Mount St. Helens, a quiet, regal peak located about sixty miles north-northwest of Portland, Oregon.
One of a chain of Cascade volcanoes, formed by the submergence of a small tectonic plate (the Juan de Fuca Plate) underneath the far larger North American plate, the mountain's volcanism was well known. St. Helens had even erupted within the short recorded history of the Pacific Northwest, throwing up embers and ash in the 1830s, 1840s, and 1850s. Since then, the mountain had been silent, becoming a recreation destination by 1980.
On March 20, 1980, that changed, as the first of thousands of earthquakes began to rumble beneath the mountain. For only the second time in the twentieth century, a Lower 48 volcano awakened from its slumber. The second would prove very different from the first.
For more information:
Fire Mountains of the West by Stephen L. HarrisWikipedia - Mount St. Helens