The Cascade Range of the Pacific Northwest began to form during the Oligocene, 36 million years ago, as the Farallon Plate subducted beneath the North American Plate. But the major Cascade volcanoes that tower above the landscape today are very young (geologically speaking), having formed within the past 2 million years.
Mt. Ranier, the largest of the Cascade volcanoes, first erupted 500,000 years ago. Much more recently, Mt. Mazama, in Oregon, erupted 6640 years ago, creating the caldera that now holds Crater Lake. The last major eruption of Mt. St. Helens (prior to the 1980 eruption) was in 1800 AD while Mt. Hood last erupted in 1805. Repeated eruptions of Lassen Peak, in northern California, occured between 1914 and 1922 and, as most of us remember, Mt. St. Helens shook our complacency in 1980.
Since the Juan de Fuca Plate, a remnant of the Farallon, continues to subduct beneath the Pacific Northwest, the Cascade volcanoes will continue to form and erupt. Our planet's natural history continues to unfold and, more than just witnesses, we are part of it.
