when did mount rainier form?

Answer 1

Mount Rainier's earliest lavas are over 840,000 years old and are part of the Lily Formation (2.9 million to 840,000 years ago). The early lavas formed a "proto-Rainier" or an ancestral cone prior to the present-day cone. The present cone is over 500,000 years old

http://en.wikipedia.org/wiki/Mount_Rainier