Geology Is Cool: The Wallowa-Seven Devils Terranes

Hells Canyon at Granite Creek Rapids

Hells Canyon has an amazingly convoluted history. Geology is messy, but Hells Canyon takes that truism to an extreme.

As geologists understand it,¹ about 150 million years ago, the Pacific Northwest ended approximately along the Idaho-Oregon state line. It was ocean, the proto-Pacific Ocean, westwards from there. The Pacific Plate, or maybe a smaller plate fragment, was sinking under the North American Plate, being “subducted,” a geologist would say. Something like the Farallon Plate today, off the coast of Oregon and Washington, being subducted under North America. There may or may not have been volcanoes, like the Cascade volcanoes today, or perhaps the melting, subducted ocean floor never emerged, but instead later became the Idaho Batholith, the large mass of granitic rock the comprises Central Idaho today.

Offshore, if you’d been standing on that ancient Permian seashore, you would have seen a series of islands, shaped in arcs. Those islands originated much further south. The volcanic islands had coral reefs, and the corals were a kind that grows in subtropical waters. As the Pacific Plate subducted under North America, those offshore islands gradually approached the coast of North America. It wasn’t fast; about two inches a year. And it wasn’t a head-on collision; the island arcs probably arrived from the west-southwest.

Serpentinite and ultramafic rock at bottom center, signature rock of ocean sea bottom. Note the CCC-built Hells Canyon Trail at the top.

As the island arcs approached North America, ocean floor got scraped up and pushed up on land. And, in very slow motion, beginning about 100 million years ago, those island arcs collided with North America, one after another. The Seven Devils Terrane was probably the first island arc to collide. Subsequent island arc collisions followed, with more ocean floor scraped up and jammed between those islands. That ocean floor included the coral reefs that surrounded the islands.

The light rock in the upper right is a chunk of ancient coral reef embedded in sea bottom, 1,500 feet above the river.

That series of slow motion collisions folded and compressed the ocean floor and the island arcs that had arrived earlier. The volcanic rocks and limestones were under immense heat and pressure, and became metamorphic rocks. They were folded, stood on end and generally put through the geologic meat grinder.

Layers of ash, basalt and limestone from one of the ancient volcanic islands, metamorphosed and turned on end. Photo at Kirkland Ranch, Hells Canyon.

Geologists are still trying to determine when the Snake River started cutting through the faults and weakened rock between the Seven Devils terrane and the various terranes further west. But we know the river was there 15 million years ago, when the Columbia basalts poured out in three major series of eruptions. Some basalt made it all the way down to the Snake River, and the river cut through them, starting about 15 millions years ago. Upstream of Hells Canyon, those same flood basalts dammed the river, and over time the river cut through those dams, leaving old lake beds behind. In a few places, the Columbia basalts flows ran down into the river, forming lava pillows, that characteristic shape of molten rock flowing into lots of water.

Basalts flows that cascaded down into Hells Canyon c. 13.5 million years ago. Note the neat, undisturbed layers of basalt flows at the summit.

Not far from those 15 million year old pillow basalts you can find 250 million year old pillow basalts, formed when the volcanoes on those islands ran down into the ocean.

The “seam” where terranes like those ancient island arcs attach to an existing plate are called “sutures,” and they are characterized by pretty battered rock and fault lines. So they erode faster than other, more intact rocks. Stream courses, including the Snake River through Hells Canyon, follows a suture line between the Seven Devils terrane and the terranes that comprise the Wallowa Mountains to the west. Similarly, the Salmon River follows the suture lines between the Seven Devils Terrane and the old North American craton.

The collision lifted up the area, much as the collision of the Indian subcontinent into Asia lifted up the Himalayas, although on a smaller scale. As the Snake River and Salmon River cut their deep canyons, the weight of the continent on the fluid-like molten rock underneath was lessened, so those areas likely rose a bit, adding to the height of the Wallowas and the Seven Devils today.

Geology wasn’t quite through with Hells Canyon. In the Pleistocene ice ages, the Great Basin held Lake Bonneville. The Great Salt Lake is the remaining fragment of Lake Bonneville. Lake Bonneville overflowed, cutting a channel through Red Rock Pass on the Idaho-Utah border. The released water, a truly epic flood, careened down the Snake River through Hells Canyon to the Columbia River in southwest Washington. Unimaginably immense amounts of floodwater. The flood (or floods) left gravel bars behind that testify to the immensity of the Bonneville Flood. The gravel bars tower 150 to 450 feet above the Snake River.

Part of a mile-long gravel bar, 200 feet above the Snake River, left by the Bonneville Flood

Today, the Snake River is more like the Snake Lake, dammed more than a dozen times, three times – Brownlee, Oxbow and Hells Canyon Dams – in Hells Canyon alone. The dams work great ecological harm, endangering fish, altering the water chemistry and warming the water. But, in geologic time, they are less than a pimple on the butt of the universe. They will be gone in a geologic eye-blink, like the basalt flows from the Columbia basalt floods 15 million years ago, and even the arrival of the Seven Devils terrane 250 million years ago. In the contest between erosion and land, erosion always wins.

But in human time, what’s left of Hells Canyon is one of nature’s great spectacles and adventures, as well as a pretty cool series of geology lessons.

 

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1. Note to any Real Geologists who wander by: Yes, WC is grossly simplifying this stuff. No, we’re not going to get in to the Wrangellia theory. Yes, WC knows about the Bonneville revisionism, but doesn’t buy it. ↩