by Allison Kubo Hutchison
About 20 million years ago, prehistoric horses grazed the flat meadows, and the now-extinct bear dogs dug caves for their young in what we now call Oregon and Washington. However, an eruption was brewing underground that stretched over 200,000 square kilometers and spanned four states. The sight of the black rock lying in thick layers over the landscape should be familiar to those who have visited the area. The Columbia River Basalt Group is an eruption of lava leading to tributaries more than 1.8 km thick. Dark basalt inundated the area, filling valleys and eventually repairing the topography like asphalt. The currents reached the ocean of eastern Oregon, forcing the Columbia River on a new course. Fifteen million years ago when it erupted the landscape was steaming with black rock, and it probably took thousands of years to cool completely. It may have looked like some of the recent outbreaks in Hawaii or Iceland, but as far as the eye can see. Given the recent eruption in Iceland, the Columbia River Basalt Group could cover all of Iceland. Twice. All in a short period of time (geologically speaking), probably only 1 million years.
The source of this voluminous melting is the same as the Yellowstone eruptions, a hot swell of the mantle known as the mantle cloud. The mantle cloud comes from the hottest part of the mantle deep in the earth and can heat the surrounding rocks to create large amounts of melt.
Volcanic eruptions of this magnitude are known as flood basalts, and the example that covers much of Oregon is only the smallest and most recent. Eruptions similar to the Siberian Traps or the Deccan Traps have been linked to mass extinctions, although this is still debated. The Columbia River’s flood basalts are linked to climate change and occurred concurrently with a period of increased atmospheric carbon dioxide and some minor ocean extinctions. These eruptions are an important link between deep earth processes and the atmosphere that protects life on earth. The Columbia River Flood Basalt remind us that our landscape is shaped by a massive movement of the earth and that we are only riding on the surface.