I remember seeing the May 18, 1980 eruption of Mount St. Helens on the news. Not too long after that, the skies darkened throughout much of the Northwest as volcanic ash choked out the sun, and over the following days, ash would fall like snow throughout neighborhoods as far away as Oklahoma. Yes, Oklahoma. In terms of measurable accumulations, the ash also traveled to central states, including parts of Wyoming, Nevada, Colorado, the Dakotas, and Nebraska. The longest distance recorded of the ash fallout was nearly 2,000 miles away from the volcano in the state of Oklahoma.
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… read more >