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

The avalanche deposits are a jumble of large and small rock fragments in a matrix of grayish-red sand (fig. 3). Many small cracks formed in the avalanche deposits during settling and compaction. The deposits are loose and porous and they slide and compact underfoot.

FIGURE 3.—Surface of avalanche deposits is hummocky and strewn with blocks. Deposit in front of the man is part of avalanche unit 3 (see fig. 7 and p. A10); deposit directly behind and to left of the man is part of unit 1.

Most of the rock fragments in the avalanche deposits are light-gray to dark-reddish-brown andesite. These fragments came from lava flows and from interbedded masses of breccia, both of which were erupted during the growth of Mount Rainier and are now exposed on the sides of Little Tahoma Peak (see Fiske, Hopson, and Waters, 1963, p. 73-75). In addition, the avalanche deposits contain blocks from an undetermined source. One of these blocks, 15 by 20 by 20 feet in maximum dimensions, lies on the terminal moraine near the northeastern edge of the avalanche deposits. A block of reddish-brown breccia about 1,700 feet upvalley from the terminal moraine has maximum dimensions of 24 by 30 by 46 feet (fig. 4), but the largest block in the entire deposit (fig. 5) lies on Emmons Glacier near the point where the White River flows from the glacier terminus. This rock has approximate maximum dimensions of 60 by 130 by 160 feet, and probably weighs at least 50,000 tons. Many blocks of reddish andesite breccia have disintegrated into piles of rubble since the avalanches came to rest; their poor consolidation illustrates the weakness of some rock layers in Little Tahoma Peak. Locally, the avalanche deposits lie on remnants of glacier ice downvalley from Emmons Glacier; these ice masses probably were present before the avalanche occurred. No glacier ice was seen in the avalanche deposits themselves.

Bridges formed of snow mixed with avalanche debris persisted over the White River at two places (figs. 6, 7) throughout the summer of 1964, and the river flowed beneath a third snow bridge between cross sections 10 and 12 until late July. The snow in these bridges had nearly the density of ice, and about 25 percent of the bridge material consisted of rock debris. This mixture probably was formed during one or more avalanches and indicates that snow was abundant in the debris when it came to rest.

The loose and porous fabric of the avalanche debris probably is due to the inclusion and the retention of much air and snow during transport and deposition. According to the records of the National Park Service, at White River Ranger Station (7 miles northeast of Little Tahoma Peak at an altitude of 3,500 ft) 13 inches of snow was on the ground on December 14, 1963. From December 15 to January 4, 1964, snow depth ranged from 7 to 15 inches at the Ranger Station, and from January 5 to the end of March, the depth ranged from 23 to 82 inches. Thus, at and subsequent to the time of the initial rockfall at least a foot of snow probably covered the area crossed by the avalanches.

FIGURE 4.—Curved furrow in surface of avalanche unit 4. Furrow is bordered on the southwest (left) side by a long curved ridge, which may represent the front of a separate avalanche. Pond on floor of furrow drains to the left. Block on ridge is about 24 by 30 by 46 feet.

The surface of the avalanche debris is very rough. If viewed from the ground, the surface between Emmons Glacier and the terminal moraine has a chaotic appearance, but if seen from above, distinct patterns are apparent. One pattern consists of a series of curved ridges and furrows between cross sections 6 and 12, just upvalley from the terminal moraine (figs. 6, 7). These ridges and furrows lie at slight angles to the trend of the valley, are in the thickest part of the deposits, and were formed as the avalanches came to rest. Another pattern lies between cross sections 3 and 6 and consists of a series of straight ridges and furrows that parallel the trend of the valley. They may have been formed by debris deposited along the margins of avalanche units 4 and 5 during movement. Shallow ponds partly occupy some of the furrows in both areas.

FIGURE 5.—Block of reddish breccia lying on Emmons Glacier (hidden beneath avalanche debris). The block is about 160 feet long and 60 feet high.

FIGURE 6.—Aerial view of downvalley part of avalanche deposits. Compare with map, figure 7. Dashed line indicates boundary of deposits. Rock fragment shown in figure 5 is circled. Photographed August 20, 1964, by Austin S. Post, U.S. Geological Survey.

FIGURE 7.—Relation of downvalley part of avalanche deposits to moraines of Emmons Glacier and to the former flood plain of White River (heavy dashed line). Black areas are shallow ponds. Numbered arrows indicate cross sections shown in figure 10. (click on image for an enlargement in a new window)

There are also straight ridges and furrows in avalanche debris on the lower mile of Emmons Glacier. Farther up, the glacier is steeper and extensively crevassed, and the avalanche deposits are thin. Even in this area, however, there are parallel strips of fairly clean ice and debris-covered ice.

Still another pattern appears if the deposits are studied on the ground. Certain areas of the deposits have a distinctive color, texture, or topography. If areas of similar characteristics are mapped, it becomes apparent that at least seven separate avalanche deposits are present; these lap over each other in a series of shinglelike lobes (figs. 6, 7), which are here called avalanche units.

The earliest two avalanche units lie along the northern edge of the deposits; they are both overlapped by a third unit, but as the exposures of units 1 and 2 are 1,500 feet apart, the time relation of these two early units is not known. For convenience of reference the one farther downvalley is designated avalanche unit 1, and the other one avalanche unit 2. Avalanche unit 1 forms a ridge along the lateral moraine on the northwest side of the valley, and blocks of a distinctive light-gray andesite litter the ridge. In July a layer of compressed granular snow as much as 6 inches thick locally separated the outer margin of this unit from the underlying moraine. Avalanche unit 2 has an overall reddish appearance, and blocks of light-gray andesite are absent. This unit was recognized only in the upper part of the valley; its downvalley extent is buried by the younger avalanche deposits.

Avalanche unit 3 cuts across units 1 and 2. It has an overall reddish appearance but does not contain as many blocks of light-gray andesite as unit 1. Unit 3 represents the largest avalanche of the group. It not only reached the terminal moraine but also poured through the river gap near the southern end of the moraine and moved about 2,000 feet farther downvalley as a narrow tongue (figs. 2,6, 7). This tongue is 300 to 500 feet wide and terminates in a lobate mass of debris 10 to 15 feet thick. On the south side of the White River, about 1,100 feet upvalley from the end of the tongue, the deposit rises abruptly about 15 feet, and farther upvalley is a series of low transverse ridges and furrows. These features may have been formed by a second pulse or wave of avalanche debris within unit 3.

On the upvalley side of the terminal moraine, trees and bushes from which bark and limbs have been partly removed flank the margin of avalanche unit 3 (fig. 8). This damage was caused by flying rock debris that now veneers the crest and flank of the moraine as much as 70 feet above the main part of the avalanche deposit. This thin veneer apparently represents an airborne curtain of sand and small rock fragments transported by a violent rush of air from beneath avalanche unit 3 as it came to rest.

FIGURE 8.—Damaged pine tree on terminal moraine. Tree has been partly stripped by airborne debris moving away from viewer. The debris forms a veneer only a few inches thick on the moraine and is visible on top of several boulders.

Another noteworthy feature of avalanche unit 3 is a scar 800 feet long that the avalanche scraped at the base of Goat Island Mountain directly opposite the terminal moraine (fig. 9). The scar is crescentic in shape if viewed from the ground, and its highest part, 140 feet above the White River, coincides with the narrowest part of the valley. To form the scar the avalanche scoured away most of the vegetation and the loose rock debris down to solid bedrock.

Avalanche unit 4 resembles unit 3 but contains fewer large blocks. Its surface is marked by a series of curved ridges; the most prominent, which is about 35 feet high (fig. 4) and 1,400 feet long, may be the front of a separate avalanche unit. Unit 5 contains more large blocks than unit 4 and cuts across the south end of the most prominent ridge in unit 4. Units 6 and 7 are differentiated by their crosscutting relation to each other and to avalanche unit 5. Neither unit 6 nor unit 7 had sufficient volume or velocity to move far beyond the terminus of Emmons Glacier.

Surveys were made in July 1964 to determine the width and thick ness of the avalanche deposits (figs. 10, 11). The survey lines followed lines surveyed across the valley by Fahnestock in August 1963. Comparison of these two surveys reveals that the avalanche deposits down valley from Emmons Glacier range in width from 900 to 1,600 feet, their broadest part coinciding with the widest and flattest part of the old valley floor. The maximum thickness of the avalanche deposits is 100 feet, at cross section 7. The greatest mass of debris lies between cross sections 10 and 12; at cross section 10, the White River is now about 50 feet higher than its pre-avalanche position and about 300 feet farther south.

The volume of avalanche debris downvalley from Emmons Glacier, estimated from the cross sections, is about 12 million cubic yards. In addition, a large volume, perhaps as much as several million cubic yards, covers an area of about 1.3 square miles on Emmons Glacier. The avalanche debris is estimated to cover a total area of about 2 square miles and to have a total volume of at least 14 million cubic yards. Although the height and the basal width of the rockfall scar are estimated to be about 1,700 and 1,800 feet, respectively, the volume of the buttress cannot be computed directly because of its irregular shape (fig. 12) and a lack of knowledge concerning its original average thickness.

FIGURE 9.—Avalanche scar, about 140 feet high, at the base of Goat Island Mountain. This scar was formed when part of avalanche unit 3 moved through the narrow gap in the terminal moraine (fig. 2). The avalanche deposit downstream (left) from the scar has been partly eroded by the White River. Area of avalanche deposits is enclosed by dashed line; arrows show inferred direction of movement of avalanche unit 3.