Mount St. Helens erupts on the morning of May 18, 1980
“This is it!”
USGS volcanologist David Johnston the morning St. Helens erupted. 

Mount St. Helens, Washington, US
From the Volcanoes of the World series
Issued by Gambia November 14, 2016

Mount St. Helens had been showing increasing activity throughout the month of March and April, culminating with a cataclysmic explosion. At 8:32am, the volcano was rocked with a 5.1 magnitude earthquake which a caused a massive landslide. Within seconds the volcano exploded into activity. 

On May 18, 1980, a magnitude-5+ earthquake was accompanied by a debris avalanche, which in turn unloaded the confining pressure at the top of the volcano by removing the cryptodome. This abrupt pressure release allowed hot water in the system to flash to steam, which expanded explosively, initiating a hydrothermal blast directed laterally through the landslide scar. Because the upper portion of the volcano was removed, the pressure decreased on the system of magma beneath the volcano. A wave of decreasing pressure down the volcanic conduit to the subsurface magma reservoir, which then began to rise, form bubbles (degas), and erupt explosively, driving a 9-hour long Plinian eruption.
1980 Cataclysmic Eruption | U.S. Geological Survey (usgs.gov)

57 people were killed in the eruption along with untold number of wild animals who were not able to escape.