Mount Rainier, highest (4,392 meters (14,410 feet)) and third-most voluminous volcano in the Cascades after Mounts Shasta and Adams, dominates the Seattle-Tacoma area, where more than 1.5 million know it fondly as The Mountain. The Mountain is, however, the most dangerous volcano in the range, owing to the large population and to the huge area and volume (92x10^6 cubic meters and 4.4x10^9 cubic meters, respectively of ice and snow on its flanks that could theoretically melt to generate debris flows during cataclysmic eruptions. In addition, sector collapses of clay-rich, hydrothermally altered debris have generated at least three huge (>2x10^8 cubic meters) debris flows in the last 5000 years. Yet surprisingly little is known of Mount Rainier's eruptive history, composition, or age. For example, probably fewer than two dozen chemical analyses of Rainier's products have been published. Probably the dominantly nonexplosive nature of past eruptions and the challenging logistics of studying the cone contribute to the relatively limited knowledge. Outstanding work, however, has been completed on its fragmental deposits, and most of what is known about the volcano derives from this work.
UnderpinningsMount Rainier is underlain by middle Tertiary volcanic rocks of the Ohanapecosh, Stevens Ridge, and Fifes Peaks Formations (Fiske et.al., 1963; Vance et al., 1987), described elsewhere in this guide. These rocks were gently warped along a northwest-trending system of folds and intruded by the Tatoosh pluton, chiefly granodiorite and quartz monzonite. The main body of the Tatoosh is 17.5-14.1 million years, but dikes, sills, and various volcanic deposits interpreted as forerunners to the emplacement of the pluton to its final level are as old as 26 million years, judging from U-Pb dating of zircons (Mattinson, 1977). Fiske, et.al. (1963) interpreted the Tatoosh to have "broken through to the surface" several times, giving rise to eruptions such as that which created The Palisades, a 250-meter-high cliff of 25-million-years-ago silicic welded tuff 5 kilometers northeast of Yakima Park (Mattinson, 1977).
Forerunner to Mount RainierNo evidence has been found for magmatism near Mount Rainier between about 14 and 3 million years. The Lily Creek Formation, a thick sequence of debris flows and related volcanic deposits (Crandell, 1963a), crops out just west of the volcano and was hypothesized by Fiske et.al. (1963) to have been erupted from the Tatoosh. However, Mattinson (1977) dated the Lily Creek as no older than 2.9 million years and therefore agreed with Crandell (1963a) that it likely was formed during the earliest activity of Mount Rainier or from a center that just preceded the cone. Stratigraphic relations with glacial deposits suggest that Lily Creek volcanism began before 0.84 million years (Easterbrook, et.al., 1981; Smith, 1987). Hornblende occurs in juvenile clasts of the Lily Creek but not in rocks from Mount Rainier (Fiske, et.al., 1983). No chemical analyses have been published for the Lily Creek.
The Main ConeMount Rainier was built on a rugged surface with more than 700 meters of relief eroded mainly into the Tatoosh pluton and the Stevens Ridge Formation. Early andesite flows from the volcano, undated but presumably several hundred thousand years old, were channeled along deep canyons, some of which were oblique to the present radial drainage pattern. The eruptions were apparently frequent, because in places one flow rests on the undissected surface of its predecessor. Laharic deposits and till locally occur between the early flows. Eventually the flows stacked up to form a mound near the main vent that became the foundation of the present cone.
Most of Mount Rainier's cone was built by hundreds of thin lava flows interbedded with breccia and minor tephra. The flows are rarely more than 15 meters thick high on the cone, where they drained down the steep slopes. They thicken near the base, and flows more than 60 meters thick occur on the apron around the cone. Some flows entered canyons radial to the volcano. Much breccia on the cone was probably derived from moving flows, but some probably reflects explosions and lahars. Radial dikes are prominent in places; possibly they fed some of the flows. The flows and dikes are petrographically uniform two-pyroxene andesite; the few chemical analyses available are medium-K silicic andesite, with three analyses marginally dacitic. The flows and breccia eventually built a cone standing 2,100-2,400 meters above its surroundings before the end of the latest major glaciation about 10 thousand years ago.
A thick pumice layer northeast, east, and southeast of the volcano may have been erupted from Mount Rainier between 70 and 30 thousand years ago. Estimates from limited outcrops suggest it is an order of magnitude more voluminous than any of the volcano's Holocene tephra layers.
Two late Pleistocene vents, Echo Rock and Observation Rock, erupted olivine-phyric basaltic andesite near the northwest base of the cone after Mount Rainier was almost fully grown. The basaltic andesite is more mafic than the cone-building flows.
Smith (1987) estimates that about 270 cubic kilometers of lava was erupted from Mount Rainier in the last 1 million years.
Postglacial Eruptive HistoryEleven tephra layers record evidence of Holocene explosive volcanism at Mount Rainier (Mullineaux, 1974). Eight of the tephras fell between 6500 and 4000 carbon-14 years B.P (before present). Only one tephra-producing eruption, between about 1820 and 1854 A.D., is known from the last 2,200 years; it was very small and left a scanty deposit that could easily be overlooked if it were older. Chemical analyses indicate that layer D is basaltic andesite and layers L and C are silicic andesite.
The tephra layers rich in lithic fragments are probably products of phreatic or phreatomagmatic eruptions. Layer F contains 5 to 25% clay-sized material, as much as 80 percent of which locally consists of clay minerals, chiefly montmorillonite, that was derived from altered rocks within Mount Rainier. Layer F and the Osceola debris flow have similar clay contents and ages, and are likely correlative. However, layer F has not been found on the Osceola and so could be slightly older.
Layer C, the most widespread and voluminous of the Mount Rainier tephras, covers the east half of the National Park with 2-30 centimeters of lapilli, block, and bombs. Overall it is the coarsest of the tephras, with 25-30-centimeter bombs 8 kilometers from the summit. A block-and-ash flow in the South Puyallup valley west of the volcano contains blocks emplaced above the Curie isotherm and charcoal dated at 2350 +/- 250 carbon-14 years. Its age and lithologic similarity suggest correlation with layer C.
Isopachs and isopleths for layer C indicate an origin at the summit of Mount Rainier, yet the layer does not occur on snow-free parts of Columbia crest cone, a young andesite cone standing 250 meters above the former summit of the volcano. Columbia Crest cone is therefore younger than about 2200 years. Crandell (1971) found numerous lahars and flood deposits in valleys surrounding the mountain that postdate layer C but predate layer Wn (1480 A.D.) from Mount St. Helens; some of the flowage deposits have carbon-14 ages older than 1000 years B.P. The eruptions that formed Columbia Crest cone likely produced some of those deposits. If so, the layer-C explosions might have initiated activity that formed Columbia Crest, and the cone would be about 2000 years old.
Crandell (1971) identified more than 55 lahars and debris flows of Holocene age from Mount Rainier. At least some were probably associated with eruptions, most notably the Paradise lahar (possibly associated with tephra layers A, L, or D) and the Osceola debris flow (layer F). The Osceola is described in the road log for Mount Rainier (see below). In general, Crandell (1971) associates lahars that lack much clay at Mount Rainier with magmatic eruptions, and those that contain much clay with phreatic or phreatomagmatic eruptions or with collapses of the hydrothermally altered edifice. Glacier-outburst floods from Little Tahoma Glacier, typically in late afternoon of warm days or after heavy rain, repeatedly scoured Tahoma Creek in the late 1960's and the middle and late 1980's. Outburst floods frequently modified many other drainages, most notably Kautz Creek and Nisqually River, during historical time.
About 20 small earthquakes occur yearly at Mount Rainier, more than at other composite cones in the Cascades except Mount St. Helens (Malone and Swanson, 1986). Trilateration and tilt networks established in 1982 indicate no definite deformation. Seven significant thermal areas above 3350 m on the volcano, including one at the summit, reflect "a narrow, central hydrothermal system ... forming steam-heated snowmelt at the summit craters and localized leakage of steam-heated fluids within 2 kilometers of the summit" (Frank, 1985).
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