Thickness of ash (and larger-sized particles within 20-40 km) that ...

Graph shows thickness of ash (and larger-sized particles within 20-40 km) 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 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 moisture in the eruption cloud (Sarna-Wojcicki and others, 1981).