29 October 2004

Remembering Mount St. Helens

Sheldon Greaves

I grew up in Salem, Oregon, which is located just off the spiney backbone of volcanoes that makes up the Cascade Range. From the roof of our home on clear days we could see three ancient volcanoes marching back into the distance: Mt. Jefferson, Mt. Hood, and, on the very horizon, Mt. St. Helens.

As fate would have it, on 18 May 1980, when St. Helens blew, I happened to be in Antwerp, Belgium. The following day I was walking down a sidewalk and saw a banner headline screaming from the masthead of a local newspaper. It read (loosely translated) "Three Northwestern States Obliterated in Volcanic Holocaust!"

"Great," I thought. "My house is probably buried under three feet of volcanic ash."

I flew home the next day. During the final leg from Chicago to Portland, the pilot addressed the passengers and told us that we were taking a circuitous route around Mt. St. Helens to avoid a plume of volcanic ash. We landed late that afternoon, and less than an hour later the airport was temporarily closed due to ash fall. After I got home, on the first clear day we went up on the roof to see if Mt. St. Helens could still be seen. No longer. If we saw anything, it was a plume of smoke from her subsequent, smaller eruptions.

At the time, a lot of people expressed surprise at the violence of the St. Helens eruption. A few professional geologists were among those killed because they underestimated the potential for explosive eruptions, in spite of the fact that to the south are found the remains of Mt. Mazama, now known as Crater Lake.

The volcanoes of the Cascade range are unlike those in the Hawaiian Islands. Volcanoes such as Mauna Loa are the result of "hot spots" in which magma from below the earth's crust flows directly to the surface. The Cascade volcanoes are a little more complicated. They roughly parallel the intersection between the North American tectonic plate and the Pacific Plate, including some smaller sub-plates such as the Juan de Fuca plate. These plates beneath the ocean slide under the North American plate. The plate is pushed under at what is called the subduction zone. Along with the plate goes a lot of sediment washed into the ocean consisting mostly of silica-based material--beach sand.

When this material reaches a critical depth of about 100 km, the plate begins to partially melt. The lighter, less dense material works its way back up toward the surface, where it emerges occasionally as part of volcanic activity. However, the composition of the molten material is different from what you find in a "hot spot" volcano. The added silica makes the magma much more viscous. It traps gas more easily, forms plugs in volcanic vents more quickly. The practical upshot is that you get volcanic systems with a nasty habit of building up tremendous pressure, then releasing it catastrophically.

One person who did not appreciate this distinction was a colorful local character named Harry R. Truman. Harry was told to evacuate from his small town near the mountain, but he refused. His rationale was that lava flows slowly, and was preceded by plenty of smoke and tremors. He insisted he could see it coming and outrun the lava if worst came to worst.

Harry died on 18 May 1980 within minutes after the mountain ripped itself open.

For obvious reasons, I am watching the events at Mt. St. Helens (albeit from afar) with considerable interest. This is a rare opportunity to observe a very important type of volcanism with very real implications for public safety.