Until recently, geologists and geophysicists believed that the young volcanoes of the Cascade Range separated everything to their west from everything to their east. In their new paper, Blakely and Sherrod tear down that wall.
“Now we're looking at the Cascades as a mountain system,” Sherrod said. He sees himself in a faction of scientists that theorizes that – from the Snoqualmie Pass South – the Cascades are only five million years old, or younger (some volcanologists put their “birth” tens of millions of years before that). That would mean they might have formed after a 10 to 15 million year long period when lava oozed across 63,000 square miles of the Northwest. Those lava flows formed the Columbia River Flood Basalts, one of the largest such flows in the world and a defining feature of the Northwest.
Many subtle clues support this position. One is part of the Pacific Northwest experience: The “rain shadow” caused by the Cascades, for example, which block moisture from passing from the Pacific Ocean to eastern Oregon and Washington. With wet wetter in one side of the Cascades but not the other, you'd expect different vegetation, as is the case today. The Columbia River Basalts, however, contain fossils of wet-weather vegetation you might find in the Great Smoky Mountains, suggesting that the Cascades weren't there to block rainfall when the basalts formed.
Over the last decade, the USGS mapped faults around the Puget Sound area west of the Cascades to identify hazards in the heavily populated area. As they did, they found fault systems that seem to link up with faults deep in the basalts of the Yakima Fold and Thrust belt. Previously, most geologists thought the Cascade Volcanoes separated the Pacific Northwest into two different tectonic regions. Knowing that the basalts existed before the volcanoes means it's likelier that the two sides are connected and part of a larger, deeper fault system than previously thought, not isolated features. That doesn't mean that all the faults will rupture at the same time if one does, but it does show a more complex interaction of seismic stresses than once taught.
“You have to view this as a whole, a whole system. you can't just kinda look at things in one little piece in isolation,” says Joan Gromberg, a geophysicist who works with Sherrod.