Tephra Distribution Downwind from Mount St. Helens on May 18, 1980

Ash Thickness and Particle Size Downwind
from Mount St. Helens, Washington, on May 18, 1980

Thickness and mean-grain diameter of tephra that fell to the ground downwind of Mount St. Helens during the eruption on May 18, 1980. The volcano ejected a minimum of 1.1 km3 of uncompacted tephra, which is equivalent to 0.20-0.25 km3 of magma or solid rock. Peak wind velocity during the eruption varied between 80 and 140 km/hour as measured 400 km downwind of the volcano at about 12 km above sea level.

Typical of tephra-fall deposits, the tephra thickness and grain size decreased rapidly downwind from Mount St. Helens. Note the increased thickness about 300 km downwind; this unusual increase in tephra thickness is thought to have resulted from the sticking together of individual grains due to


Sarna-Wojcicki, A.M., Shipley, S., Waitt, R.B., Dzurisin, D., and Wood, S.H., 1981, Areal distribution, thickness, mass, volume, and grain size of air-fall ash from six major eruptions of 1980, in Lipman, Peter W., and Mullineaux, Donal R. (eds.), 1981, The 1980 eruptions of Mount St. Helens, Washington: U.S. Geological Survey Professional Paper 1250, p. 577-600.