Idaho’s geology: Beautiful and problematic

The hillside along U.S. Highway 95 at milepost 188, where rockslides that continue to block the road occurred earlier this month, is seen last week from Pollock Road.

The most recent landslide along U.S. Highway 95 south of Riggins may have only happened a couple of weeks ago, but the failure was primed by events that date back tens of millions of years.

Claudio Berti, a geologist for the state of Idaho, said several factors contribute to the geological instability there.

For example, there are foliations, where the rocks split into thin sheets, that weaken their structure, as well as faults and fractures that contribute to their collapse. There are also outcrops of “ultramafic” or oceanic rocks that, when subjected to intense heat or pressure, metamorphose into talc.

Talc, a major ingredient in baby powder, is a very soft clay mineral that can act as a lubricant to facilitate landslides.

Taken together, Berti said, these characteristics create “really the worst possible conditions for stability.”

“That particular area has a geological setting that’s favorable to slides,” he said.

That’s evident from the history of landslide activity along U.S. Highway 95 from Grangeville south to New Meadows. Rockfalls are common along the steep canyon walls, and slides big enough to interrupt travel happen every five to 10 years.

The most recent landslide took place July 3, covering about 100 feet of highway in debris as much as 40 feet thick. Although the road temporarily reopened July 8, it closed again the next day after renewed movement was detected. Officials continue to monitor the area to determine when road crews can safely begin clearing the debris.

Some of the faulting and foliations that contribute to that instability are more than 100 million years old.

Berti said much of the landscape along Idaho’s western margin is part of the North American accreted terrane province, an extremely complex and varied group of rocks that stretch all the way from Mexico to the Arctic Circle. It includes most of Oregon, Washington, British Columbia and Alaska.

“These rocks really aren’t part of (ancient) North America,” Berti said. “They may have formed as microcontinents, island arcs, submarine volcanoes or as slices of ocean floor.”

In Idaho, these “exotic” terranes run from the Hells Canyon region north toward Orofino and Lewiston. Some of the rocks are more than 250 million years old. They subsequently accreted, or collided, with North America more than 100 million years ago.

“The history of the rocks before they accreted is quite complex,” Berti said. “We don’t have a good idea where they originated.”

Nevertheless, the foliation in the rocks, which makes it easier for them to split along certain planes, likely predates the collision with North America, he said. The faulting and fracturing, by contrast, probably happened during or after the collision.

“The rocks definitely aren’t very stable or resistant,” Berti said. “That’s why the river carved a steep gorge, and why the road was built there. It’s why that stretch of road is an engineering nightmare.”

However, while Idaho’s geological history set the scene for today’s transportation challenges, it’s also responsible for the state’s dynamic landscape.

“It’s part of the beauty of Idaho,” Berti said. “But part of living in a mountain environment is that it comes with natural hazards. When we forget that, nature has a way of reminding us — and then we have to scramble to deal with it.”

Spence may be contacted at bspence@lmtribune.com or (208)-791-9168.