Volcanic ash of St. Helens

Volcanic ash is a fine mixture of minerals and rock fragments thrown out of a volcano during exposive volcanic eruption.

Here is an example of ash from the volcano that is responsible for the most deadly eruption in the history of USA. The volcano is of course Mount St. Helens and it happened in May 18, 1980.

Volcanic ash is so fine that with a naked eye you only see dark to light gray powder. What is it made of? Lots of it is glassy froth – pumice as a rock type (white grains). Mineralogically it is composed of plagioclase (gray blocky crystals), quartz (lots of silica is glassy, not quartz), amphiboles, and pyroxenes (black grains). All these components together in one rock is called dacite. This is a fairly silicic volcanic rock which explains why the eruption turned out to be so violent. Silicic magma is highly polymerized and holds the gases which escape from the rising melt but can not break free. So the whole thing expands like a fermented dough until KABOOM! In this particular case the story is more complicated. There was a flank collapse which triggered the eruption but that is not so important here.

This sample is collected just one day after the eruption and more than 100 kilometers away in Yakima County, Washington.

Volcanic ash collected in Yakima County 120 kilometers away from the Mount St. Helens in May 19, 1980 — just one day after the catastrophic eruption. The width of the view is 4 mm.