when did mount rainier form?

Answer 1

Between 840,000 years and 500,000 years ago.

Answer 2

I'm not sure of the actual years, I would guess around 3-4 million years ago.

It formed because the pacific plate was subducting off the west coast of the North American plate. This is true for all other Cascades Volcanoes like Mount St. Helens, The Three Sisters, Mount Shasta, Mount Lassen, and others.

Although the subduction is not as wide spread as it once was, the Mountains are evidence of how large it used to be- extending from Canada into the northern parts of California!

there is still a small piece called the Juan de Fuca plate which is subducting off the coast of Washington state (allowing for activity in St.Helens, and Rainier.


Did you know- scientists believe the most dangerous volcano in the Cascade range is not St. Helens, but rather Mt. Rainier! due to the large amounts of snow on its summit that could form a large mudslide if an earthquake caused it to slide.


this site says it formed 500,000 years ago, oh well only a couple million years off! :o)
http://www.americanparknetwork.com/parkinfo/content.asp?catid=85&contenttypeid=30