Freeze-thaw cycle is cause of rockfall at Mount Rainier

The ongoing process of freezing and thawing led to the rockfall from Mount Wow that destroyed a pickup truck and is forcing managers to relocate the parking area at the end of Westside Road at Mount Rainier.

That is the assessment of Scott Beason, geologist at Mount Rainier National Park.

The rockfall took place on the morning of May 15, and forced the closure of the road for almost one month. The road reopened June 14.

In that time, Beason and other park staffers walked among the boulders that came to a rest on the roadway, and he flew in a helicopter to determine where the boulders broke away from on the side of 6,040-foot Mount Wow.

An estimated 8,500 cubic feet of Ohanapecosh Formation andesite broke away from 4,000 feet up the mountain. About half of it made it all the way to the road a quarter mile away. One boulder was large enough to make a crater 8 feet wide by 3 feet deep. Another boulder, which did not make it to the road, measures about 30 feet wide by 20 feet long by 5 feet high.

The rock is from the Oligocene age, making it about 30 million years old.

“I haven’t seen an event like that since I started here in 2004,” Beason said. “But if you look at the source area, this is something you should expect. When you look at the talus field, you know it’s happened before in the mountain’s history.

“It’s just a freeze-thaw situation,” he added. “It seems like this year was a pretty intense freeze-thaw cycle. It seems like there has been a lot of rock fall this year. I don’t if it’s because it is something we’re looking for or if there is an increase in activity.”

During the helicopter flight, however, there was no indication there would be more rocks coming down in that area.

Still, Beason said, geologists are recommending park managers move the parking area lower on Westside Road.

The road is currently barricaded just south of the Dry Creek crossing to prevent cars from parking in the rockfall zone. Visitors who want to hike up the old roadway have to park south of the barricade and walk through the rockfall zone.

In the coming weeks, a permanent gate will be installed just north of Dry Creek to allow room for vehicles to turnaround. New signs will mark the parking area, and more permanent signs will be installed warning of the rockfall hazard zone and advising people to proceed through the area with caution.

Beason said there has been some discussion of leaving some of the boulders where they came to rest.

“There were some pretty big rocks there. When you see that in the field, you think ‘There’s a lot of power behind these things,’” he said.

Jeffrey P. Mayor: 253-597-8640