Which plate boundary formed "Mount St. Helens" volcano in the Pacific Northwest region of the United States?

Mount St. Helens is a volcano in Washington, near the Oregon border, in the Cascade Range, a segment of the Pacific Ring of Fire. It lies 52 miles (83km) northeast of Portland, Oregon and 98 miles (158 km) south of Seattle. Mount St. Helens Volcano, which stretch all the way from British Columbia through Northern California, are stratovolcanoes that have formed inland from a convergent plate boundary, where ocean crust is subducting below the continent. This subduction zone, where oceanic crust and continental crust collide, disturbs the balance in magma chambers and set off a volcanic eruption. The plate margin that created Mount St. Helens was Juan de Fuca plate, subducting beneath the North American continent, producing volcanoes along the Cascade Mountain Range.

Mount St. Helens is best known for its major eruption on May 18, 1980, the deadliest and most economically destructive volcanic event in U.S. history. 57 people were killed; 200 homes, 47 bridges, 15 miles (24 km) of railways, and 185 miles (298 km) of highway were destroyed. A massive debris avalanche, triggered by a magnitude 5.1 earthquake, caused a lateral eruption that reduced the mountain's summit from 9,677 ft (2950 m) to 8363 ft (2,549 m), leaving a 1 mile (1.6 km) wide, horseshoe- shaped crater. Despite this, it is a popular hiking spot and is climbed year-round.

In 1982, the Mount St. Helens National Volcanic Monument was established and is maintained by the U.S. Forest Service.