Lahars are common at Mount Rainier, because its mantle of snow and ice provides water when melted, and parts of the upper flanks of the volcano contain abundant loose, weak, hydrothermally altered rock.
Hydrothermal alteration occurs where hot, sulfur-rich volcanic gases encounter groundwater. The sulfur gases dissolves into the groundwater creating sulfuric acid that attacks and leaches chemical components from the rock. Commonly, the reactions replace much of the rock with clay minerals that are weak and water saturated. The presence of abundant soft, wet clay aids in mobilizing the collapsed material, allowing it to flow like a liquid. These flows are called lahars, sometimes referred to volcanic mudflows. Additional general information about lahars can be found in the Volcano Hazards Program section on lahar hazards.
Prior to the Osceola Mudflow 5,600 years ago, three notable lahars, each originating as an avalanche that included abundant hydrothermally altered rock, swept down the south flank of Mount Rainier. The younger two of these, the Reflection Lakes and Paradise lahars, prominently blanket the Paradise Park area; and the older and larger of the two topped a low divide in Mazama ridge and spilled into Reflection Lakes basin. The younger Paradise lahar deposited colorful altered rocks with hues of orange, yellow, or red that are visible near the Paradise visitor center. All of these lahars flowed tens of meters (hundreds of feet) deep through the Nisqually River valley system reaching at least 38 km (24 mi) downstream to National, WA.
The Osceola Mudflow of 5,600 years ago was Mount Rainier's signature event during the Holocene. During a period of eruptions, avalanches caused 2 to 3 km3 (0.5 to 0.7 mi3) of mainly hydrothermally altered material from the volcano's summit and northeast slope to slide away. This lahar swept down the west and main forks of the White River, passing the location of current day Enumclaw before reaching the Puget Sound near present-day Auburn. The Osceola collapse left a 1.8 km-wide (1 mi-wide) horseshoe-shaped crater, open to the northeast, almost the same size as the crater produced by the 1980 eruption of Mount St. Helens. Most of the Osceola crater has been filled in by subsequent lava eruptions, most recently about 2,200 years ago.
Osceola deposits cover an area of about 550 km2(212 mi2) in the Puget Sound lowland, extending at least as far as the Seattle suburb of Kent, and to Commencement Bay, now the site of the Port of Tacoma. The communities of Orting, Buckley, Sumner, Puyallup, Enumclaw, and Auburn are also wholly or partly located on top of deposits of the Osceola Mudflow and, in some cases, of more recent lahars as well.
Round Pass Mudflow
Early in the Summerland eruptive period (2,500-2,600 years ago), the Round Pass Mudflow began as a sector collapse of hydrothermally altered rock from Sunset Amphitheater and Tahoma Glacier headwall on Mount Rainier's west flank. This edifice collapse removed a bite-shaped section from the Osceola Mudflow crater, so that subsequent eruptive products could spill west as well as northeast. Just west of Round Pass, deposits of the mudflow crop out 300 m (984 ft) above adjacent valley bottoms. The lahar descended the Puyallup River drainage at least 30 km (19 mi), where it left hummocky-surfaced deposits as much as 30 m (98 ft) thick. It also descended the Nisqually River valley via Tahoma Creek as far as National, WA.
Late in Summerland time, the explosive eruption that generated the C tephra 2,200-2,300 years ago also triggered lahars at the heads of several drainages (White, Cowlitz, and Nisqually), including one named the National Lahar, which moved for at least 100 km (62 mi) down the Nisqually River valley. It is among the largest of many lahars that originated during eruptions when hot rock mixed energetically with snow and ice to form sudden floods of water and debris. As they descend, these sudden watery floods incorporate considerable sediment from debris- mantled slopes and valleys to become voluminous lahars. Energetic mixing of hot rock and ice during eruptions is the most common cause of voluminous lahars at Mount Rainier.
The Electron Mudflow began as an avalanche of hydrothermally altered rock high on Mount Rainier's west flank near Sunset Amphitheater, but its onset cannot be correlated with volcanism. Possible triggers include an eruption so small its tephra is not preserved, hydrothermal explosions, or an earthquake. Regardless of triggering mechanism, the Electron avalanche was aided by hydrothermally weakened and voluminous water-saturated clay-rich rocks west of the summit area. The lahar was highly fluid and flowed 100 km (60 mi) downstream to the Puget Sound lowland. When it entered the Puget Sound lowland in the community of Electron, it was 30 m (98 ft) deep.
The Electron Mudflow reminds us of the possibility that, occasionally, lahars may have non–eruption origins and thus may occur with little conventional volcanic warning. D-Claw computer simulations provide examples of how unheralded lahars of differing sizes move and flow, with detailed information about anticipated depth, speed, and range of movement in the densely developed valleys of the Puyallup and Nisqually rivers that drain the western side of Mount Rainier.