Global Volcanism Program | Image GVP-04572
Deep glacial valleys cut the western flanks of Mount Jefferson volcano. An original andesitic-dacitic volcano that was at one time perhaps higher than the current volcano was extensively eroded by glaciers prior to formation of a younger dacitic volcano on top of the older edifice. Several dacitic lava domes, accompanied by major ash eruptions and pyroclastic flows, were associated with the younger volcano, which ceased activity during the late Pleistocene.
Photo by Lee Siebert, 1996 (Smithsonian Institution)