LAVA FLOWS AND PYROCLASTIC ERUPTIONS
Most of Mount Rainier's lava flows are older than the last major glaciation (Hopson and others, 1962) and therefore are probably more than 25,000 years old. One possible exception is a lava flow in the valley of the West Fork of the White River just north of Winthrop Glacier (fig. 2; Fiske and others, 1963), whose topographic position on the valley floor suggests a relatively young age. The youngest lava flows at Mount Rainier form the present summit cone, which is indented by two overlapping craters (fig. 3). The slopes of the cone are smooth and unmodified by erosion; this condition may be partly due to volcanic heat which hinders the accumulation of deep snow and ice on the flanks of the cone. Even so, the lack of erosion suggests that the summit cone is no more than a few thousand years old, and possibly the cone is only a few hundred years old. The two craters of the summit cone most likely were formed during a single episode of volcanic activity. Fumaroles are present today on the slopes and along the rims of both craters, although the highest apparent temperatures are associated with the east crater (Moxham and others, 1965). The overlapping relation of the craters probably represents only a small eastward shift of the eruptive conduit near the summit of the volcano.
Future lava flows limited to the summit cone would not be direct hazards, although rapid melting of snow and ice by lava could cause disastrous floods and debris flows on valley floors. Lava flowing from the flanks of the volcano would threaten existing campgrounds in the valleys of the Nisqually and White Rivers and Tahoma Creek if the flows reached a distance of 5 miles beyond the base of the volcano, but they should present a minimal direct hazard because their relatively slow rate of movement would permit ample warning. Such flows could, however, also cause disastrous floods and debris flows if the lava were erupted onto glaciers or snowfields.
Pyroclastic eruptions at Mount Rainier have included the ejection of both molten and solid rock, which formed, respectively, deposits of pumice and dense rock fragments. Although pumice is readily recognized as a product of an eruption, it is so easily transported by wind that care must be taken to determine whether or not a specific deposit was actually erupted by Mount Rainier volcano. For example, some layers of pumice in the park were erupted by Mount Rainier, but others were brought in by wind from other Cascade volcanoes (table 1). Air-laid deposits consisting of large fragments of volcanic rock are also evidence of eruptions, but fine-grained deposits of similar material can also originate as windblown sediment derived from moraines and alluvium and from dust clouds that accompany large rockfalls.
The various pyroclastic deposits form thin but widespread layers that are interbedded locally with organic material and elsewhere with other deposits such as debris flows. Radiocarbon age determinations of the organic material permit age bracketing of the pyroclastic deposits by absolute dates. Some debris flows have been dated directly by radiocarbon; others, by their stratigraphic relation to pyroclastic deposits that have been dated elsewhere.
The most extensive and youngest known pumice layer erupted by Mount Rainier is layer C (table 1), which is widely distributed east and northeast of the cone. Crandell, Mullineaux, Miller, and Rubin (1962) estimated its age to be 1,000-3,000 years. Subsequently, wood fragments from above and below the layer near Mystic Lake (fig. 2) were found to have radiocarbon ages, respectively, of about 2,000 and 2,450 years. (All the radiocarbon age determinations cited in this report were made in the laboratories of the U.S. Geol. Survey under the supervision of Meyer Rubin.) In addition, wood fragments from above and below the layer at Huckleberry Park were found to have ages, respectively, of about 1,500 and 2,350 years. Thus, layer C is approximately between 2,000 and 2,350 years old. Crandell and Waldron (1956) described the layer in a measured section on the north east side of Mount Rainier as brown and light-gray pumiceous cinders, and others (Hopson and others, 1962; Fiske and others, 1963) have described it as coarse-grained tawny-brown to light-gray pumice that crunches underfoot.
Layer C forms a mantle as much as a foot thick at the northeast base of the volcano; it diminishes in grain size and thickness northward and eastward and extends beyond the boundaries of Mount Rainier National Park. The layer contains pumice lapilli as large as 2 X 3 X 4 inches, at the east end of Burroughs Mountain, as well as angular fragments of dense gray andesite 1-2 inches in diameter that apparently were erupted at the same time. Fragments of pumice as large as 1 foot in diameter have been found near the west end of Goat Island Mountain. On the south and west sides of Mount Rainier, layer C is limited to the immediate flanks of the conea distribution that suggests southwesterly winds during the pumice eruption.
Layers D, L, and R (table 1) also were erupted by Mount Rainier and seem from preliminary field and laboratory studies to be similar to layer C in thickness, grain size, and mineral content. Layers D and L are thickest and most extensive directly east and southeast, respectively, of the volcano. Layer R seems to have nearly the same pattern of distribution as layer C but is not nearly so widespread.
At some time between 5,000 and 6,600 years ago, as dated with respect to the Osceola Mudflow and layer O (table 1), the Yakima Park area (fig. 2) was blanketed by a few inches to as much as 3 feet of explosion rubble that consists of angular rock fragments in a matrix of reddish-brown sand and silt (fig. 4). The fragments are as large as 1.5 feet in diameter and are derived from the volcano. A thin rubble of comparable rock type lies on the summit and upper flanks of Goat Island Mountain. The distribution and composition of the rubble, plus the fact that no new pumice or scoria seems to be associated with it, indicate that the rubble probably originated in one or more steam explosions that blew out part of the northeast flank of Mount Rainier.
TABLE 1.Postglacial eruptions, debris flows, and avalanches at
Note: The North Puyallup River valley and the North and South Mowich River valleys have not yet been studied. Italics indicate debris flows of very large volume, as shown by long distance of travel or by temporary filling of valley to depth of hundreds of feet. Dating of most ash layers is based on radiocarbon age determinations by Meyer Rubin, U.S. Geological Survey, of inter-bedded organic deposits (Crandell and others, 1962). Age of other events is based on the stratigraphic relation of their deposits to pyroclastic deposits and to the Osceola Mudflow
Layer Y, which is locally nearly 2 feet thick and is the most voluminous postglacial pumice deposit in Mount Rainier National Park, came not from Mount Rainier but from Mount St. Helens volcano, 50 miles to the south-southwest (Crandell and others, 1962; Mullineaux, 1964). Layer Y underlies layer C and is between about 3,000 and 3,500 years old. Layer Y apparently is the sand- to granule-size pumice deposit that was described by Hopson, Waters, Bender, and Rubin (1962, p. 641) as the main ash fall that forms the "major part of the youngest ash blanket from Mount Rainier"; they believed this deposit to be no more than 600 years old. Our interpretation that layer Y is a product of Mount St. Helens is based chiefly on the fact that this deposit coarsens and thickens to the south-southwest toward that volcano. A few miles northeast of Mount St. Helens the layer, as much as 10 feet thick, consists of pebble- and cobble-size pumice lumps.
Layer W (table 1), like layer Y, coarsens and thickens to the south-southwest, its thickness ranging from 1-3 inches at Mount Rainier to more than 10 feet on the north flank of Mount St. Helens. The pumice of layer W also originated at Mount St. Helens volcano.
Layer O, which forms a discontinuous blanket a few inches thick over the whole park, was erupted by Mount Mazama at Crater Lake, Oregon, about 6,600 years ago (Wilcox, 1965). The deposit from Mount Mazama is the most widespread postglacial ash layer in the Pacific Northwest, blanketing the region from western Washington and Oregon eastward to Montana (Powers and Wilcox, 1964). Wide distribution of the ash from Mount Mazama makes it particularly useful as a marker horizon in postglacial deposits in the park as well as elsewhere in the region.
Future eruptions of pumice from Mount Rainier, even on the largest scale recorded by postglacial deposits, would be troublesome but not catastrophic. This does not mean that a larger eruption could not occurat least once during the time just preceding the last glaciation, for example, a much more voluminous eruption of pumice did occur at Mount Rainier. Nevertheless, the absence of any large pumice eruption in postglacial time and the similarity of recurring small pumice eruptions suggest that a disastrous event of this nature is not likely.
If a future volcanic explosion caused an eruption of previously solidified rock on a scale similar to that represented by the explosion rubble at Yakima Park and Goat Island Mountain, it would be a serious hazard to life within a radius of perhaps 5 to 8 miles from the summit of the volcano. Such an explosion would probably be strongly directional and would affect only a small sector of the volcano.
Last Updated: 28-Mar-2006