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Black Buttes - 495,000 to 290,000 years ago

Black Buttes (jagged and pointed peaks to the left) and Mount Baker...
Black Buttes (jagged and pointed peaks to the left) and Mount Baker (snow-covered peak) in Washington. (Credit: Post, Austin. Public domain.)

Black Buttes, an arc-shaped ridge of craggy peaks, is the remnant of a large stratovolcano that stood in approximately the same location as Mount Baker, only higher. Its amphitheater shape was formed as glaciers carved out the altered core of the old volcano, while leaving the outer part of the edifice in place. The highest point is 2,851 m (9,355 ft), and Deming Glacier now occupies the amphitheater and former central vent.

Eruptions at Black Butte occurred between approximately 495 and 288 ka as stacks of andesite lava flows that are up to 600 m (1950 ft) thick where preserved. The base is widely covered by ice, so the flow stacks could be thicker. The initiation of activity in this region with robust production of lava flows demonstrates the unmistakable development of the southwestern focus of volcanic activity. Most Black Buttes eruption products yield ages in the interval of 350 to 330 ka.

Black Buttes' Satellite Volcanoes, 460 to 296 ka

At the same time as the Black Buttes' cone was forming, at least four sets of lava flows erupted from flank vents outside the main eruptive focus. The earliest three units are andesite and the most recent is a basaltic andesite. The andesite of Lava Divide (460 to 296 ka) was the earliest and longest-duration eruptive unit and likely produced a voluminous cone, but it has since been glacially stripped to a single ridge. From 455 to 366 ka, andesite lava flows erupted from a vent now concealed beneath Mount Baker, and at least two of the flows contain subhorizontal columns suggesting they were emplaced next to ice, likely a glacier.

Remnants of five different lava flows that postdate Black Buttes volcano and predate Mount Baker have been mapped, though source vents are certain for none of them. Composition for these lavas ranges from rhyodacite to basaltic andesite. The single rhyodacite (Boulder Ridge, 199 ka) is the only one known to have erupted in the area since the last of the Kulshan rhyodacites at 0.99 Ma.

Black Buttes (jagged peaks in the foreground) and Deming Glacier at Mount Baker, Washington. View to the east. (Credit: Scurlock, John. Public domain.)