Mount St. Helens produces small to large explosive eruptions, which send varying quantities of ash and tephra into the atmosphere.
The May 18, 1980 tephra plume lasted for about eight hours and the plume top ranged from 14–18 km (8.5–11 mi) high. Ash fallout caused major problems in communities up to 600 km (370 mi) away.
The major hazards associated with eruption of tephra result from suspension of the abrasive, fine particles in the air and water, burial of transportation routes and vegetation, and loading on roofs or other structures. Volcanic ash may pose hazards hundreds of kilometers downwind from source, directly after accumulating at the surface and later, when particles are remobilized by wind or passing vehicles. Airborne ash causes eye and respiratory irritation, damages unprotected machinery (especially internal–combustion engines), and often causes short circuits in electric–power transmission and distribution lines. It also endangers aircraft, which may completely lose engine power if they fly through ash clouds. Ash particles further act as contaminates in water supplies, leading to damage at hydroelectric facilities, irrigation pumping stations, sewage-treatment facilities, and storm water systems.
Volcanic explosions can propel rock fragments on ballistic trajectories that may differ from the wind direction. These events may occur without warning and in the absence of a larger magmatic eruption. A blast related to the emplacement of the Sugarbowl dome on the north flank of Mount St. Helens about 1,200 years ago propelled ballistic fragments as large as 5 cm (2 in) as far as 10 km (6 mi) from the vent. More typically, ballistic projectiles are limited to within about 5 km (3 mi) of vents.