Twenty nine years ago, on a beautiful spring Sunday after church, I watched the darkest clouds I had ever seen sweep toward my hometown in eastern Washington State. We didn’t know it at the time – communication systems were still primitive by modern standards – but Mt. St. Helens had erupted catastrophically a couple hours earlier, sending a plume of volcanic ash high into the atmosphere.
When the cloud of volcanic material reached us around noon, we were plunged into darkness. Worse than the darkness was the fact that the ash in the air made breathing almost impossible. I remember well walking home with an old mitten from my coat pocket held over my nose and mouth to act as a crude filter. I was nevertheless hacking and sputtering when I got inside the house. By then the world was as dark as midnight, and all the streetlights had automatically come on.
We didn’t see any daylight until the next morning when the sun illuminated a moonlike, ash-gray landscape. The next few weeks were a nightmare of billowing ash clouds stirred up by every strong wind. Our stores were unable to keep enough facemasks on the shelves to meet demand, and cars and trucks were disabled by the ash filling their air intake systems.
So I send my sympathy to those living downwind of Mt. Redoubt, the volcano that recently started large-scale eruptions in Alaska. Volcanic ash from Redoubt has reached 60,000 feet in the atmosphere. Some airplane flights have been cancelled, others rerouted because flying into such an ash cloud could cripple jet engines. So far, a great deal of the material erupted has been steam, carbon dioxide and sulfur compounds – rather than the pulverized rock fragments known as ash. But that could change quickly.
Redoubt is the same type of volcano as St. Helens – the nasty and unpredictable kind that we geologists call a stratovolcano. To explain why stratovolcanoes are so violent, let me dredge up childhood memories I hope you have of playing with a bottle of soda pop, especially a warm or a shaken bottle. When you first pick up such a container, the gases inside are under pressure – enough pressure they are actually dissolved in the liquid. But when you open the top, you hear the “ffffpt” as the pressure drops, and the dissolved gases rapidly leave the liquid – expanding a lot as they make bubbles.
If you shake the bottle or heat it, the effect is even more dramatic. If you were lucky as a kid, you were pointing the shaken bottle away from yourself (and perhaps toward a sibling) when you got the top off.
Volcanoes are a good bit like pop bottles. The lava of stratovolcanoes is stiff, not fluid, and the gases in the stiff lava are at high pressure. When the lava moves upward and gets near the surface of the Earth, pressure drops. The gases come out of the lava just like the tiny bubbles come out of soda pop. And then they expand, and expand, and expand some more. Viola! Entire mountainsides can be blasted apart by the force liberated by the expanding gases coming out of a stratovolcano lava.
The volcanoes in Hawaii are relatively “gentle” because the lava there is a different type and very fluid. The fluid lava allows the gases to escape bit-by-bit, rather than explosively. I do wish that here on the continent our volcanoes were like that.
Another hazard of stratovolcanoes in both the Northwest and Alaska is mudflows. Warmth from erupted steam and ash rapidly melts the snow and ice on the volcano’s steep slopes. The water can quickly produce mudslides and a thick mixture of mud, ash and water heads downhill. People, livestock and buildings can be swept along in rapidly-moving mudflows that inundate valleys. Here in Washington State, we actually have outdoor loudspeakers permanently set up to blast warnings day or night to people in the path of likely mudflows at the foot of Mt. Rainier.
The size of geologic forces is staggering. We veterans of major eruptions know that in our bones, and we wish Alaskans well as they cope with their unsettled volcanic neighbor.