DESCRIPTION:
Round Pass Mudflow, Mount Rainier, Washington

Mount Rainier, the highest and third most voluminous volcano in the Cascade Range, is potentially the most dangerous volcano in the range because of the large population living around its lowland drainages. These areas are at risk because of the mountain's great relief and the huge area and volume of ice and snow on the cone (92x10^6 square meters, and 4.4x10^9 cubic meters, respectively) that could generate lahars during eruptions. In addition, large (>2x10^8 cubic meters) sector collapses of clay-rich, hydrothermally altered debris from the cone have occurred at least 3 times in the last 6,000 years ( Osceola, Round Pass, and Electron mudflows. ...

Wood from buried treed in the Round Pass Mudflow has been dated at 2,600 Carbon-14 years B.P. This clay-rich diamict is characterized by great thickness (locally >250 meters), limited downvalley extent, hummocky terrain, and megaclasts of lithologically homogeneous material. It probably began as a debris avalanche of hydrothermally altered material from high on the western slopes, and most of it was deposited in the upper 20 kilometers of the Puyallup River valley.

Roadcuts at Round Pass expose a mudflow as much as 25 feet thick, which overlies pyroclastic layer Y and underlies layer W. The color of the mudflow is purplish to pinkish gray where seen in the roadcuts. The deposit is here named the Round Pass Mudflow; its type locality is at Round Pass. ...

Because remnants of the Round Pass Mudflow can be found on the slopes of the volcano at least as far as the east end of Emerald Ridge and to an altitude of nearly 7,000 feet on the ridge that extends from St. Andrews Park to Puyallup Cleaver, I presume that the mudflow originated somewhere on the flank of the volcano upslope from these points. The mudflow's clay minerology suggests derivation from masses of altered and partly altered rock similar to those now present in the east wall of Sunset Amphitheater. The mudflow probably originated as a massive avalanche or series of avalanches of this kind of rock. The distribution of the mudflow as a veneer on older deposits in the valleys close to Mount Rainier indicates that the mud was very fluid and that almost all of it drained away after the crest of the mudflow passed downvalley.

The heights reached by the mudflow suggest that the flow had enough volume to fill temporarily the upper South Puyallup and Tahoma Creek valleys to a depth of at least 1,000 feet, either as a flowing stream of mud or as one or more large waves. The mass of the mudflow needed to form a deep flowing stream of mud probably is adequately represented in the Puyallup River drainage by extensive deposits which probably have a volume of at least 200 million cubic yards in the area west of the park. Because of its water content, the lahar must have had a substantially greater volume while moving. Deposits of comparable volume have not been found in the Tahoma Creek valley, and the mudflow apparently had a depth of only a few tens of feet at a point just 4 miles downvalley from Round Pass. These relations may have resulted from movement of the mudflow down Tahoma Creek valley as a single huge transient wave, which progressively decreased in height, similar to that inferred for the Paradise lahar. Substantial amounts of the mudflow were left only in areas where a low slope did not permit it to drain away; elsewhere veneers only a few thick were left on the sides and floor of the valley. Different volumes of the mudflow in the three valleys may have resulted form slightly different directions of initial downslope movement of successive avalanches from Sunset Amphitheater, which caused unequal amounts of debris to move into respective valleys. Or, the different volumes could have been caused by unequal lateral distribution of rock debris within a single enormous avalanche.

Round Pass Mudflow - Crandell 1971

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