What the Columbia River Flood Basalts Teach Us
Wednesday, April 28, 2021
by Allison Kubo Hutchison
Approximately 20 million years ago, prehistoric horses grazed on the flat grasslands and the now extinct bear-dog dug burrows for their young throughout the lands we now call Oregon and Washington. But below the ground, there was an eruption brewing that would shape over 81,000 square miles (200,000 square kilometers) reaching across four states. The sight of the black rock lying in thick sheets across the landscape should be familiar to those who have visited the area. The Columbia River Basalt Group is an eruption of lava resulting in inflows with a thickness of more than 5,900 ft (1.8 km). Dark basalt flooded the area filling in valleys and eventually repaving the topography like asphalt. The flows reached the ocean from Eastern Oregon and forced the Columbia River into a new course. Fifteen million years ago when it was erupting the landscape steamed covered in black rock that likely took thousands of years to fully cool. It may have looked like some of the recent eruptions in Hawaii or Iceland but for as far as the eye could see. For a perspective considering the recent eruption in Iceland, the Columbia River Basalt Group could cover all of Iceland. Twice. All within a short (geologically speaking) period of time likely only 1 million years.
The source of this voluminous melting is the same as the Yellowstone eruptions, a hot upwelling of the mantle called a mantle plume. The mantle plume comes from the hottest part of the mantle deep within the Earth and can heat the surrounding rocks to generate large amounts of melting.
Volcanic eruptions of this magnitude are dubbed flood basalts and the example which covers much of Oregon is only the smallest and most recent. Similar eruptions such as the Siberian Traps or the Deccan Traps have been associated with mass extinctions although this is still debated. The Columbia River flood basalts are associated with climate change and occurred at that same time as a period of elevated atmospheric carbon dioxide and some lesser marine extinctions. These eruptions are an important link between deep Earth processes and the atmosphere which shelters life on Earth. The Columbia River Flood basalts remind us that our landscape is shaped by a massive movement of the Earth and we are just riding on the surface.