Ash/Tephra Fall

Mount St. Helens' high frequency of eruptions during the recent geologic past and its two eruptive episodes of the past three decades indicate a high probability of renewed eruptive activity.

Magma began intruding into the Mount St. Helens edifice in the late winter and early spring of 1980. By May 18, the cryptodome (bulge) on the north flank had likely reached the point of instability, and was creeping more rapidly toward failure.

When a volcano erupts explosively, an ash cloud will be produced. Its size and travel-distance are determined by the amount of material erupted, the height of the cloud, plus the wind directions and speeds. Knowing where the ash cloud might travel is critical for managing air space and warning downwind communities to be ready for possible ash fall.