Oregon Coast Notebook: An Embarrassment of Volcanics


Heceta Head, north of Florence, Oregon

Most readers will have heard of the Oregon Flood Basalts, the series of volcanic events between 17 and 11 million years ago that covered much of Oregon and Washington, and a chunk of Idaho, under immense lava flows. What they may not know is that two of the lava flows actually ran all the way from the source rifts in far northeastern Oregon to the Oregon Coast. Heceta Head, shown above, is a remnant of that epic volcanic event, basalt that is chemically identical and radioisotope dated to the source rock north of the Wallowa Mountains. The Grand Ronde flow, in particular, was extraordinary.

Map showing the approximate extent of the Grand Ronde lava flows, 15 to 13 million years ago

Those lava flows were so voluminous that after 15 million years of erosion by the waves of the North Pacific Ocean, at Heceta Head they are still more than 250 feet thick. That, as WC’s geology professor put it, is a “Whole lotta rock.”

Confusingly, there are volcanic rocks along the central Oregon coast that are much, much older than the Grand Ronde basalts. Some seem to be as much as 80 million years ago. It’s a puzzle that geologists are only recently beginning to tease apart. The older basalts appear to be part of a suspect terrane, a chunk of continental plate that drifted in and accreted to North America from out in the ocean somewhere. The terrane has been named Siletzia, and is believed to extend from the tip of Vancouver Island all the way down to southern Oregon.

Rotation of Siletzia (green) about a northern pivot point. The Klamath Mountains(blue) rotated with Siletzia, having been formerly adjacent to the Blue Mountains (also blue, and also since rotated) near the Idaho Batholith (right edge). Red dashed line is the Olympic–Wallowa Lineament. Original image courtesy of William R. Dickinson.

Siletzia carried its own volcanoes with it, likely a back arc, where some other chunk of ocean bottom had subducted under Siletzia, creating a set of Cascade Mountain-like volcanoes. The details are still being sorted out, but the obvious conclusion is that Oregon imported volcanics from out in the Pacific Ocean somewhere and added them to its coastal volcanic landscape.

The geology didn’t stop there. Based on paleomagnetic analysis – fossil magnetism preserved in the Siletzian rocks – the southern two thirds of Siletzia, after docking against North America, rotated clockwise some 35-75 degrees. Why? How? Gosh, those are good questions. One theory, and it is still very much a theory, is that the Yellowstone Plume fired up a lot earlier than previously believed, before the North American Plate moved west over it. When that geologic blow torch was under the recently created Siletzian-North American seam, it blew it apart.

Whatever the mechanism, it’s pretty clear that the Blue Mountains, in northcentral Oregon, are chemically and physiographically identical to the Siskiyou Mountains in southwestern Oregon. Something forced them a couple of hundred miles apart. Much of the evidence for whatever rotated Siletzia is now buried under thousands of feet of flood basalts. Whatever that force was, there were volcanic events attendant to it as well, evident on the central coast of Oregon today.

Flood basalts from hundreds of miles away. Imported volcanics from suspect terranes. Landscapes spinning like waltzing dance partners triggering still more basalt flows. All present today in the narrow little band of the Oregon central coast. You’ll agree that’s a whole lot of volcanics.

And the southern Oregon coast is even more dramatic. But that will have to be a subsequent blog post.

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