Columbia Plateau

The Columbia Plateau is a broad region of the northwestern United States underlain largely by successive flood-basalt lava flows of the Columbia River Basalt Group.

Columbia River Basalt Group: A sequence of flood-basalt lava flows erupted across parts of the Pacific Northwest during the Miocene. These flows form the plateau’s underlying volcanic foundation.
Basalt: Basalt is a dark, fine-grained volcanic rock produced when low-silica lava cools. The plateau’s extensive bedrock consists chiefly of this lava-formed rock.
Columbia River Basalt eruptions: The Columbia River Basalt eruptions were immense Miocene episodes of lava outpouring across the Pacific Northwest. They built the basalt province that defines the plateau’s geology.
Palouse: The Palouse is a rolling agricultural region of southeastern Washington and adjacent Idaho, known for fertile loess soils. Windblown soils over basalt support the plateau’s extensive dryland farming.
Flood basalt: Flood basalt is an extensive province of solidified lava formed by voluminous, widespread eruptions. The plateau is a major example of terrain built by flood-basalt eruptions.
Fissure eruption: A fissure eruption releases lava through a long crack rather than a single central vent. Long fissures supplied many of the region’s extensive basalt flows.
Miocene: The Miocene is a geological epoch spanning about 23 to 5.3 million years ago. Most Columbia River Basalt Group eruptions occurred during this epoch.
Columbia Basin Project: The Columbia Basin Project is a federal irrigation project that supplies water to farmland in central Washington. It transformed parts of the plateau’s dry interior into irrigated cropland.
Snake River: The Snake River is a major river flowing through the northwestern United States, including Idaho and Washington. Its canyon cuts through basalt near the plateau’s eastern margin.
Grande Ronde Basalt: Grande Ronde Basalt is the thickest and most extensive formation within the Columbia River Basalt Group. Its immense volume accounts for much of the plateau’s basalt bedrock.

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