Stop #3 — Geologic History

Geologic History

The Columbia River Gorge is up to 4,000 feet deep and stretches 80 miles from Troutdale through the Cascade Range to The Dalles, Oregon.

The Columbia River Gorge began forming as far back as the Miocene (roughly 17 to 12 million years ago), and continued to take shape through the Pleistocene (2 million to 700,000 years ago), forming a huge basalt plain.

But the Columbia River Gorge wasn’t created by volcanic flows or giant floods. The river was here before the mountains. The river cut a path as the land was gradually pushed up by tectonic forces.

As recently as 165 million years ago, most of western North America was still part of the Pacific Ocean. The Yellowstone hotspot passed through the Snake River Plain about 10 million years ago.

Smith Rock is a remnant of a huge “super-volcano“, some 40 miles in diameter that erupted in Central Oregon 29 million years ago. A large caldera was formed when overlying rock collapsed into an underground lava chamber.

You can easily see layers of volcanic flows (pdf), topped by layers of flood sediments all along The Gorge.

During a 10–15 million-year period, lava flow after lava flow poured out, eventually reaching a thickness of more than 5,900 ft. The 1980 Mount St. Helens eruption had a Volcanic Explosivity Index of 5, with 0.29 cubic miles of volume ejected. Crater Lake was formed after a VEI of 7, with 62 cubic miles of ejecta.

The basalt cooled, creating hexagonal shapes as it shrunk. As the molten rock came to the surface, the Earth’s crust gradually sank into the space left by the rising lava.

The Cascade Mountain range was pushed up later by tectonic forces. The primordial Columbia River gradually carved a path through the rock in the Cascade Mountains over millions of years.

Hells Canyon on the border of Oregon and Idaho is the deepest river gorge in North America, 2,000 feet deeper than the Grand Canyon.

The waters of the Snake River began carving Hells Canyon out of the plateau about 6 million years ago and now flows more than 1 mile (1.6 km) below the canyon’s west rim on the Oregon side.

Some 12,000 years ago, near the end of the last ice age, the Bonneville Floods rushed down the Snake River, while the Missoula Floods occurred in the same period, but farther north.

The Washington Geological Survery has created a breathtaking journey through the ice-age floods that shaped the landscape of Montana, Idaho, Oregon, and Washington.

The Missoula Floods were a series of cataclysmic floods that swept periodically across eastern Washington and down the Columbia River Gorge at the end of the last ice age, between 15,000 and 13,000 years ago.

The ice sheet retreated 15,000 years ago causing enormous ice dams to break, around the time humans first appeared in the area in the current Holocene era.

The ice-age floods came long after the volcanic basalt was deposited…and did their share of remodeling the region.

(more at page bottom)

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