Geological Survey Bulletin 1221-A Rockfalls and Avalanches from Little Tahoma Peak on Mount Rainier, Washington

The topography and geology of Little Tahoma Peak form an ideal setting for rockfalls; in fact, it is surprising that large rockfalls are not more common there. Yet, one is led to wonder whether some specific event might have triggered the first rockfall from the peak, which apparently occurred about noon on December 14. Although perhaps not the largest of the series of falls, this first rockfall probably was the most important for it almost certainly removed support from other masses of rock, leaving them highly unstable.

A sound heard at the time the first mass of rock is believed to have fallen suggests that a steam explosion may have occurred at Little Tahoma Peak. On December 14, Benjamin E. Cottman (oral commun., Aug. 25, 1964) and Richard J. Allen (oral commun., Aug. 17, 1964), Forest Rangers of the U.S. Forest Service, were at the Crystal Mountain ski area, about 12 miles northeast of Little Tahoma Peak. Shortly before noon they both heard a very loud, sharp boom in the direction of Mount Rainier. Jet aircraft were not heard before or after the loud noise, and Cottman and Allen concluded that the noise had originated in some way at Mount Rainier rather than as a sonic boom. Although the noise possibly resulted from the rockfall itself, the fact that it was a single sharp boom may suggest that it originated from an explosion rather than from a rockfall and avalanche. Visibility was poor at the time because of clouds and snow, but as the clouds lifted from time to time during the afternoon of the 14th, Cottman and Allen observed that a large part of Emmons Glacier had become covered with dark rock debris below Little Tahoma Peak. According to Allen, fresh rock debris also fell from the peak onto new snow several times during the following 2 months, but the size of the later rockfalls is not known.

If an earthquake had occurred just before or at the same time as the first rockfall, one might conclude that the shock triggered the fall. According to Prof. Norman Rasmussen (written commun., July 17, 1964) of the Department of Geology at the University of Washington, the most recent quake that preceded the rockfall was recorded on a seismograph at Longmire, 8.5 miles south of Little Tahoma Peak, on December 13, at about 8:20 a.m. This earthquake had a duration of about 40 seconds and occurred at an estimated distance of about 11 miles from Longmire in an undetermined direction. No earthquake was recorded on the seismograph near noon on December 14, when the initial rockfall apparently occurred; however, according to Professor Rasmussen, at 4:40 p.m. on that day the seismograph recorded "a disturbance not containing the usual character of an earthquake. This event is of very small amplitude and short period suggesting a local disturbance and may be from sliding rock against rock * * *" This small seismic disturbance may have been caused by one of the rockfalls and avalanches that followed the initial rockfall, possibly by avalanche unit 3, the largest of the group. There is no evidence that an earthquake shock was directly responsible for the initial rockfall.

Another possible trigger might have been freezing and thawing of ice in cracks in the rock, either of which would have caused a slight weakening of the rock. At White River Ranger Station, the maximum temperature was 34° on December 14 and the minimum 28°F It seems unlikely that the temperature reached above freezing on that day at altitudes of 9,000-11,000 feet on Little Tahoma Peak. Although repeated freeze and thaw undoubtedly contributed to progressive weakening of the rock over many years, these processes probably were not the immediate cause of the rockfall on December 14.

A third way in which the initial rockfall might have originated could have been by a small steam explosion at or near the base of Little Tahoma Peak, caused by water beneath the surface coming into contact with hot rock in a confined space.

If one or more steam vents had been active between December 14 and the end of January, they probably would not have been observed because of the generally cloudy weather during this period; however, William Pope and Ross Gregg (oral commun., July 1964), residents of Crystal Mountain ski area, observed clouds at Little Tahoma Peak from Crystal Mountain on clear days during the winter of 1963-64. Some of these appeared to be dust clouds because of their grayish-brown tinge; others were thought to be clouds of water vapor.

Clouds of "steam" also were frequently reported at Little Tahoma Peak during July and August 1964 by observers at Yakima Park and Burroughs Mountain, but owing to the distance of these points from Little Tahoma Peak it was not possible to be sure that these clouds were water vapor rather than the dust plumes that were a daily feature of the rockfall scar during those months.

On the afternoon of July 9, Crandell and Ranger Naturalist Arthur Haines observed the rockfall scar on Little Tahoma Peak through binoculars from a point near Mount Ruth. Plumes of water vapor could be seen rising from near the base of the west side of the rockfall scar (figs. 13, 16), forming a small white cloud that hovered along the north side of the peak all afternoon. Meanwhile, no water vapor could be detected coming from other obviously moist parts of the scar, and no other clouds were visible in the sky. On the basis of this observation, Crandell concluded that an unusual source of heat was present at that time near the base of Little Tahoma Peak, and that this source of heat might have caused a steam explosion in December 1963.

In the late summer, the U.S. Forest Service was asked to conduct an aerial examination of Little Tahoma Peak with infrared heat-sensing equipment that is normally used to detect and outline the extent of forest fires. The investigation was made on September 3 and 4, 1964, and the results indicated that the temperature of the rockfall scar was then no warmer than normal. Furthermore, snow that fell in late August and early September seemed to disappear from all parts of the scar at the same rate, indicating approximately equal temperatures throughout. Thus, if a source of heat did exist near the base of the peak from December to July 1964, it apparently had disappeared by late August or early September.

Small steam explosions probably are not unusual occurrences on Mount Rainier, but owing to the inaccessibility of most of the volcano during much of the year, they are rarely observed. Such an explosion, however, occurred during the summer of 1961. Luther G. Jerstad (written commun., Sept. 16, 1964) of Eugene, Oregon, was at that time guiding parties to the summit. According to Jerstad, he was awakened one night at Camp Muir (fig. 1) by a loud noise and shaking of the ground. There was bright moonlight, and a great cloud of dust could be seen at the south end of Gibraltar Rock, caused by continuing rockfalls which could be plainly heard. The next morning, Jerstad observed rock fragments strewn over the surface of Cowlitz Glacier for a distance of at least three-quarters of a mile below Gibraltar Rock, and in the southern side of Gibraltar Rock there was a fresh scar about 150 feet wide, 150 feet deep, and 100 feet high. Steam was spouting 200 feet into the air from a vent in the scar, apparently under great pressure, and making a noise like a high wind. According to Jerstad, the steam continued to issue from the vent all summer, although it seemed to decrease in volume and pressure over a period of about 5 weeks. It was no longer active by the summer of 1962.

FIGURE 16.—Water vapor issuing from western base of rockfall on July 9, 1964. White area at upper right is a glacier. (See fig. 13)

This example demonstrates that small steam explosions are normal and expectable phenomena on the flanks of Mount Rainier. Such explosions in unstable areas undoubtedly have contributed substantial quantities of rock debris to glaciers on the volcano in the past. Although the evidence is not conclusive, the rockfalls and avalanches from Little Tahoma Peak possibly were initiated by such a mild steam explosion.