The ash produced in the 1980 eruption of Mount St. Helens was composed a
a rock type called Dacite. Dacite is rich in silica so the magma is
much thicker than Basalt a type of magma you might have heard from in
Hawaii. The Hawaiian lavas are more fluid and flow along like liquid.
At Mount St. Helens our Dacite magma is so thick that our lava flows
tend to ooze very slowly (like crusty toothpaste from a tube). Instead
of fluid lava flows we get piled up mounds of lava called lava domes.
Because our magma is so thick the gasses that are trapped in it tend to
burst out explosively like bubbles from a pot of thick oatmeal. The
fine ash that is produced is the very tiny rock fragments from the
bursting bubbles (just like what happens when you get sprayed with fine
spray when a soap bubble pops). ENJOY geology--the earth and the
processes that shape it are fascinating!
Peter Frenzen, Mount St. Helens Nat'l Volcanic Monument
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Jessica