Mount Rainier is an old volcano, not dead, merely dormant, built up by its own actions, that is, by successive lava flows intermittent with more explosive eruptions that threw out great quantities of pumice and other volcanic ash. The lava, barely fluid, came from a central vent on a rather low base of granite about fifteen miles to the west of the Cascade Range and long after that range was formed. (Fig. 1.)
This lava, largely andisite, did not flow far before solidifying and consequently succeeding flows piled up forming a great cone sixteen or seventeen thousand feet in height with a small crater on the summit. (Fig. 2.)
Several thousand years ago but very recently geologically, during some very violent eruptions some two or three thousand feet of the summit were blown away leaving a crater on top about three miles across. (Fig. 3.)
Due to a very heavy snowfall often totaling more than 100 feet in Paradise Valley, glaciers were formed and for thousands of years they have been rapidly cutting the peak away. As the heaviest snowfall comes around ten thousand feet elevation the ice is thickest there and the cutting greatest so the peak is actually being undermined and is as a result becoming steeper.
Inside the old crater at a comparatively recent date a small cinder cone has been built up on top of which is a distinct volcanic crater today about half a mile across and funnel-shaped. This is the highest point 14,408 feet above sea level. Most of the old crater rim has been broken down by the action of the ice. (Fig. 4.)
As this glacial action continues unless there are subsequent lava flows, the peak will gradually take on the shape of Mount Hood and eventually that of the Matterhorn. (Fig. 5.)
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