Tuesday, July 4, 2017

Chasing Volcanoes and Overthrusts: Exploring the Pacific Northwest and the Northern Rocky Mountains

Mt. Shasta from Interstate 5 in the vicinity of Red Bluff and Corning. Photograph by Mrs. Geotripper
A few weeks ago I had the privilege of attending a field trip on Kilauea volcano on the Big Island with Don Swanson and Tina Neal of the Hawaii Volcano Observatory (and I'm probably not done with those blogs yet, by the way). Along the way, Don parked our van at the end of a road, and then made a seemingly useless U-turn before turning off the vehicle. He said in passing that when studying active volcanoes, he preferred to park facing away from the volcano...saved time when needing to evacuate in a hurry. With the stories he told, his habit seemed to make a lot of sense.

I was thinking about that as we drove north on Interstate 5 in the northern Great Valley on the first day of our exploration of the Pacific Northwest and northern Rocky Mountains. We were a band of 19 students and staff on a two week journey, camping our way across some of the most spectacular landscapes on earth. Looking out the windshield I couldn't help but think that we were headed right at some particularly young and active volcanoes, such as Mt. Shasta (above).
The Sutter Buttes north of Sacramento in the Great Valley. Photo by Mrs. Geotripper.
These volcanoes are part of the Cascades Range, which extends from Lassen Peak at the south end, to the vicinity of Garibaldi Peak in British Columbia. The "fire" beneath the volcano is derived as the oceanic crust off the coast sinks into a vast subduction zone. When the sinking slab reaches a depth of several tens of miles, water is liberated which changes the melting point of the minerals in the rock in the crust above. Buoyant magma rises into the crust, and eventually some of it leaks out, producing the eruptions that build these volcanoes.

The subduction zone is disappearing in geologic terms as it replaced by the growing San Andreas fault (don't panic too much, it's happening at a few inches a year). Volcanoes at the south end have been going dormant and then extinct. Once possible example is the unusual set of hills in the midst of the Great Valley north of Sacramento. The Sutter Buttes are the eroded remnants of a volcano that erupted around 1.6-1.4 million years ago. They're west of the main axis of the Cascade range, but then again so is Mt. St. Helens. Most research suggests that the volcano is more closely related to volcanic fields in the California Coast Ranges.
Lassen Peak from the west. Photo by Mrs. Geotripper.
Soon after passing Sutter Buttes, Lassen Peak became visible off to the east. The 10,457 foot high volcano is a smaller type of volcano called a lava dome or plug dome composed of viscous silica-rich dacite lavas. The original volcano erupted about 27,000 years ago, but it reawakened in 1914, had a major eruption in 1915, and continued to sputter for a few more years after that. It was made into a national park soon after. It was not on our itinerary on this trip, but we spend time there on our fall semester explorations.
Black Butte on the flank of Mt. Shasta. Photo by Mrs. Geotripper.

We passed one more prominent volcano before we reached the end of the road on the flanks of Mt. Shasta. Black Butte is a large looking volcano when viewed from Interstate 5 around Mt. Shasta City, but is a very small volcano compared to the adjacent edifice of Mt. Shasta  (it can be considered a part of Shasta). Like Lassen, it is a plug dome that erupted about 9,800 years ago.
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