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Formation and Early Activity
Mount Rainer started forming 500,000 years ago during the Pleistocene era. The volcano was incredibly active at this time, with lava flows rapidly accumulating. It was during this period the high peak was formed, and the pyroclastic (lava rock) deposits suggest the volcano grew to its present height quickly between 500,00 and 420,000 years ago. Lava flows during this era cooled to create formations like the Burroughs Mountain, Grand Park, and Old Desolate.
Burroughs Mountain Trail
Rampart Ridge
Declining Activity (420-280 thousand years ago)
During this period the volcano became much less active. It had very infrequent eruptions, and the eruptions it did have were small. Because of this, there is very little geologic evidence of this period apart from Rampart Ridge (formed ~380,000 years ago). The Ridge cooled quickly as lava hit the walls of a glacier, which caused the smooth texture of the trail.
Fluctuating activity (280-11 thousand years ago)
280 thousand years ago volcanic activity picked up, but it did not reach the activity levels of its early formation. Eruptions were much more frequent and larger than they were previously. Lava flows from this period created Klapatche Point, Sunset Park, and Meany Crest. Starting 160 thousand years ago, activity decreased, and flows from this time created Little Tahoma Peak. 40 thousand years ago, the eruption rates began to slightly increase again.
Meany Crest
From 11,000 years ago to now, Mt. Rainier's Volcanic activity can be broken down into four main periods: Sunrise, Cowlitz Park, Osceola, and Summerland. Eruptions in this timeframe shaped the park through deposited lava flows and lahars/mudflows. You can learn more about lahars/mudflows by clicking here
Image showing the stratigraphic summary of the prominent deposits from this era
Sunrise
11,000 years ago
Multiple eruptions spread ash and volcanic rock across the eastern side of the park. At least one major lahar (mudflow) moved through Van Trump, Paradise Parks and Nisqually River.
Cowlitz Park
7,400-6,700 years ago
At least 4 major eruptions happened during this time. Hot volcanic debris interacting with snow and ice caused lahars and flows during this period.
Osceola
5,600-4,500 years ago
Eruptions during this time caused the Osceola Mudflow, which is one of the most significant events in the park's history. The mudflow travelled over 40 miles and reshaped the Pudget Sound region of the park
Summerland
2,700-2,000 years ago
In this time period eruptions resumed again. These eruptions reshaped the summit of Mount Rainier and deposited lava flows that now are under glaciers.
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