READING THE ROCKS

 

What was once a lava-filled submarine canyon of the Columbia River is now a spectacular mountain chain slashing through Oregon’s northern Coast Range. How did that happen?

Born of lava eruptions hundreds of miles away

Fifteen million years ago the largest lava flows ever recorded on Earth emerged in what is now Idaho, covering the Pacific Northwest, filling the prehistoric Columbia River channel, and plunging deep into the ocean. At that time the ocean shoreline was closer to the Willamette Valley than it is today and what is today Oregon’s coastline was underwater. The mouth of the Columbia River was farther south, closer to Nehlem Bay.

These lavas spewed from fissures in the Earth: weak spots where two land masses had collided eons before. They shot lava fountains as much as 5,000 feet into the air and flooded molten lava over 65,000 square miles in a matter of weeks. Though we don’t have the records to show it, these events must have sent poisonous gasses around the planet. It was, by any measure, an epic global event.

The lavas filled the channel of the Columbia River, pushing the entire river channel north. When the lava reached the ocean, it continued to follow the undersea river channel. Mighty rivers such as the Columbia have so much force that when they reach the ocean, they continue to flow and erode the seabed, creating massive submarine canyons. The modern Columbia River submarine canyon, off Astoria, is today as much as six miles wide and three thousand feet deep.

When the lavas filled the submarine canyon, they cooled rapidly, creating massive piles of glassy, fragmented rubble mixed with soft marine sediments. The result was a massive basaltic dike in the Pacific Ocean.

Enter colliding tectonic plates

Due to the fault line off our coast, where tectonic plates are colliding, the continent—including the continental shelf—began to rise some 13 million years ago. That uplift continues to this day. Slowly Oregon’s modern coastline began to emerge, and the inverted submarine canyon of the Columbia River rose out of the ocean, creating one of the tallest mountain chains on the Oregon Coast. Saddle Mountain, Sugarloaf Peak, Onion Peak, Angora Peak, Neahkahnie Mountain, and Haystack Rock were all created by this river of lava flowing into the ocean.

As the peaks of these solid, glassy rubble piles rose, they emerged as islands, separate from one another. As uplift continued, they emerged as a mountain chain. Slowly the soft marine sediments that surrounded them eroded away, leaving the peaks isolated, like sky islands. As soils developed, plants and animals began to colonize these rock islands. Isolated from surrounding land, new species began to evolve on the highest of these peaks, species that exist only on Saddle Mountain, Angora Peak, and Onion Peak and nowhere else on Earth. These extraordinary mountains—legacy of an epic volcanic event 15 million years ago—continue to spell out their geologic origin story in the carpet of rare wildflowers that appear every spring on these iconic peaks.

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