Field Notes: Challis Volcanics and the East Fork of the Salmon River

The East Fork of the Salmon River cuts against a cliff of Miocene Challis Volcanics

By anyone’s measure, the Salmon River is one mighty twisty stream. Flowing along geologic faults, twisting among the weaker rocks in the mass of granodiorite plumes that make up the Idaho Batholith, it flows north, east, west and south through deep, steep-walled canyons as it wends to its confluence with the Snake River in central western Idaho, just below Hell’s Canyon. From its headwaters below Galena Pass to the confluence with the Snake River, the Salmon is about 170 miles as the Golden Eagle flies. But it’s 425 miles of river, and all those river miles are located wholly within the state of Idaho.

One tributary of the Salmon is the East Fork.1 That tributary is only 34 miles long, beginning at the confluence of the West Fork of the East Fork Salmon River and the South Fork of the East Fork Salmon River.2 It flows between the White Cloud Mountains on the west and the Boulder Mountains on the east. In the lower reaches of the East Fork, it cuts into the Challis Volcanics, yet another volcanic field in a state that has an embarrassingly large number of volcanic fields.

The Challis Volcanics in central Idaho are the largest of several Eocene (51 to 40 million years ago) volcanic fields in an arc across the Pacific Northwest and Canada. Other fields from the same period include two in British Columbia, another in northeastern Washington, the Clarno field in north-central Oregon, the Absaroka field in Montana and Wyoming and many more. Geologists disagree on what caused the 11 million year-long volcanic episode. Two models have been proposed to explain all that Eocene volcanism. In one model, subduction of a very shallow dipping Farallon plate, or perhaps the vanished Kula Plate, beneath the North American craton created a wide volcanic arc. In the other model, the volcanic activity resulted from rifting as a result of collision between the Pacific and North American plates. Or it may be that the mechanism wasn’t the same across all of the provinces. It’s “poorly understood.”

Today, the Challis Volcanics in east central Idaho are a bit battered by 40 million years of subsequent geology. Part of that subsequent geology was the last of the intrusion of the granodiorite plumes – a failed volcanic eruption, as it were – that created the Idaho Batholith. Other events include the docking of a couple of additional terranes to the west, the immense flood basalts to the west, Basin and Range faulting and significant mountain glaciation. Today, where exposed, the Challis volcanics are a little the worse for wear.

In the photo above, the originally horizontal layers of ash have been tilted by about 30 degrees, broken by faults and even folded a bit. A poorly defined sill traces upwards in the middle left, broken by subsequent faulting.

East Fork canyon wall showing tilted and warped layers of welded tuff and rhyolite

Remember layers of volcanic ash like this welded tuff are originally laid horizontally. The 20 to 30 degrees of tilt occurred later, although there doesn’t seem to be consensus among geologists as to how much later. The multiple layers speak to multiple eruptions over a considerable amount of time, although WC couldn’t find any sign of soils between the layers he could reach. So the interval between eruptions doesn’t seem to have been long enough for erosion or wind deposited soils to develop.

Looking north, downstream in the East Fork Valley; note the Challis Volcanics at the crown of the ridge in the background

The Challis Volcanics are thousands of feet deep. in the area of the East Fork, the rivers have cut more than 2,000 feet down through the formation, and the riverbed still rests in Challis rocks. When you consider the formation was likely a high, level plain when eruptions ceased, at or above the ridge in the background in the third photo, the amount of missing rock, eroded away downstream, is nearly as impressive as what’s left. Presumably, the parts that didn’t wash down into the Pacific Ocean are now buried under the subsequent Columbia River basalt flows.

The upper parts of the East Fork Valley are on WC’s list of sites to visit now. WC only visited the lower couple of miles. Stay tuned.

1 Not to be confused with the East Fork of the South Fork of the Salmon River. Idaho’s utter failure of imagination in place names is demonstrated yet again.

2 See Note 1, supra.

One thought on “Field Notes: Challis Volcanics and the East Fork of the Salmon River

  1. Lived 10 years in a tipi on a ranch a few miles downriver, hunted elk up on that ridge. Did a fall float from there to the Columbia in ’81 – an old WW2 raft on the Salmon, homemade redwood canoe on the Snake. The Salmon River country does have some amazing geology. Bonus – lots of hot springs. A most favorite place, but appalling politics. Thanks for the post of old stomping’ grounds.


Comments are closed.