Re: Material ejected 5/18/80 How much was magma mixed with gas, is this what made the dust. What happened to the solid rock that made up the mountain prior to eruption, was it blasted out in big chunks or was it reduced to dust. It is hard to see a mechanism for the latter, but their is a big hole in the mountain and not many boulders lying around, or so it seems. Thanks

rocky Bob Eichler

Dear Bob,

About 0.25 cubic mile (1.1 cubic kilometer) of tephra (mostly ash) was erupted on May 18th.

About 0.55 cubic mile (2.3 cubic kilometer) of mountain was removed by the giant landslide (debris avalanche). Much of the rock was pulverized to dust-sized particles but some blocks up to 560 feet (170 m) survived. The small particles are derived from the collisions of larger pieces of rock. Such a huge mass moving at more than 110 mph (50 m/s) has a tremendous amount of energy. Some of the energy was used to crush up rocks.

To see the deposits look for hummocky hills in the north fork of the Toutle River.

Steve Mattox, University of North Dakota

Source of Information:
Lipman, P.W., and Mullineaux, D.R., (eds.), The 1980 eruptions of Mount St. Helens, Washington, U.S. Geological Survey Professional Paper 1250. 

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