when was the last time mount saint helens exploded
1980 eruption of Mount St. Helens On May 18, 1980, a major volcanic eruption occurred at Mount St. Helens, a volcano located in Skamania County, in the State of Washington. The eruption (a VEI 5 event) was the most significant volcanic eruption to occur in the contiguous 48 U.S. states since the much smaller 1915 eruption of Lassen Peak in California.[1] It has often been declared as the most disastrous volcanic eruption in U.S. history. The eruption was preceded by a two-month series of earthquakes and steam-venting episodes, caused by an injection of magma at shallow depth below the volcano that created a large bulge and a fracture system on the mountain's north slope.
1980 eruption of Mount St. Helens Suddenly, at 8:32 a.m., a magnitude 5.1 earthquake centered directly below the north slope triggered that part of the volcano to slide,[24] approximately 7–20 seconds (about 10 seconds seems most reasonable) after the shock.[6] The landslide, the largest in recorded history, travelled at 110 to 155 miles per hour (177 to 249 km/h) and moved across Spirit Lake's west arm. Part of it hit a 1,150-foot-high (350 m) ridge about 6 miles (10 km) north.[6] Some of the slide spilled over the ridge, but most of it moved 13 miles (21 km) down the North Fork Toutle River, filling its valley up to 600 feet (180 m) deep with avalanche debris.[24] An area of about 24 square miles (62 km2) was covered, and the total volume of the deposit was about 0.7 cubic miles (2.9 km3).[6]