It’s not a desert, although the area only gets about seven inches of precipitation a year, on average. It’s a playa, the dry bed of a former lake, and it’s about 8 miles wide and 70 miles long. The surface of the dry lake bottom is absolutely, unnervingly flat, and bare of any visible trace of life. WC was there last just week.
The playa surface is at an elevation of about 4,060 feet. It’s high desert. The block fault face of Steens Mountain towers over the playa, rising more than a mile overhead. Steens Mountain was intermittently glaciated in the late Pleistocene and Pliocene, from 3.5 million years to 15,000 years ago. No ordinary lake can survive for 3.5 million years; they fill up with sediment. But the Alvord Playa is part of a graben, a fault-bounded block of crust that has sunk several thousand feet as Basin and Range faulting has lifted Steens Mountain up.
The area is still geologically active, but it has a 17 million year of catastrophic geology, quite possibly longer. Beginning about 16.5 million years ago, the area was ground zero for the beginning of the Columbia River Basalt flows, the long series of flood basalt eruptions that covered eastern Oregon and parts of Idaho and Nevada in thousands of feet of molten rock. That was followed by a series of rhyolite flow, the last of which, the Rattlesnake Formation, formed a tuff that caps much of the terrain today.
About 7 million years ago, basin and range faulting started and over the next five million years or so, the easterly edge of Steens Mountain rose up like an opening trap door. The westerly side has a gentle slope of about 3 degrees or so down to the Malheur Basin. The east face, the rising lip of the geologic trap door, was a sheer cliff, dropping nearly straight down to the Alvord Desert. The Rattlesnake Formation tuff, the mostly topmost layer of rock, was hoisted more than a mile above where it was deposited.
Steens Mountain rose high enough that during the Ice Ages it developed considerable glaciation. While the most impressive glaciers were on the westerly side, carving the Blitzen, Fish Creek and Kiger Creek Gorges, massive, glacier-carved canyons. But glaciation also carved up the easterly face. leaving it deeply dissected today. The Columbia River Basalts have been lifted thousands of feet up, and the underlying rock – the surface before the flood basalt events – is exposed. And that exposed basement rock is . . . andesite, another kind of volcanic rock, ejected from long-vanished volcanoes.
Since Steens Mountain rose up, and the relatively wetter Ice Ages ended, the Pacific Ocean moisture moving east across Oregon gets sucked out by the Coast Range, and then the Cascade Range, and then Steens Mountain. Which is the principle reason that, today, it’s called the Alvord Desert. But until about 10,000 years ago, it was Alvord Lake; small remnants remain today. Alvord Lake didn’t just evaporate, though; in the late Pleistocene there was at least one major flood event, when Alvord Lake cut through the tuff and ash and drained 10,000 cubic feet per second down Crooked Creek to the Owyhee River to the Snake River. There my have been more than one flood event; the thing about catastrophic floods is they tend to obliterate the signs of any previous ones.
So if you visit the Alvord Playa some day, and sit out on the hard-packed clay and salt surface, consider the geology that has gone before: huge flood basalts, gigantic, incinerating ash flows, mountains springing up in a geologic eye blink, big glaciers, even bigger lakes and stupendous floods. A lot of geology that came before the quiet playa that’s there today.