Geology of Mount Rainier
MPG V2.8, January 2013

What can I say that hasn't already been said, that isn't flowery hype about the mountain and National Park's beauty or isn't over technical with the details of its geologic history? Truthfully, not much beyond what I think is important to know or interesting about the mountain and the NP. I've been hiking the NP on and off since 1977, mostly from the early 1990's to today, and reading some of the general liternature on the mountain and NP, I've found there is much that hasn't been said in the books which I think helps one to understand the mountain.


Since Mountain Rainer was originally "discovered" and described in literature in the journals of northwest explorers, quite a bit has been written about its majesty and magnificence, especially during the period before and after the effort to establish the mountain as a national park. And while there are incidental descriptions of the geology of the mountain, it wasn't until the explorations during the 1880-90's that the mountain and surrounding area came into scientific view.

Beyond the USGS expedition in 1896 and the occasional observation of visiting geologist and the observations of the Nisqually Glacier, nothing much was done conducting serious scientific and environmental research until the 1910-1920's when scientists and rangers began measuring the glaciers and other scientists began the longterm study and inventory of the natural resources of the NP. The importance of this work became clear when a glacial outburst flood occurred on the Nisqually glacier October 14, 1932 which destroyed the Nisqually bridge to Paradise, and other studies were conducted as later floods became the most destructive recent events in the NP.

That's perhaps a generalization but few books outside the scientific and professional publications describe Mount Rainer sufficiently for people to understand its more complex history, structure, and nature. I don't know why this is, it's confusing to comprehend why writers focus on its beauty but not how it got there, to help understand the mountain they see during their visit(s) more completely.

And so I will give a little more, which I find interesting and different from prevailing popular literature about Mt. Rainier. You can get more comprehensive and detailed information from the links below or the books listed in the bibliography.

Interesting Information

Mt. Rainier is defined as a stratovolcano, as are all of the Cascade Mountain volcanoes. Mt. Rainier, though, doesn't necessarily have the full classic shape of most because of its geologic composition and history. While its structure is the result of volcanism, its present day shape is the result of glaciation and mass wasting.

The most obvious fact, as with most of the Cascade volcanoes in Washington but not those in Oregon, is that Mount Rainier does not sit on the crest of the Cascade Mountains, but is 11 miles west of the Cascade crest at Chinook pass This means all the rivers draining Mt. Rainier flow into either the Columbia River (Cowlitz River) or the Puget Sound (Nisqually and Puyallup Rivers), see map of basins.

Mount Rainier is not necessarily a tall volcano. While it's crest is 14,410 feet above sea level, second highest mountain in the conterminous states after Mt. Whitney, it's really only about 6-7,000 feet above the surrounding area and the basement rocks of the Cascade Mountain batholith. This is common for all of the Cascade volcanoes except with their height above the batholith.

The Cascade Mountain Range is a south-to-north upward tilting batholith which is 5-6,000 feet below sea level at Mt Shasta and 6-8,000 feet above sea level at Mt. Baker, crossing sea level underneath Mt. Hood. Much of the time before recent was a series of volcanism, sinking and inundation by seas, and uplight, which went on for millions of years as the Cascade Mountain Range developed and uplifted.

The Mt. Rainier of late isn't old, only a few hundred thousands years, during a period of volcanism and building, before the mountain we see today with all it's features, which is less than 10,000 years old after episodes of volcanism, erosion by stages of Puget Sound and Cascade glaciation and many small to major mass wasting events.

Mt. Rainier is a good example of dynamic geologic processes constantly at work. What you see during your visit will never been seen again. The mountain is in constant flux, the state between the underlying volcanic processes which built it and continues to build and heat it and the current period of glaciation, in constant change growing and shrinking glaciers along with the mass wasting events.

Mt. Rainier, with its frequent small earthquakes monitored by the USGS and the University of Washington Seismology office, is considered an active volcano, but it's unsure if it will enter into another period of significant eruptions or lava flows. They are sure that it is the most potentially dangerous volcano in the Cascade Mountains from eruptions and mass wasting events, especially in proxmiity of the southern Puget Sound development.

Some of the material around Mt. Rainier didn't not originate from Mt. Rainier, but is volcanic material erupted from other Cascade Mountain volcanoes, some from Mount St. Helens and some from Mount Mazama, now Crater Lake. The prevailing wind patterns predominately spreads erupted material on the east side of the mountain, which has the most layers of deposition, but the west side has the thicker layers of material.

What surprises people is that the loss of the older summit wasn't from eruptions similar ot other Cascade volcanoes, but from mass wasting. Mt. Rainier has a geologic problem. It does not have a lot of hard rock inside it and it's been thoroughly penetrated by water over its geologic history. As one geologist said, "It's a mountain of mush.", meaning it's a mountain of very fractured or soft rocks and material with lots of water inside and glaciers outside, and the continuing Pacific Northwest climate with 500-1,000 inches of snow annually.

The summit is two craters with a permanent snowfield overlying a heated core with numerous caves and steam vents. Climbers stranded on the summit overnight have found shelter in the tunnels to survive, barely as the steam is hot and soaks the clothes and body of the climber and the air temperature is subfreezing. You burned or froze and you dared not go to sleep. And these climbers didn't have the advantage of modern clothing and shelter technology.

Mount Rainier has three types of glaciers. see map. The first are the summit glaciers, those with the head at or very near the summit, such as the Carbon, Tahoma, Winthrop and Emmons glaciers. Second are the lower glacier, which arise secondary to the upper glaciers, such as Mowich and Fryingpan glaciers. And third are the lowest glaciers, arising from the slopes of the mountain, in the 8-10,000 foot level.

As mentioned the current peak of Mt. Rainier wasn't the result of volcanic buildup but the opposite, mass wasting. The peak was once about 16,000 feet above sea level, about 2,000 feet higher than today, when the northeast and east facing slope collapsed and the entire material of the peak and faces flowed down slope and down the White River to as far as the Puget Sound. T his is known as the Osceola Lahar which left the peak we see today.

Much of the lower slopes of Mt. Rainier have been destroyed and/or (re)created by mass wasting processes such as landslides and debris and mud flows, see history of mud and debris flows. Even small ones occur recently, as with the Kautz Glacier in August 2001 (PDF pages 17-19).

In the end Mt. Rainier's beauty is the result of mountain buiding with the Cascade Mountains, the volcanism underlying the Cascade Mountains in the chain of volcanoes from Mt. Shasta to Baker, and glaciation from the recent, in geologic terms, and current climate. This means that what you see during your visit the mountain and the NP is different than it was before and will be different after your visit. Everything you see is unique for the time you're there.

Additional Resources

If you want to learn more, below are some resources, along with the books listed on the books Web page.

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WSR V2.8, January 2013