Geologists find extinct volcano near Snoqualmie Falls
October 22, 2009
By Laura Geggel
Tourists flock to Snoqualmie Falls to see its water cascade over the 268-foot tall precipice and hear the roar of the plunging river.
Little do they know they’re standing near a 20-million-year-old volcano.
A team led by Washington’s Department of Natural Resources geologists recently mapped the area and discovered that volcanic rock in Snoqualmie Valley was more local and younger than previously thought.
Until now, geologists had attributed the Valley’s volcanic rock to Mount Persis, an extinct volcanic area to the north in Gold Bar.
“It was very poorly understood,” DNR geologist Joe Dragovich said.
A closer examination showed the Valley’s volcanic rocks looked different than rock from Mount Persis. After testing, geologists learned it was also about 20-million-years-younger than the 40- to 50-million-year-old volcanic rock of Mount Persis.
A few telltale signs indicated to geologists that the Valley’s volcanic rock was created locally.
When volcanoes spew lava, gas and earth, the heavier material can’t travel as far as the lighter stuff. This heavier spewed content — called volcanic bombs if they’re larger than two and a half inches in diameter — acquire a round shape during flight or upon landing. Geologists found hundreds if not thousands of volcanic bombs near the falls, suggesting the center of the volcano was nearby.
The Snoqualmie Falls area is also dominated by lava flows, “and flows usually don’t move very far from the volcano,” Dragovich said.
More recent seismic and glacial activity make it hard to pinpoint the location of the volcano, but geologists think its center is near Snoqualmie Falls.
Visitors can see evidence of the volcano by walking down to the wooden platform across from the base of the falls. A careful eye will spot volcanic bombs across the Snoqualmie River.
Don’t worry about eruptions, though.
“It’s extinct for sure,” Dragovich said. “Volcanoes tend to last a couple of million years. It’s like acne, it moves.”
Volcanoes become extinct when they lose their magma source. When the Juan de Fuca plate descends below North America, it melts and eventually forms magma. Volcanoes form when magma reaches the surface. Volcanoes in the Cascade Range commonly erupt explosively.
Local geologist Heather Littke worked with Dragovich during the mapping process. The 2001 Mount Si High School graduate grew up in Fall City, “pretty much at the bottom of the falls,” she said.
Littke’s family shared a love for geology, and she grew up rock hounding until she made it official by studying earth and space sciences at the University of Washington.
When the geology team studied the Valley for volcanic evidence, they toured the tunnel at the bottom of the Falls. Inside, Littke saw breccia, a smaller volcanic bomb.
“The tunnel was fascinating in itself because you see the multicolored breccia flow — it was green and red — really intense,” Littke said.
Littke added she has always enjoyed the falls, and knowing it flows near a 20-million-year-old volcano is icing on the cake.
“I think it’s amazing,” Littke said. “Snoqualmie Falls is a magical place and (now we know) there’s a river going through a faulted volcano. It’s been a really great experience to be a part of this, especially since I’m from here.”
DNR’s Division of Geology and Earth Resources, headed by Dragovich and assisted by geologists from King County, the Pacific Northwest Seismic Network, Colorado College and Washington State University, spent three years mapping the area’s geologic features.
The project was paid for by the federal U.S. Geological Survey and DNR.
Laura Geggel: 392-6434 ext. 221 or firstname.lastname@example.org. Comment at www.snovalleystar.com.