Exploring exposed subduction zone rocks in the Olympic Mountains

September 6, 2022

by Anna Ledeczi

This August, Harold Tobin (UW Faculty and PNSN Director), Peter Lindquist, and I (Anna Ledeczi, PhD students in ESS) spent 7 days in Olympic National Park in an effort to access remote outcrops that can provide insight into processes occurring today at the Cascadia Subduction Zone, where the Juan de Fuca plate is subducting underneath the North American Plate, and where potentially giant earthquakes can occur. Due to their remote and inaccessible location, the structural and fault-related features of outcrops in the high Olympic Mountains, particularly in the area surrounding Mt. Olympus and Mt. Tom, have not been studied since the 1970s.

With this in mind, we set out on a 50-mile backpacking and mountaineering trip with the goal of reaching the Snow Dome Research Station, our base for three days of geology in the mountains. Due to federal wilderness protection laws, helicopter operations within the park are severely restricted, and our inquiries about hitching a ride to the research station were denied. Instead, we began our trip on the Hoh River Trail, a trail through the Hoh Rain Forest in Olympic National Park that terminates on the lateral moraine of the Blue Glacier. From here, we took the climber’s route towards the summit of Mt. Olympus, traversing across the main portion of Blue Glacier and up the steep side of the top of the glacier, known as Snow Dome. In order to traverse on the ice and snow, the National Park Service (NPS) required us to receive glacier crevasse rescue training, which we completed the week prior with the American Alpine Institute near Easton Glacier on Mount Baker.

Figure 1: Anna and Peter collect snow to melt into drinking water at sunrise.

Figure 2: Peter and Anna hike up Snow Dome. Blue Glacier is visible in the back.

Figure 3: An ice fall off Blue Glacier at night. View from Snow Dome Research Station.

The Snow Dome Research Station was built in the 1950s for the International Geophysical Year and remains to host researchers who wish to access this part of the Olympic Mountains. The hut, which is nestled beneath Panic Peak around 6600 ft in elevation, provided cozy accommodations for us while at Snow Dome - complete with a propane stove, mattresses, and lots of relics from the 1950s, such as SPF 4 sunscreen and old sardines. Today, Snow Dome is also home to a UNAVCO GPS station and a PNSN seismometer. The batteries on the seismometer needed servicing, and we were able to exchange the two batteries. Because the seismometer is deemed necessary to public safety, and because the batteries are prohibitively heavy, they were dropped off via helicopter. Once we followed the detailed instructions for battery replacement provided to us by PNSN, Harold performed a “stomp test,” jumping up and down on the floor of the generator shack to ensure the seismometer was in working order once again.

Figure 4: Peter, Anna, and Harold in front of the Snow Dome Research Station.

Figure 5: The helicopter dropping off batteries for the seismometer.

Figure 6: The results of the stomp test performed by Harold on the seismometer after replacing the batteries.

The helicopter made several additional appearances throughout our days at Snow Dome (and the pilots served as the only people we saw while on top!) due to the mountain goat relocation program which the NPS has been operating. Mountain goats, which were introduced to the Olympic Mountains in the 1920s and are thus not considered native, thrived there until the beginning of the translocation program, which transports them to North Cascades National Park, where natural goat populations were declining. The relocation involves netting the goats and transporting them via helicopter and truck across the state. The four-year program to relocate all goats in the Olympics is nearing its end in 2022, and we saw some of the last helicopter surveillance flights in this area.

In the high Olympic Mountains, there are exposures of highly chaotic and jumbled masses of sedimentary rock, deformed by the process of subduction, known as mélange. These rocks were deposited as sediments around 30 million years ago, which were then subducted and transferred to the overriding North American plate in a process called “underplating,” which basically means they were plastered onto the bottom of the plate, compressed into rock, and uplifted and exposed as mountains over time. The fault between the Juan de Fuca and North American plates at the subduction zone is the source of the potential magnitude 9 Cascadia earthquake and tsunami which threatens coastal communities in the Pacific Northwest today. The mélange rocks we examined in the high Olympics originated at this plate boundary, and can thus illuminate features of the seismogenic zone through their structures, mineral compositions, and chemistry.

Figure 7: Peter and Harold on an outcrop of phyllite.

The hiking was difficult, but our reconnaissance of these outcrops was useful, and may serve as the basis of future field projects to study the geologic record of subduction seismicity. We collected more than a dozen rock samples for further analysis in the laboratory to shed light on how the rocks were deformed and under what conditions of stress and temperature. Stay tuned!

Figure 8: Harold astride an exposure of a fault zone.

Exploring exposed subduction zone rocks in the Olympic Mountains