Hazards

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.

Lava flows from Mount St. Helens typically affect areas within 6 mi (10 km) of the vent. However, two basalt flows erupted about 1,700 years ago extended about 10 mi (16 km) from the summit; one of them contains the Ape Cave lava tube.

Mount St. Helens produces small to large explosive eruptions, which send varying quantities of ash and tephra into the atmosphere.

During historical eruptions of Mount St. Helens, pyroclastic flows have originated from collapsing eruption columns and from gravitational or explosive disruption of growing lava domes.

Pyroclastic surges are less dense than pyroclastic flows, but are highly turbulent mixtures of gas and rock that flow at rapid velocities just above the ground surface.

Mount St. Helens is susceptible to lahars because the vent, and site of recent lava-dome building, is surrounded by Crater Glacier.

We know that Mount St. Helens is the volcano in the Cascades most likely to erupt again in our lifetimes. It is likely that the types, frequencies, and magnitudes of past activity will be repeated in the future. However, neither a large debris avalanche nor a major lateral blast like those of May 18, 1980 is likely now that a deep crater has formed.

Excess sediment is a lasting legacy of eruptions, especially those that destroy vegetation or deposit significant volumes of debris in watersheds.

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.