Geological History of Mount Rainier

Mount Rainier

Mount Rainier

The geological history of Mount Rainier goes back thousands and millions of years. For many people a trip to Mount Rainier National Park means a nice day out with the family, some beautiful scenery, perhaps a picnic lunch and then back home in time for supper. Other people like to get into the back of beyond of Mount Rainier National Park and really pit their wits against nature with a little back country camping. Others rise to the challenge of climbing Mount Rainier, it’s tough but it’s worth it. Some like to hike, some like to cycle, some like to ski, some like to ride on horses and some people just like to drive on by.

More current news and information on the Mount Rainier National Park Facebook page.

Mount Rainier National Park is a magical land of dense forests, carpets of dazzling wildflowers, snowfields, glaciers, the deafening crash of waterfalls . . . and towering above it all . . . the mighty Mount Rainier itself. 

There are some people who really want to learn about the geological history of Mount Rainier . . . where did it come from . . . how was it formed . . . has it always been so beautiful?

We’re not talking boring old school stuff here, this is very real and exciting, so if anybody is really interested in the geological history of Mount Rainier, read on . . .

Mount Rainier is quite simply the product of fire and ice. The mountain of Mount Rainier, a volcano born of fire, built up over the years due to successive and repeated eruptions and laval flow. In volcano years Mount Rainier is just a baby, only about a million years old, as opposed to the mountains of the Cascade mountain range which date back around 12 million years and appeared as the result of the Earth’s surface buckling, folding and uplifting periodically. Mount Rainier volcano certainly isn’t an isolated case, there are others like Lassen Peak in California right down in a line to Mount Garibaldi in British Columbia . . . they may be sleeping but could awaken at any time. Mount Rainier is actually considered to be a very dangerous volcano indeed, with large amounts of glacier ice it has the potential to produce massive lahars (aka volcanic mud flows) and threaten the entire Puyallup River valley.

Makes you think don’t it . . .

As fast as the fire is building up these volcanoes, the snow has been busy shaping and forming them. Glaciers are formed from snow which doesn’t arrive and thaw each and every year, but builds up to greater and greater depths. The sheer weight of this snow eventually presses out all air, packing it tightly down and compressing until ice is formed. Gravity begins to play a part pulling these great ice sheets down the mountainside, freezing and thawing, breaking off rocks and carrying them along . . . these forces are way beyond our control and continue to shape and re-shape the Mount Rainier which we know and love today – make the most of it, it could be gone in another million years or so.

The Making of Mount Rainier

We’ve already established that Mount Rainier is a volcano, and a very potentially dangerous one at that, but don’t imagine that it appeared overnight, ‘cos that ain’t how it happened at all.  Mount Rainier was not born out of one great eruption, but is the result of very many years of volcanic activity. Geologically speaking it is a stratovolcano, which means that it has been built up with layers (or strata) of lava flows and eruptions of rock and ash.

  • Although volcanoes had been erupting in the area for around 40 million years, only 500,000 years ago (give or take a couple of years) molten rock was thrust through a weak spot in the earth’s crust, the hole oozed with lava, rock and pumice were spewed out and a volcanic cone was born. It’s estimated that Mount Rainier at this time was around 16,000 feet above sea level (it’s a little less than 14,500 feet at the moment).
  • 5,700 to 6,600 years ago, violent explosions and eruptions within Mount Rainier caused the mountainside to collapse with tons of mud, rock and other debris sent rolling down the northeast flank. They say that a wall of mud 100 feet high cascaded like wet cement across 125 square miles . . . this mudslide, known as the Osceola Mudflow was where the towns of Kent, Auburn, Sumner and Puyallup now stand.
  • Since that time, small and medium sized eruptions have occurred at Mount Rainier every few hundred years or so. The last large eruption was around 1,000 years ago , but a smaller eruption happened as recently as 150 years ago.
  • The last known eruption of Mount Rainier happened in 1894 when a minor emission of smoke and ash was documented and confirmed by a climbing party. Only time will tell when the time will come for Mount Rainier to erupt again, spewing ash and steam and causing floods and mudflows. No wonder scientists and geological types are keeping an eye on Mount Rainier.


Don’t worry, it’s not real . . . yet!