The sudden removal of the upper part of the volcano by the landslides triggered the almost instantaneous expansion of steam and gases within the volcano. The abrupt pressure release uncorked the volcano. Although the lateral blast began some seconds later than the debris avalanche, the blast's velocity was much greater, so that it soon overtook the avalanche. 

The blast was widely heard hundreds of miles away in the Pacific Northwest. A strong, vertically directed explosion of ash and steam began very shortly after the lateral blast and rose very quickly. In less than 10 minutes, the ash column reached an altitude of more than 12 miles and began to expand into a mushroom-shaped ash cloud. 

The 5.1 magnitude earthquake caused the gravitational collapse of Mount St. Helens' north flank, which produced the debris avalanche and triggered the ensuing violent lateral and vertical eruptions. The blast, ash, lightning, lava, mudflows and floods cauSed by the eruption caused widespread and extensive damage.