May 18, 1980 -- A day that lives in infamy
It was the eruption heard 'round the world, or at least 'round the U.S. Mount St. Helens erupted in Washington State on May 18, 1980, killing 57 and resulting in an estimated $1 billion in damages.
Mount St. Helens is located in southwestern Washington extremely close to the border of Oregon, and nearby other massive Cascade peaks Mount Hood (Oregon), Mount Adams (Washington) and Mount Rainier (Washington). Mount St. Helens is part of the Cascade Volcanic Arc, which, in turn, is part of the Pacific Ring of Fire. Other well-known U.S. peaks that are part of the Cascade Volcanic Arc include Mt. Hood, Mt. Adams, Mt. Rainier, Mount Shasta (California), Mount Baker (Washington), and Glacier Peak (Washington).
On May 18, 36 years ago, a magnitude-5.1 earthquake struck in southern Washington, which triggered a eruption that literally blasted off a huge section of Mount St Helens. The eruption was massive, generating a huge lateral blast that obliterated the northern side of the stratovolcano, flattening millions of trees over more than 370 square miles of nearby terrain, and sending ash all the way to northeastern regions of the U.S.
Significant seismic activity had been occurring near Mount St. Helens for weeks leading up to the blast. On March 20, 1980, magnitude-4.2 quake hit beneath the peak, and by the end of April 1980, the northern side of the mountain was showing signs of bulging.
Then, on May 18, 1980, a magnitude-5.1 quake struck and quickly set off the chain of events that ultimately led to the collapse of the northern flank of the volcano. The eruption and eventual collapse of the northern side of the Cascade peak resulted in the largest landslide-debris avalanche in history.
— USGS (@USGS) May 18, 2016
Recent (2016) seismic activity at Mount St. Helens
From March through mid-May 1980, hundreds of small earthquakes struck Mount St. Helens. In late March 1980, for example, scientists reported that magnitude-4 quakes were shaking the mountain at a rate of roughly 3 per hour. Eventually the magnitude-5.1 quake hit on May 18, and the rest is history.
Recently, the U.S. Geological Survey (USGS) scientists reported that more than 100 small earthquakes have shaken Mount St. Helens since mid-March (2016). USGS scientists called the recent seismic events "volcano-tectonic in nature" and noted that there are "no signs of an imminent eruption." According to the USGS, the event is more of a "recharging," which is something that can occur for many years beneath a volcano without an eruption.
So, despite the recent earthquake activity shaking beneath Mount St. Helens -- coincidentally occurring really close to the anniversary of the historic eruption -- officials and scientists are not particularly concerned. All told, however, the recent events in southern Washington are bringing a renewed focus on volcano danger that is ever present in the Pacific Northwest.