Volcanics All the Way Down

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There’s some very interesting geology in Oregon; unfortunately, it’s all buried under 2,000 feet of basalt. – Eldridge Moores

Moores was being ironic when he said that to WC’s geology class in 1971. But when you get to southeastern Oregon, he was also profoundly understating the case. The Owhyee Plateau, a seriously dissected volcanic platform in the southeastern corner of Oregon, tops out with at least 6,000 feet of volcanoclastics. WC says “at least” because in almost all of the plateau, even after 17 million years, erosion still hasn’t cut all the way down through the volcanic pile. Pretty much every form of volcanic product can be found in the Owyhee Plateau. We’ll look at a small sample, but first some background.

Whatever the underlying rock may have been, it was buried in the massive Steens Mountain flood basalts about 17 million years ago, the earliest part of the Columbia Flood Basalt Group. At Steens Mountain itself, at the base of the tilted fault block, there’s an exposure that suggests that the underlying rock may have been . . . wait for it . . . even older basalt, but geologists aren’t sure. In southeastern Oregon, subsequent eruptions from calderas like the McDermitt and Lake Owhyee Volcanic Fields buried the Steens Basalt in basalt, rhyolite, tuff, cinders and ash. Some of the calderas were 30 miles in diameter, implying truly massive explosive events. Almost all of those calderas were buried by still more volcanism.

From the distance of 10 millions years or so, the volcanism might seem continuous, but the geology reports long gaps when soils developed, plants grew and animals prospered. Only to be buried in subsequent volcanics. As ash and magma blasted out of the earth, the overlying rocks collapsed along faults, creating grabens. Most of which filled later with still more volcanics.

With that background, here are a few glimpses of a few of the volcanic deposits left behind.

Paleosol between two layers of rhyolite, Owhyee River, Oregon

A paleosol is soil that developed on top of earlier volcanic deposits, only to themselves be buried in later lava flows. Here, 10-12 inches of soil have been cooked to crumbly rock between two flows of rhyolite. Soil takes millennia to build up on bare rhyolite; there was a lot of time between these eruptions. Bright red bands in volcanic rocks are often paleosol, although you need to lens them to be sure. Iron-rich and water-altered volcanics can also be red.

Ash layers that slumped after deposit, then buried and welded together, Owhyee River, Oregon

The view here is about a meter across. There are hundreds of layers of ash, thin bands indicating multiple small eruptions. Those layers of ash later slumped downslope, bending into the curves you see here.

Landslide debris, mixed ash and cobbles, very poorly welded, Owhyee River, Oregon

This is a surficial landslide or mudslide, fairly recent and easily broken apart; there’s no sign of burning or heating or welding, as you might expect in a lahar.

Probable lahar, Owhyee River, Oregon

A lahar is a red-hot landslide, filled with steam, hot ash, mud, rocks and sometimes moving hundreds of miles an hour on a bed of red-hot steam. When it cools, many of the bits are welded together, and incinerated organic debris may be incorporated. That’s all present in this outcrop. Under a loupe, there were small bits of black carbon as well as the tell-tale red color.

Rhyolite dike pushed through loosely welded tuff; note the altered stuff on each side of the dike, welded by the heat of the intrusive magma, Succor Creek, Oregon

Any new surface eruptions of magma have to force their way through the thousands of feet of pre-existing volcanic debris. Here you can see a rhyolite dike that rose through existing welded tuff. We know the dike came later because it melted the tuff in thin bands on either side of the dike. As you would expect in a plateau that sustained repeated episodes of volcanism, the Owhyee Plateau is lousy with dikes and sills, their horizontal cousins.

Volcanic ash slide, buried, exposed and eroded, Owhyee River, Oregon

Volcanic ash is the most common product of rhyolitic eruptions. It piles up in thick layers and, when on a slope, eventually gravity pulls it down. This ash slid down a slope and was then buried by later eruptions which very slightly welded it. Eventually erosion exposed the welded ash, and has since eroded it.

Rhyolite eroded into hoodoos, Succor Creek, Oregon

Succor Creek is the next drainage east from the Owhyee River. While the volcanics of the Yellowstone hotspot are bimodal – sometimes sticky, viscous rhyolite and sometimes more fluid lavas – the upper layers, those exposed to the world today, are largely rhyolite. Some of is old, and has been exposed to erosion for a long time. When that happens, Rhyolite can erode into hoodoos like those you see here.

Coarse-grained tuff, poorly welded, Succor Creek Oregon

When tuff is green, it usually means there is chlorite present. Chlorite is not volcanic; it means that in making its way to the surface the hot rock passed through basement rocks containing chlorite. So, under the immense amount of volcanic rock, we can infer there is something else buried down there.

Owhyee Reservoir and Indian Creek Campground, Oregon

Overall, the site is pretty spectacular, even if you aren’t an amateur geologist. At mid-summer, it’s hot and crowded with fisherman and boats, but spring is delightful and quiet. Recommended, and not just for the rocks.