more steam explosions.
During its movement, the Osceola Mudflow submerged the White River valley at the site of White River campground beneath at least 500 feet of mud and rock debris. It traveled 40 miles downvalley to the mountain front, then spread out in a lobate mass that covered 65 square miles in the Puget Sound lowland. There it buried the sites of the present communities of Enumclaw and Buckley under as much as 70 feet of mud and probably extended at least as far northwest as the present town of Auburn (fig. 1).
At about the same time, a similar avalanche and debris flow swept down and across Paradise Park and Paradise Valley on the south side of the volcano (see frontispiece) and temporarily filled the Nisqually River valley at the site of Longmire to a depth of at least several hundred feet (Crandell, 1963a).
Another very large mudflow originated less than 3,500 years ago on the west side of the volcano. It temporarily filled the valleys of Tahoma Creek and the South Puyallup River to a depth of more than 700 feet, so that mud spilled from one valley into the other through Round Pass. At the pass, remnants of the mudflow as much as 15 feet thick overlie pumice of layer Y. This mudflow also buried the northern part of Indian Henrys Hunting Ground (fig. 2), which is 800 feet above the adjacent valley floor. It is possibly the same mudflow as that exposed at the junction of the Mowich and Puyallup Rivers (fig. 1), from which wood about 2,200 years old has been obtained (Rubin and Alexander, 1960). Because of the large size of the mudflow, its distribution, and the apparent lack of any other source of adequate volume, we think it had an origin similar to that of the Osceola Mudflow.
A younger mudflow, which has been named the Electron Mudflow, originated on the west side of Mount Rainier within the Puyallup River drainage basin; the flow extended about 40 miles downvalley to the outskirts of Sumner (fig. 1) and buried the floor of the Puyallup River valley at Orting under 15 feet of rock debris and mud (Crandell, 1963b). Wood from this mudflow has been radiocarbon dated as about 500 years old.
A debris flow between 400 and 500 years old temporarily flooded the valley floor at Tahoma Creek campground to a depth of at least 200 feet. This flow contains clay and fragments of altered rock similar to that cropping out in Sunset Amphitheater (Fiske and others, 1963) and probably originated in avalanches at that locality.
It should be noted that the larger debris flows from Mount Rainier apparently did not form permanent fills in valleys to the maximum height of their remnants on the valley walls. Instead, these remnants probably mark transient flow crests, analogous to those of stream floods.
The formation of a large debris flow depends partly on the availability of a large source of water. A potential source on the flanks of the volcano and in the craters is the extensive cover of snow and ice, parts of which would melt during an eruption. Heat production at the summit craters (Moxham and others, 1965) even now probably is adequate to convert part of the snow and ice to water. In this regard, Flett (1912) stated that a member of a climbing party descended about 85 feet into an ice cave along the rim of the eastern crater, threw stones farther down into the cave, and heard splashes as the stones fell into a body of water. More recently, Louis W. Whittaker (oral commun., Aug. 5, 1966) of Tacoma, Wash., and his brother James W. Whittaker of Seattle descended into the east summit crater along an ice cave on a summer day in the mid-1950's. Using oxygen masks, they climbed down along the crater wall, which had a slope of 30° or less and was studded with fumaroles and areas of hot rock. When they reached a depth of about 450 feet vertically below the crater rim, they threw stones farther down the ice cave and heard splashes as the stones struck a body of water.
A significant increase of heat caused by molten rock rising in the volcano could further melt the ice and snow in the summit craters to form lakes perhaps several hundred feet deep. Such lakes in the two craters could contain several hundred million gallons of water. If this water were expelled during an eruption, it would most likely spill down the east, south, or west side of the volcano, pick up loose rock debris on the flanks and on valley floors, and create floods and large debris flows.
Some debris flows form during very heavy rainfall or rapid melting of snow; others are caused by outbursts of water from within, under, or on top of glaciers. Flows of these kinds are not directly connected with any kind of volcanic activity, unless they result from excessive melting of ice due to volcanic heat. Debris flows caused directly by heavy rainfall pick up rock debris mainly from masses of loose glacial drift. Although these flows are of rather limited size, they occur more often than the vastly larger flows such as the Osceola and Electron. Debris flows occurred in October 1947 in the Kautz Creek valley during a period of very heavy rainfall. As runoff from valley sides and Kautz Glacier swept downvalley, it formed a series of debris flows that came to rest in a broad fan at the lower end of the valley (see sketch, p. 15). Grater (1948) estimated that 50 million cubic yards of rock debris was carried by the debris flows. Deposits of at least six previous but similar debris flows are exposed in the banks of Kautz Creek.
Even though they do not result in debris flows, some large rockfalls and avalanches of rock debris from high parts of the volcano are also a potential hazard on the slopes of the volcano and on adjacent valley floors. Avalanches that resulted from rockfalls at Little Tahoma Peak in December 1963 moved as much as 4.3 miles and stopped only 2,000 feet from White River campground (fig. 5). These rockfalls may have been triggered by a small steam explosion on the north side of Little Tahoma Peak (Crandell and Fahnestock, 1965).
Last Updated: 28-Mar-2006