From Washington State University:
Gases from ancient Inland Northwest volcanic eruptions blocked out sun, cooling planet
The Pacific Northwest was home to one of the Earth’s largest known volcanic eruptions, a millennia-long spewing of sulfuric gas that blocked out the sun and cooled the planet, Washington State University researchers have determined.
Only two other eruptions — the basalt floods of the Siberian Traps and the Deccan Traps — were larger, and they led to two of the Earth’s great extinctions.
“This would have been devastating regionally because of the acid-rain effect from the eruptions,” said John Wolff, a professor in the WSU School of the Environment. “It did have a global effect on temperatures, but not drastic enough to start killing things, or it did not kill enough of them to affect the fossil record.”
The research, which was funded by the National Science Foundation, appears in Geology, the top journal in the field. Starting 16.5 million years ago, they say, vents in southeast Washington and northeast Oregon put out a series of flows that reached nearly to Canada and all the way to the Pacific Ocean. The flows created the Wapshilla Ridge Member of the Grande Ronde Basalt, a kilometer-thick block familiar to travelers in the Columbia Gorge and most of Eastern Washington. The researchers say it is “the largest mapped flood basalt unit on Earth.”
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