The Pacific Northwest, a geographic region in western North America, was home to one of the planet’s most powerful known volcanic eruptions, according to a team of scientists from Washington State University (WSU) and the University of Auckland.
A smoking volcano. Image credit: Bela Marie.
Starting 16.5 million years ago, 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.
“It is the largest mapped flood basalt unit on Earth,” said WSU Professor John Wolff and co-authors.
The researchers estimate that, over tens of thousands of years, the floods put out between 242 and 305 billion tons of sulfur dioxide.
“This would have been devastating regionally because of the acid-rain effect from the eruption,” Professor Wolff said.
“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 Wapshilla eruption blanketed the Earth in an aerosol veil, creating the ‘Year Without A Summer’ and food shortages across the northern hemisphere. 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.
Davis et al sampled sulfur dioxide trapped in rock near lava vents to gauge how much gas was put in the air by massive basalt flows 16.5 million years ago. The inset map shows the extent of the flows, with the region in the black line showing the Wapshilla Ridge flow that emitted more than 200 billion tons of sulfur dioxide. Image credit: Washington State University.
Most of the lava’s gases were released during the eruption, but some of the gas remained trapped in crystals near the volcanic vents.
“We sampled melt inclusions and host glasses preserved in near-vent phreatomagmatic deposits associated with the voluminous lavas,” the scientists said.
“Sulfur (S) contents of melt inclusions range up to 0.19 wt% S, while host glasses are variably degassed with 0.01-0.13 wt% S. Incomplete degassing of glassy lapilli is attributed to phreatomagmatic quenching in the vent.”
“The magmatic S contents in the very voluminous (40,000 km3) Wapshilla Ridge Member scale up to 242-305 Gt SO2 release to the atmosphere over a maximum time period of 94,000 years.”
The Wapshilla eruption also provides an insight into the workings of climate change.
“It took place in what is known as the Miocene Climatic Optimum (MCO), when some 50 million years of cooling was interrupted by 5 to 6 degrees Fahrenheit of warming,” the authors said.
“But at its peak, the MCO had a brief cooling period that coincides with the Wapshilla eruption and its profusion of sulfur dioxide.”
The research is published in the journal Geology.
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Klarissa N. Davis et al. Sulfur release from main-phase Columbia River Basalt eruptions. Geology, published online October 2, 2017; doi: 10.1130/G39371.1
