At least 22 layers of tephra--volcanic ash and coarser airfall pyroclastic debris--can be recognized among the postglacial deposits in Mount Rainier National Park. Each of these tephra layers records a separate eruptive event. Eleven of the layers were erupted from Mount Rainier; 10 originated at Mount St. Helens volcano 80 km (50 miles) south-southwest of Mount Rainier; and the other came from prehistoric Mount Mazama at the present site of Crater Lake, Oreg., 440 km (275 miles) south of Mount Rainier. Two tephra beds, layer 0 from Mount Mazama (6,600 years old), and layer Yn from Mount St. Helens (3,400 years old) covered the entire park. These two form highly conspicuous light-colored layers among the darker tephra erupted from Mount Rainier. They are especially valuable marker beds that delineate the boundaries of three nearly equal segments of the ap- proximately 10,000 years of postglacial time. Layer Yn, locally more than 30 cm (12 in.) thick, is the most voluminous tephra deposit in the park. Only one other tephra bed from Mount St. Helens is conspicuous, layer W, about 450 years old; the others are thin and obscure.
All but one of the tephra layers erupted from Mount Rainier contain abundant vesicular fragments that were formed by explosive eruptions of molten lava. The thickest and coarsest three layers from Mount Rainier (layers L, D, and C) form well-defined lobcs that cxtend east of the summit vent; layers L, D, and C are nearly absent in the western part of the park. All three, where thick, are conspicuous deposits that can be recognized in a variety of environments. Seven smaller, generally thinner deposits form less well defined lobes east of the summit and are recognized only where exposures are relatively good. One of these smaller deposits is the only layer composed wholly of solid rock fragments and, s5, does not record eruption of molten lava. Another one, which was erupted early in the 19th century, represents the most recent eruption of Mount Rainier that resulted in a recognizable deposit.
One other layer from Mount Rainier (layer F) is a widespread, generally thin ash that consists of three overlapping ash beds. In contrast to most of the tephra layers, which contain little or no clay minerals, the lower and upper beds of layer F are rich in montmorillonitic clay. The middle bed, however, consists of mineral crystals and pumice and contains little or no clay. Layer F was laid down at the time the Osceola Mudflow originated at Mount Rainier. The eruptions that formed layer F may have triggered the mudflow. Color, size, thickness, gross composition, and stratigraphic position permit field iden- tification of most of these layers where the sequence is fairly complete. The content and refractive index of iron-magnesium minerals, however, are particularly useful for initially establishing the sequence of beds present and for identifying beds that occur alone. Three mineral pairs are especially common: (1) hypersthene and augite, generally occurring with olivine and less commonly with hornblende; (2) hypersthene and hornblende; and (3) cummingtonite and hornblende. The hypersthene-augite suite is found in all tephra from Mount Rainier; olivine is absent in only two of those layers, and hornblende occurs in three. The other two pairs of minerals commonly occur without other iron-magnesium minerals and are characteristic of Mount St. Helens ash layers.
The tephra record shows that Mount Rainier has erupted at very irregular intervals throughout the last 10,000 years. The earliest postglacial eruption was more than 8,750 years ago, then the volcano was relatively quiet until about 6,500 years ago. It was especially active between about 6,500 and 4,000 years ago, when 8 of the 11 known tephra layers from Mount Rainier were ejected, including 4 of the 6 most voluminous deposits. No eruption of Mount Rainier is recorded in the tephra sequence between 4,000 and 2,500 years ago. The most voluminous postglacial tephra deposit was erupted between 2,500 and 2,000 years ago. Since that time, the 19th-century eruption is the only one known to have produced a tephra layer.
Large tephra deposits from Mount Rainier that consist chiefly of pumice and scoria probably resulted from eruptions that were preceded by the rise of molten rock into the volcano from a considerable depth. Such an upward movement of magma expectably would be accompanied by earthquakes and by increased flow of heat and gases from the volcano. Thus, an initial explosive eruption probably would be preceded by recognizable signs. A pyroclastic eruption that ejected only fragments of previously solidified rock, however, might occur with no preliminary movement of magma, and perhaps without even significantly increased heat or gas emission. Such an eruption might occur without any kind of recognizable preliminary events.
Hazards from pyroclastic eruptions include the effects of impact and accumulation of tephra fragments, of volcanic gases, and of secondary landslides, mudflows, and floods set off by the eruptions. Eruptions of molten material, because they probably would be preceded by warning signs, are potentially less dangerous to persons than nonmagmatic eruptions that might occur with no warning.
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