If you’re ever lucky enough to fly into Portland International Airport (PDX) on a clear day, chances are you’ll be treated to a scenic view of Mount Hood. At 11,240 feet (3426 m) this stratovolcano is the highest point in the state of Oregon. Located only about 50 miles (80 km) east of Portland, Mount Hood dominates the city skyline.
Mount Hood is only one of many large volcanoes found throughout the Cascade Range from northern California to British Columbia. All of them have been formed by the same basic geologic process. For the last few millions of years the North American tectonic plate has slowly overridden the Juan de Fuca plate to the west. As the lighter continental crust has sunk the denser oceanic crust beneath it, the ocean crust has gradually melted and risen to the surface of the overlying continent. All of this ascending molten rock has lead to a chain of composite volcanoes along the margin, creating today’s Cascade Range.
These volcanoes have all been active over the last few thousand years, and the most notable recent event was the catastrophic eruption of Mount Saint Helens in 1980. While many of these volcanoes are in relatively remote areas, some are very close to major population centers. Mount Rainier is only about 50 miles from Seattle, and Mount Hood is only about 50 miles from Portland. If either of these volcanoes experienced an explosive eruption it would threaten the lives of millions of people and cause billions of dollars in damage. Because of this potential the United States Geological Survey (USGS) maintains a close watch on these behemoths of nature.