DESCRIPTION:
Paradise Lahar, 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. ...

Post-glacial deposits at Mount Rainier are dominated by lahars; over 60 have been identified. Although relations between Holocene tephra and flowage deposits remain speculative, at least some lahars were probably eruption induced, most notably the Paradise lahar and the Osceola Mudflow, which has been dated at 5,040 Carbon-14 years B.P., had a volume >10^9 cubic meters, and a profound geomorphic effect on the Puget Sound shoreline, over 100 kilometers from the mountain. As interpreted from well logs (neglecting minor relative sea-level changes), syn- and post-Osceola sedimentation has pushed the shoreline seaward 25 and 50 kilometers, respectively, in two Puget Sound embayments and added approximately 460 square kilometers of new land surface. ...

The northeast part of Mount Rainier slid away about 5,600 years ago as part of a catastrophic collapse similar to, but much larger than, that of May 18, 1980 at Mount St. Helens. Debris from this collapse created the Osceola and Paradise mudflows that traveled down the White and Nisqually Rivers, reaching Puget Sound and pushing out the shoreline by as much as several miles. The scar from this collapse was a horseshoe-shaped crater, about 1.25 miles wide, open to the northeast. Since the collapse, lava flows and avalanches of hot lava fragments have erupted from the crater and largely filled it, forming the present summit cone of Mount Rainier.

Ridges and valleys between Panorama Point and Ricksecker Point are veneered with a yellowish-orange deposit of angular to subrounded rock fragments in a plastic matrix consisting of sand, silt, and clay. Although originally the deposit was informally designated as the Paradise debris flow (Crandell, 1963a), its name is changed here to Paradise lahar because of the textural variability within it. The lahar is younger than pyroclastic layer O and predates layer D; it was formed some time between about 5,800 and 6,600 years ago.

Debris flows are one of the most common and devastating geologic phenomena in the postglacial history of the volcano. The largest debris flows from Mount Rainier probably originated in volcanic explosions that caused large-scale avalanching of rock debris. Other debris flows were caused by such factors as heavy rainfall and rapid snowmelt, which are unrelated to volcanism; their occurrence at Mount Rainier results from the availability there of large quantities of loose rock debris on steep slopes. ... The largest postglacial mudflow, which is estimated to have had a volume of nearly half a cubic miles, is the 5,000-year-old Osceola Mudflow ...

At about the same time, a similar avalanche and debris flow swept down and across Paradise Park and Paradise Valley on the south side of the volcano and temporarily filled the Nisqually River valley at the site of Longmire to a depth of at least several hundred feet (Crandell, 1963a). ...

It should be noted that the larger debris flows from Mount Rainier apparently did not form permanent fills in valleys to the maximum height of their remnants on the valley walls. Instead, these remnants probably mark transient flow crests, analogous to those of stream floods.

Paradise Lahar - Crandell, 1971

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