Steens Basalt

Porphyry

Photo from 2019

Lake Albert in southern Oregon USA occupies a graben, a down-faulted trough that began to form 10-15 or so million years ago. The Albert Rim (at left in the photo above) is 17-million-year-old Steens Basalt, so the faulting is younger than that. The lake is a remnant of a much larger pluvial (sustained by rainfall) lake that was here during glacial times.

Plagioclase feldspar is diagnostic as huge crystals (to 3 cm or more in my specimen above) in the Steens Basalt. Specifically, the plagioclase is toward the anorthite end member, i.e., more calcium than sodium in this aluminosilicate. Based on published analyses of other specimens it is probably in the labradorite-bytownite range. The crystals (phenocrysts, from Greek words meaning showing [visible] crystals) probably grew slowly down inside the magma chamber before the magma erupted. Then, when it reached the surface, the whole mushy package cooled quickly forming the very fine-grained basaltic groundmass of the rock which also has the big early-formed crystals in it. So it’s definitely fair to call this a porphyritic basalt. Porphyry is a rock with two distinct grain sizes; its name is from Greek porphyra, purple, as in the color of royalty, from a purple rock with large feldspar crystals found along the Red Sea in Egypt. Today in geology, the color is irrelevant, and the word refers only to rocks with two discrete grain sizes.

The Steens Basalt erupted about 16.8 to 16.7 million years ago, the first outpouring of magma in the Columbia River basalts. Later eruptions continued until about 6 or so million years ago. The Steens Basalt is one of the smaller members, but it reaches 3,000 feet thick and covers 19,000 square miles, extending from Oregon into California and Nevada.

Map of the northwestern U.S. from USGS (public domain), showing the approximate locations of Yellowstone hotspot volcanic fields (orange) and Columbia River Basalts (gray). Boundary of Yellowstone National Park is shown in yellow. Modified from Barry et al. (GSA Special Paper 497, p. 45-66, 2013), Smith and Siegel (Windows into the Earth: the geologic story of Yellowstone and Grand Teton National Parks: Oxford University Press, 2000), and Christiansen (USGS Professional Paper 729-G, 2001).

The timing of the Steens Basalt eruption is very close to the outbreak of the earliest known caldera associated with the Yellowstone Hot Spot, the McDermitt Caldera beginning at about 16.5 million years ago. That was just west of the common corner of Idaho, Oregon, and Nevada – also fairly close in space to the Steens Basalt. There’s almost certainly a close connection between the Columbia River basalts and the Yellowstone Hot Spot, but details remain enigmatic and controversial.

Pleistocene pluvial lakes in the Great Basin, from Santi and others, 2019, Lake level fluctuations in the Northern Great Basin for the last 25,000 years: Desert Symposium 2019: Exploring Ends of Eras in the eastern Mojave Desert, at: Zzyzx, California.

Marion, Brian, 2016: Spatiotemporal Slip Rate Variations Along Surprise Valley Fault in Relation to Pleistocene Pluvial Lakes. 10.13140/RG.2.2.17619.27689. M.S. thesis, Central Washington University. “ARF” is Albert Rim Fault, whose scarp is visible in my photo at the top.

Modern Lake Albert is a remnant of Lake Chewaucan, a Pleistocene pluvial lake that developed during the glacial period. Chewaucan included the modern Summer Lake, northwest of Albert, but it was small in comparison to the lakes in the main portion of the Great Basin, including Lahontan in western Nevada and Bonneville in western Utah.

Until July 2019, the only time I had been here was in November 1973, when it was 2:00 a.m. and cold; there was absolutely no one on the road besides me and my green Volkswagen Beetle. I was on my way home from Missoula, Montana, to Davis, California. That Thanksgiving was when Bob & Marian Lankston and I decided to definitely make the Tobacco Root Geological Society happen, which we did the following March.