What’s next at St. Helens?
Standing in the summit crater of Mount St. Helens, you look up to see sky; prior to May 18, 1980, your vantage point was inside the mountain, buried under a quarter mile of rock.
The explosion that began that day’s eruption triggered an avalanche of rock and ice that roared downward at 150 miles an hour.
Suddenly free of the pressure that had kept them in solution, gases exsolving from the magma exploded outward at 700 miles an hour. The blast blew down 150,000 acres of timber, uprooting trees or snapping them like match-sticks. The hot wind, many times faster than any hurricane, claimed most of the volcano’s 60 victims.
From Northern California to British Columbia, North America and the floor of the Pacific are in collision. The less-dense continent rides up and over the ocean floor, which continues pushing eastward, even as it dives down (subducts) into Earth’s mantle.
Rich in uranium and thorium, the mantle is hot. At a depth of maybe 80 miles – a hundred or so miles in from the coast – it bakes water out of the subducting ocean rock. The water seeps into and weakens the chemical bonds of the rock above. The wet rock melts, the magma floats upward.
Earth’s crust between San Francisco and Salt Lake City is expanding roughly 3 feet per century, pushing on the crust to the northwest. This pressure, acting with collisional forces from the west, has broken the crust of the Pacific Northwest into huge blocks.
In southern Washington, a block of rock drops downward between neighboring blocks. The faults along which it slides have provided paths for magma to reach the surface. To the east, eruptions build Mount Adams; to the west, Mount St. Helens.
In the 40,000 years since Mount St. Helens began growing, magma has reached the surface in 10 eruptive periods, the last beginning in 1980.
Now the mountain is rumbling again. Is this merely the dregs of magma that erupted in the ’80s? Or is it a new pulse of magma, the beginning of yet another eruptive – likely explosive – period?
Volcanoes explode when magma approaches the surface, reducing pressure on its dissolved gases – not unlike a bottle of warm soda from which the cap’s been removed.
Aircraft sampling the gases escaping from Mount St. Helens – “gas flights” – have determined that St. Helens has been releasing about 70-150 tons of gas per day – much less than the thousand or so tons that might be expected from fresh magma. Perhaps the magma reaching the surface is, indeed, the dregs of the 1980s, which, like warm soda, has gone flat.
Or perhaps the magma is fresh, but has become bottled up and, inside the mountain, pressure is building.
Ultimately, we need a sample of the lava that’s just made it to the surface, to see if it’s the same as 1980s rock, or contains a new mix of minerals. A robot geologist would here be ideal, though not a Mars-type rover like Spirit or Opportunity, which were designed for the relatively smooth terrain of the Martian desert. To navigate the rough terrain of the summit crater, would require much larger wheels – or legs.
One of the most challenging tasks a child must ever master is walking. Several years ago, engineers tested a prototype-walking robot in a volcano. The robot stumbled, fell, and had to be rescued.
Geologists are now debating whether a human geologist might safely (safely enough) enter the St. Helens summit crater and grab a sample.
My thanks to USGS geologists Mike Clynne and Tina Neal.
Alan Stahler trained as a biologist and is an amateur astronomer. He teaches enrichment classes for children and adults at Sierra Friends Center. His science programs can be heard at noon on alternate Tuesdays on KVMR-FM (89.5).
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