Around 500,000 years ago, Mount Rainier started to grow atop the eroded remains of an earlier ancestral Mount Rainier that was active 1-2 million years ago. The modern edifice grew as a series of four alternating stages of volcanic activity, averaging a little more than 100,000 years duration. During stages of high output lava flows travelled up to 24 km (15 mi) from the summit, and in times of modest volcanic output, including today, the lava flows rarely exceed 8 km (5 mi) from the summit. Glaciers deeply filled the surrounding valleys over most of the volcano's history. The style of volcanic activity remains the same, with no evidence of diminution.
500 to 420 ka - Old Desolate – Rapid accumulation of lava
500,000 years ago marks the beginning of the voluminous and continuous volcanic rock record that we call Mount Rainier. For the first approximately 80,000 years the new volcano was highly active, producing a thick apron of pyroclastic flows that are well exposed above Glacier Basin. These pyroclastic–flow deposits are preserved as high as Steamboat Prow (3,000 m or 9,640 ft) where they dip directly away from the present summit, indicating that the new volcano quickly grew to a height similar to that of today. The pyroclastic-flow deposits are capped by thick lava flows, such as at Burroughs Mountain, Grand Park, and Old Desolate. The Sunrise visitor facilities sit atop the Burroughs Mountain lava flow, the lower end of which is visible around elevation 1,500 m (4,800 ft) along the road to Sunrise, where ice-chilled lava columns jut from the tip of the lava flow. The side of the lava flow forms the high cliff above the White River campground.
420 to 280 ka - Rampart – Reduced rate of lava accumulation
Geologists know less about this time period in Mount Rainier's history because infrequent and small eruptions left little evidence within the geologic record. An exception is the Rampart Ridge lava flow, which erupted 380,000 years ago. Visitors get a dramatic view of this ridge from Longmire, where they observe it from the valley floor once filled by glacier ice. Because of infrequent eruptions, the upper mountain was probably eroded substantially during this time.
280 to 160 ka - Mowich – Rapid accumulation of lava
Eruption rates increased again around 280,000 years ago and persisted to about 180,000 years ago, as shown by extensive lava flows on the flanks of the volcano, including on the west at St. Andrews Park, Klapatche Point, and Sunset Park, and on the east at Meany Crest. The volcano grew to perhaps its greatest height, as shown by 200,000 year-old rocks preserved high on the Mowich Face on the volcano's upper west flank. Heightened magma input rates created dikes that intruded into the upper west and east flanks and fed large lava flows from flank vents.
160 to 40 ka - Little Tahoma – Reduced eruptions and erosion of the upper mountain
Eruption rates waned gradually after 160,000 years ago, and erosion removed much of the upper north and south flanks reducing the summit elevation. At the start of this phase, dikes and vents on the upper east side of the volcano feed the lava flows that constructed Little Tahoma Peak. At about 105,000 years ago new vents opened on the lower northwest flank and lava flows of basaltic andesite (hotter, more fluid magmas than typical for Mount Rainier) spread across the Spray Park and Mist Park areas from cinder cones preserved as Echo and Observation Rocks. A similar eruption occurred 130,000 years ago from a vent far to the north of the volcano at Windy Gap, from which lava flowed against ice filling the present Carbon River valley, producing the lava flow of Bee Flat. These north and northwest flank vents were probably fed from great depth, rather than laterally from the nearby Mount Rainier magmatic plumbing system.
40 to 15 ka - Point Success – Reconstruction of the upper mountain
Eruption rates increased again around 40,000 years ago, though not to the degree of the earlier high-effusion stages. With the exceptions of the Mowich Face and Little Tahoma Peak, most of the upper headwalls and ridges on the volcano were constructed during the period 40,000 to (roughly) 15,000 years ago. These relatively youthful lava flows include those exposed on Liberty Ridge, Willis Wall, upper Curtis Ridge, Gibraltar Rock, Wapowety Cleaver, Success Cleaver, and Tahoma Cleaver, as well as the Tahoma Glacier Headwall. The only flank lava flowfrom this time is that of Ricksecker Point (40,000 years ago) that also supports Narada Falls. This stage of reconstruction of upper Mount Rainier overlaps the most recent period of thick glaciers known as the last major ice age that peaked at 20,000 years ago. The rough ridge and headwall form of the upper mountain results from thin lava flows encountering thick ice, as well as to simultaneous rapid glacial erosion and lava effusion. Growth of upper Mount Rainier is also roughly synchronous with growth of upper Mount Adams (less than 30,000 years old), as well as the beginning of growth of present-day Mount St. Helens (earlier volcanoes were present at that location).
Eruptions occurring after the last ice age are better preserved and greatly expand the understanding of the volcanic history of Mount Rainier. The Holocene eruptive periods were determined from studies of volcanic ash layers where scientists derive the age of ash layers by measuring the age of wood within or adjacent to them. Because of the wealth of knowledge, details about Holocene activity are included as a separate web page.