Mount Rainier, Washington

Mount Rainier

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.

Underpinnings

Forerunner to Mount Rainier

The Main 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 History

Table 1: Holocene tephras from Mount Rainier, from: Mullineaux, 1974

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).

• Trip Mileage 0.0 - USFS Station in Packwood, Washington.

• Trip Mileage 24.3 - Junction Highway 410 with road to Sunrise.

• Osceola Debris Flow

  Trip mileage (13.9 from junction of Highway 410 with road to Sunrise) - STOP 36: gravel pit, off Road 120: ... Observe a cross section of the Osceola debris flow. The Osceola has a high clay content, presumably from hydrothermally altered rocks on the volcano, her here 50-60% of the clasts were derived from stream gravel and till, no the volcano. Here the upper 1-3 meters of the Osceola is yellow, and the lower 2-4 meters pinkish gray; this reflects ground-water oxidation in the aereated zone and is not a contact. The Osceola here overlies iron-oxide-coated hardpan on outwash gravel of probably Evans Creek age (ca. 18-15,000 years). The hummocky terrain 200 meters northwest of the gravel pit was once thought to be a separate lahar (the Greenwood lahar [Crandell, 1971]) but is now considered a marginal facies of the Osceola (J.W. Vallance, oral commun., 1988).

  Trip mileage 49.6 - STOP 39: Sunrise: ... Flag pole at Sunrise, 1950 meter elevation. The 10-12 centimeter-thick layer of crunchy brown pumice belongs to tephra layer C, erupted from Mount Rainier 2,200-2,300 years B.P. (Mullineaux, 1974). Walk to Emmons Nature Trail Overlook above White River for view of the source area of the Osceola debris flow (2 cubic kilometers) that poured down the White River and West Fork White River and spread across more than 260 square kilometers, mostly in the Puget Sound lowland, 4-70 kilometers from the volcano (Crandell and Waldron, 1956; Crandell, 1971). Logs in the Osceola give a bristlecone-corrected Carbon-14 age of about 5,700 years B.P. (Crandell, 1969, 1971).

  The Osceola everywhere has a high content of clay (6-12% [Crandell, 1971]), presumably derived from hydrothermally altered rocks high on the volcano. The source area is now hidden beneath the broad upper part of the Emmons and Winthrop Glaciers and possibly includes material removed from a former summit region above 4,250 meters (Crandell, 1963b; Crandell, 1971). As Crandell and Waldron (1056) pointed out, "the very existence of this broad expanse of ice suggests the possibility that the ice occupies a depression that resulted from the explosive destruction of part of the volcano, or from eruptive action followed by partial collapse of this area." The Osceola occurs on the ridgetops above Glacier Basin and on top of Steamboat Prow (2,957 meters), so clearly it originated even higher on the volcano (Crandell and Waldron, 1956; Crandell, 1971). A remnant of the debris flow is visible from here as a terrace along Inter Creek just upstream form the White River. Conceivable the debris flow could have been generated by collapse of the altered flank of the cone without accompanying eruptive activity. Whether an eruption occurred is uncertain. Tephra layer F (Mullineaux, 1974) from Mount Rainier is clay-rich like the Osceola and ha a corrected Carbon-14 age of 5,700-5,800 years; it has been found neither above nor below the Osceola, however, perhaps owing to unfavorable wind directions. A reasonable supposition is that layer F records phreatic activity preceding or accompanying generation of the avalanche and resulting debris flow. The Osceola is 2-7 meters thick along its margins but presumably much thicker in its center. Remnants of the debris flow on the sides of the White River and West Fork White River valleys show that the thickness aw locally at least as much as 150 meters while the debris flow was moving. A three-dimensional model in the visitor center helps to visualize the geometry and enormity of the Osceola. A measure of its size is the profound effect it had on Puget Sound more than 100 kilometers from the volcano; the shoreline was displace seaward 27-50 kilometers as more than 460 square kilometers of new land was created!