Happy anniversary Mount St Helens! Thirty-one years ago today (May 18, 1980), she blew her top in a catastrophic explosive eruption that should still remind us of the hazards of Cascades Range volcanoes.
I love this iconic image below - Mount St Helens prior to 1980. A beautiful vacation spot in the mountains of southern Washington State.
People lived there, sailed boats on Spirit Lake, and logged the nearby forests. Then the rumbling began and the northern side of the mountain started bulging. At 8:32 am, it blew.
The lateral blast wave caused extensive damage on the northern side of the volcano. Volcanic ash mud flows (lahars) and debris ripped down the river valleys taking out everything in their paths.
Now it's not quite so pretty. Here's me at the Johnston Ridge Observatory with the remnant of the mountain five and a half miles behind me (geologist David Johnston was killed on this ridge when the volcano blew).
Walking with my kids in the same general area a few years earlier than the previous picture (they're 10 years old now). Note the steam coming off the lava dome which was actively growing at the time. I always feel like a bad dad when I see this - "Hey kids, let's go for a walk near an active volcano!"
Hard to get a sense of scale in this image. We're over five miles from the mile wide crater and that lava dome is 1,000 feet off the crater floor!
Why did Mount St Helens erupt? The answer lies in the understanding the plate tectonic configuration of the Pacific Northwest.
Just offshore is a small tectonic plate, called the Juan de Fuca plate, composed of basaltic seafloor crust. There's a spreading ridge on its western side generating new seafloor from rising magma and pushing the plate to the east. As the plate moves east, it hits up against the thicker continental crust of the North American plate. With nowhere else to go, it slides down a seafloor trench under the continent - a process geologists term subduction (the trench is called the Cascadia subduction zone).
Subduction is a slow process and the plate often gets stuck. Stresses build up until the strength of the rock is exceeded and then it breaks and slips all at once in an area. Bang. An earthquake. On February 28, 2001, for example, a magnitude 6.8 earthquake struck 32 miles below Seattle. What's 32 miles below Seattle? The subducting Juan de Fuca plate.
When the subducting plate gets deep enough (~150 km), it starts to melt and generate magma. This magma, being less dense than the surrounding mantle rock, slowly makes its way up to the surface where it forms volcanoes. A whole line of volcanic mountains forms on the continent east of the subduction trench - geologists call this a continental volcanic arc (the Andes Mountains of South America have the same origin due to subduction of the Nazca plate down the Peru-Chile trench). The continental volcanic arc of the Pacific Northwest is called the Cascades Range.
Note that volcanoes in the Cascades have periodically erupted. They will continue to periodically erupt. (By the way, there are no volcanoes in central and southern California because that subduction zone plate boundary turns into the San Andreas fault system - a transform plate boundary where plates slide sideways past each other resulting in earthquakes but not generating magma).
Seattle is scenic, isn't it?
One day Mount Rainier will also blow, just as it's done many times in the past. Here's a picture of me up on Mount Rainier (above Spray Park, if you're familiar with the mountain). See the reddish rock. All pumice from a previous eruption.
Will geologists be surprised and shocked the next time a Cascades volcano blows causing death and destruction in the Pacific Northwest. Unfortunately not.