Passage VII

Early on May 18, 1980, Mount St. Helens violently erupted, expelling a huge cloud of volcanic ash. Most of the ash fell to Earth on May 18, as the cloud moved rapidly away from the volcano. The following studies examined the ashfall.

Study 1

Researchers measured the depth of uncompacted ash, in millimeters (mm), at hundreds of stations in Washington, Idaho, and Montana. Only ash that had fallen on manufactured objects such as roofs and cars was measured. The results are shown in Figure 1.

Study 2

The thickness of the ash layer on manufactured objects was measured at 14 stations along a north-south line passing near Moscow, Idaho (see Figure 1).

Researchers found that ash deposits east of Yakima, Washington, were composed of two different layers. The thin lower layer (1-2 mm) was made of dark gray ash composed of sand-sized particles of fragmented volcanic rock, and, above that, a thicker layer of lighter-colored ash was composed of smaller (silt-sized) particles (see Figure 2).

It was believed that the dark ash formed during the initial eruption, when rock from the mountain was pulverized. Whereas the lighter-colored ash formed later in the eruption as molten rock (magma) inside the volcano was violently expelled.

Study 3

Samples of volcanic ash were collected along a line of stations stretching from the volcano to western Montana. Figure 3 shows the calculated average particle size of the samples at each station.

(Note: The average grain size in Figure 3 includes particles of both dark and light ash.)

Figures adapted from A. M. Sarna-Wojcicki et al., "Areal Distribution, Thickness, Mass, Volume, and Grain Size of Air-fall Ash from the Six Major Eruptions of 1980.” ©1982 by the U. S. Geological Survey.