We know more about the recent volcanism at Mount Rainier because deposits postdate extensive glaciation and therefore are well preserved. Eruptions group into 4 eruptive periods and several isolated episodes between about 11,000 years ago and now.
11,000 years ago - Sunrise eruptive period
Several eruptions deposited volcanic ash and pumice across the eastern portion of Mount Rainier National Park. At least one lahar, apparently of landslide origin, moved across Van Trump and Paradise Parks and then down the Nisqually River. Tephra layers indicate isolated eruptions a thousand and more years after the chief eruptive events.
7,400 to 6,700 years ago – Cowlitz Park eruptive period
A number of tephra layers in alpine meadows east of the volcano indicate a succession of eruptions, at least 4 of which were moderately explosive, distributed pumice and ash downwind, and produced lahars that descended the White and Nisqually River valleys. Interaction of hot pyroclastic rock with snow and ice probably caused three or more lahars, one of which traveled along the White River and South Prairie Creek as far as Orting 70 km (43 miles) downstream. Around 7,300 years ago, an eruption produced a thin ash layer and caused a collapse of hyrdothermally altered rock on the south flank of the volcano. The collapse generated the Reflection Lakes lahar that swept across the Paradise area and down the Nisqually River valley; the lahar also spilled over a low divide in Mazama Ridge and moved as far as Reflection Lakes. Deposits of the lahar now crop out in cuts along Skyline and Golden Gate trails above Panorama Point and nearly to the crest of Mazama Ridge.
5,600 to 4,500 years ago – Osceola eruptive period
One to two hundred years prior to the Osceola Mudflow, a portion of the edifice collapsed to produce the Paradise lahar, which swept down the mountain's south side along the Nisqually River at least as far as the village of National. Deposits now crop out in the Paradise area but not to such high levels as those of the previous Reflection Lakes lahar.
About 5600 years ago Mount Rainier's seminal postglacial event occurred on its northeastern flank and summit when an eruption triggered an edifice collapse of weakened hydrothermally altered rock that contained sufficient widely dispersed water from the hydrothermal system to generate the enormous, 4 km3 (1 mi3) Osceola Mudflow. The lahar washed across Steamboat Prow and Glacier Basin and then ran up to about the 6400-foot level of Goat Island Mountain and Sunrise Ridge. It then descended the White River valley 80 to 150 m (260- 490 ft) deep, spread out over 210 km2 (82 mi2) of Puget Sound Lowland 70-100 km (44-62 mi) from source, and flowed into Puget Sound, moving underwater up to 20 km (12.4 mi) to the present sites of Tacoma and the Seattle suburb of Kent. The contemporaneous phreatic and phreatomagmatic explosive eruptions blew hydrothermal clay and mud northeastward across Sunrise Ridge and spread pumice across an arc from south to northeast of the volcano. The Osceola edifice collapse left a horseshoe-shaped crater open to the northeast at Mount Rainier, much like the open crater formed at Mount St. Helens in 1980.
After the Osceola Mudflow, the White River abandoned its old course flowing westward along South Prairie Creek and established a new channel flowing northwestward across the lowland to Puget Sound. Much of the Duwamish arm of Puget Sound was filled in as a result of post-Osceola erosion and sedimentation. The upper White River system responded with tens of meters of aggradation. At the volcano, numerous subsequent, thin tephra layers suggest sporadic cone building during a period of several hundred years.
2,700 to 2,000 years ago – Summerland eruptive period
About 2,700 years ago Mount Rainier began to erupt again, producing tephra and lahars that flowed northeastward into the White River valley. Within a few tens of years, resumed eruptions generated tephra, lahars to the northeast, and a landslide-induced laharcalled the Round Pass mudflow, which swept westward into the Puyallup River drainage and the Nisqually River drainage via Tahoma Creek. Continued growth of the edifice during this period opened previously cutoff drainages to the west and south to flows originating from the summit. A sequence of tephra falls, pyroclastic flows and lahars followed, indicating continued eruptions separated by time intervals ranging from several tens to hundreds of years. Next Summerland lava flows descended from the summit area's west crater and now underlie much of the upper Emmons, Winthrop and Tahoma Glaciers. The upper portion of Camp Schurman sits atop lava erupted from the summit at this time. The largest Holocene tephra eruption (Layer C: ∼0.1 to 0.2 km 3 or 0.02 to 0.04 mi3) occurred about 2,200 years ago. It covers areas near Sunrise visitor center, as well as Burroughs Mountain. Lahars caused by the layer-C eruption descended the White, Cowlitz, and Nisqually River drainages. Summerland lahars and associated sediment accumulated to depths of several meters thickness at sites as far downstream as Auburn and downstream along the Duwamish River toward Elliott Bay. Lava flows that rim Mount Rainier's east crater, Columbia Crest, followed the eruption of layer C. By the end of the Summerland eruptive period the summit of Mount Rainier had grown to its present form and altitude.
1,500 years ago – Twin Creek eruptive episode
Several thin ash deposits restricted to sub-alpine meadows and far-traveled lahars of the Twin Creeks assemblage show that small eruptions took place about 1,500 years ago. Although these events involved the eruption of new magma, no lava flows were produced.
1,100 to 1,000 years ago – Fryingpan Creek eruptive period
About 1,100 years ago, one or possibly two lahars flowed down the White River as far as Auburn. These lahars may have been generated as pyroclastic flows descended the Emmons and Winthrop Glacier and incorporated and melted snow and ice. Reworking of the lahar sediment caused extensive aggradation of the Duwamish River valley as far as Puget Sound and the southernmost Seattle suburbs. No lava flows were erupted.
500 years ago – Electron Mudflow
Around 500 years ago, an avalanche of hydrothermally altered rock from the west side of Mount Rainier caused a lahar known as the Electron Mudflow (0.26 km 3 or 0.06 mi3) that swept down the Puyallup drainage at least as far as Sumner. There is no evidence to indicate whether an eruption or some other event triggered thelahar. Somewhat younger lahars descended the Nisqually and White River drainages at about this time.
1840's to 1890's – Steam explosions?
Early pioneers to the Puget Sound region wrote of occasional dark clouds at the summit of Mount Rainier that they interpreted as small eruption plumes. The most complete reports came in November- December 1894 when the Seattle Post-Intelligencer and Seattle Press-Times printed accounts of steam and "black smoke" rising from the summit. These accounts were not, however, universally accepted, with a competing newspaper, the Seattle Telegraph, discounting the reports. The Seattle Post- Intelligencer supported an expedition to the mountain in late December 1894. On December 24th expedition members reached the northern foot of the mountain where they reported seeing small plumes of white steam and a single darker plume rising slowly from the distant summit, but a clear photograph of the volcano taken from Tacoma on December 29th shows no eruptive activity whatsoever. No tephra or other deposits have been found that would confirm an 1894 eruption, and sparsely scattered pumicepreviously interpreted as due to an eruption between 1820 and 1850 (the X tephra) is now known to be pumice from the widespread 2,200 year old C tephra fall from the Summerland eruptive period, later redeposited by snow avalanches and other processes onto younger glacial moraines. Evidence of 19th century steam explosions or eruptions from the summit of Mount Rainier in the 19th century is both scanty and tenuous.