Fossil Lakes and Tuff Rings

Fort Rock, aerial photo from the northeast, via WikiCommons and Atlas Obscura

During the Pleistocene, as recently as 12,000 years ago, central Oregon had a number of large lakes. One of them, Fossil Lake, located in south-central Oregon, in northern Lek County, was, at peak, about 30 miles wide and perhaps 250 feet deep. It’s vanished now. The last two ponds evaporated about 1877. But during the time the lake existed, a part of it was the site of intense, if episodic. volcanism.

Fort Rock from ground level, about 1.5 miles away

Fort Rock, an impressively large tuff ring near the unincorporated Oregon community of the same name, is the product of that underwater volcanism. As the hot magma at a vent contacted the bottom of the lake, the water exploded into steam, blasting fragmented basement rock, ash, baked lake mud and steam away in a rough circle. The hot rock, ash and mud welded into tuff, a soft rock. Over thousands of eruptions over thousands of years, the piles of volcanic debris rose above the surface of the lake, all the while being eroded by the windblown lake water. As the climate changed at the end of the Pleistocene and the lake receded, the volcanic vent erupted less frequently, leaving a very odd, ring-shaped rock structure, open to the south. The Fort Rock Tuff Ring is about 4,500 feet in diameter and the higher parts of the ring are about 200 feet above the central floor.

Easterly end of Fort Rock; note the wave-cut terrace in the middle right
Westerly end of Fort Rock

The southerly portion of the ring is missing, eroded away; the prevailing winds, during the Pleistocene and now, are from the south. The wind-generated waves of Fossil Lake eroded the soft tuff of the southerly side of the tuff ring away. Perhaps two-thirds of the ring remains. The westerly end shows the individual layers of erupted ash and rock, the distinct layers created. When you consider that the tuft ring stands hip deep in eroded debris, the number of eruptive events is pretty impressive.

Close up of the Fort Rock tuff

In this macro shot of the tuff wall, you can see the dark gray shattered fragments of erupted magma, brownish mud from the lake bottom and occasional fragments of the native rock, all in a matrix of poorly cemented volcanic ash. You can also see that it is weathered and crumbly.

Tuff layering in the north wall for Fort Rock

Again, the layering in this outcrop shows nicely the number of eruptive episodes from this particular vent. Geology, as WC has said before, is messy, but once in while you can see a really clear example of how a part of its works.

The Fort Rock Tuff Ring is near the easterly end of the Brothers Fault Zone, which traces a system of volcanic episodes that are progressively younger as you move northwesterly. The Newberry Craters are the most dramatic recent volcanic event in that progression. But the Brothers Fault Zone will have to be the object of another blog post.

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