Columbia Plateau

The Columbia Plateau is a massive geologic and geographic region in the United States.

It spans across portions of Washington, Oregon, and Idaho. The plateau sits between two major mountain ranges. These are the Cascade Range and the Rocky Mountains. The Columbia River flows through the middle of this wide area. This region is a flood basalt plateau. This means it was formed by massive amounts of lava. It is part of a large igneous province. This term describes a large area of the Earth's crust covered by volcanic rock.

During the late Miocene and early Pliocene times, the landscape changed forever.

Lava flows engulfed much of the Pacific Northwest during this era. This process lasted for about 10 to 15 million years. Flow after flow of molten rock poured onto the surface. These layers eventually reached a thickness of more than 6,000 feet. As the lava rose, the Earth's crust began to sink. This happened because the crust subsided into the space left by the rising lava. This subsidence created the large, slightly depressed plateau we see today.

The volcanic activity produced the Columbia River Basalt Group. This group consists of seven distinct formations. These are the Steens Basalt and the Imnaha Basalt. They also include the Grande Ronde Basalt and the Picture Gorge Basalt. The final three are the Prineville Basalt, the Wanapum Basalt, and the Saddle Mountains Basalt. Many of these formations are further divided into specific members and flows. These layers of rock tell the story of the massive eruptions. They show how the lava moved across the ancient landscape.

The advancing lava also changed the path of the water. The ancient Columbia River was forced into its current course by the northwesterly lava flows. As the lava moved, it filled old stream valleys. This created natural dams across the landscape. These dams caused impoundments, which are large bodies of water or lakes. These ancient lake beds became places where history was preserved. Scientists find fossil leaf impressions and petrified wood in these layers. They also find fossil insects and the bones of vertebrate animals.

Scientists want to know why so much lava erupted here. The plateau is far from a plate boundary. This makes the massive outpouring of lava very unusual. Researchers established hardening dates for many individual lava flows to find answers. They discovered a specific pattern in the ages of the rocks. The youngest volcanic rocks are clustered near the Yellowstone Plateau. As you move toward the west, the lavas become older.

This pattern suggests a moving source of heat beneath the crust.

A probable explanation involves a hot spot. A hot spot is an extremely hot plume of deep mantle material. This plume rises from the base of the lithosphere. The lithosphere is the layer of crust and upper mantle that forms tectonic plates. This heat source likely starts at the boundary between the Earth's core and mantle. A temperature instability there triggers a plume hundreds of kilometers wide. This plume ascends directly to the surface of the Earth. The track of this hot spot sweeps from the west up to Yellowstone National Park.

While the hot spot remains stationary, the North American plate moves over it. This movement creates a record of the rate and direction of plate motion. We can see evidence of this heat through steaming fumaroles and geysers.

Today, many people live within this unique landscape. In Washington, cities include Spokane, Yakima, and Pasco. In Oregon, cities like Pendleton and Hood River sit on the plateau. Idaho cities include Lewiston and Coeur d'Alene. The area is part of the Nearctic temperate and subtropical grasslands, savannas, and shrublands ecoregion.