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. 160 to 40 ka - Little Tahoma – Reduced eruptions and erosion of the upper mountainĮruption rates waned gradually after 160,000 years ago, and erosion removed much of the upper north and south flanks reducing the summit elevation. Heightened magma input rates created dikes that intruded into the upper west and east flanks and fed large lava flows from flank vents. 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. Andrews Park, Klapatche Point, and Sunset Park, and on the east at Meany Crest. 280 to 160 ka - Mowich – Rapid accumulation of lavaĮruption 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. Because of infrequent eruptions, the upper mountain was probably eroded substantially during this time. Visitors get a dramatic view of this ridge from Longmire, where they observe it from the valley floor once filled by glacier ice. An exception is the Rampart Ridge lava flow, which erupted 380,000 years ago. Geologists know less about this time period in Mount Rainier's history because infrequent and small eruptions left little evidence within the geologic record. 420 to 280 ka - Rampart – Reduced rate of lava accumulation The side of the lava flow forms the high cliff above the White River campground. 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 pyroclastic-flow deposits are capped by thick lava flows, such as at Burroughs Mountain, Grand Park, and Old Desolate. 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. 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. (Public domain.)ĥ00,000 years ago marks the beginning of the voluminous and continuous volcanic rock record that we call Mount Rainier.
Ka indicates thousands of years before present. Map shows simplified geology of Mount Rainier volcano, Washington.