During the Pliocene and Pleistocene, eruptions continued, accompanied by large scale faulting and elevation of the mountain chain. The lavas became more felsic and the eruption style of the mountains became more violent. Mt. Mazama apparently became larger and larger, built up by alternating flows of molten rock and pyroclasts. It often had more than one vent, and we think it was never a pleasingly symmetric volcano, but rather a series of overlapping cones. Glaciation during the Pleistocene altered the face of the mountain, and glacial deposits are found on the flanks of the old volcano.
Some glaciers appear to have been more than ten miles long and to have cut deeply into the mountain.
During the last ice age, smaller cones on the flank of the mountain were built by basalt flows. Then came the big event about 5,000 years ago. Note that a date so close to the present means there were probably human witnesses to the eruption. That is in accord with legends of the Klamath Indians who speak to volcanic destruction from the mountains. The ash can be found today over an area of 350 square miles.