The Columbia River Basalt Group

The Columbia River Basalt Group

Seventeen million years ago, huge amounts of molten basalt poured across the countryside. These eruptions were located in a 150 square mile area in South East Washington, NE Oregon, and Western Idaho. There were approximately 300 different eruptions. These eruptions stopped about 6 million years ago. These flows eventually covered 50,000 square miles. Over 20 of these flows reached the Pacific Ocean. In the Tri-Cities area, the basalt is 2 ½ miles thick.

Lava Flows

The lava flows that occurred from 17 to 14 ½ million years ago represented a high percentage of the total amount of lava produced. These early flows were not only larger and covered more area than the later flows, they were also more frequent. These early flows were approximately 10,000 years apart. The later, smaller flows were often hundreds of thousands of years apart. During the long periods of time between these later flows, rivers deposited much sediment and formed shallow lakes in the low areas found on the surface of the previous flow.

Tectonic Forces & Columbia Basin Formation

Regional tectonic forces; before the great lava flows began, stretched and fractured the crust while forming the Columbia Basin. These effects enabled the magma to more easily reach the surface and thereby contributed to its non-explosive eruptions. In addition, the magma's low silica content and low amounts of explosive gases, also contributed to the non-explosive nature of these eruptions.

Ridges & Valleys  Formation

After the flood basalts were complete, regional tectonic forces compressed the Columbia Basin from the north and south. These forces resulted in the creation of a series of east-west ridges and valley systems within the Columbia Basin. During this time much tectonic fracturing of the basalts occurred. In addition, as the basalt layers cooled, they produced cooling fractures.

Ice Age Floods

The fractures caused both by tectonic forces and by basaltic cooling and shrinking, made the basalt particularly susceptible to the erosive properties of the Ice Age Floods. This basalt erosion is responsible for the dramatic coulees, scablands and other geological features found throughout the flood paths.

Images of the Formation

View images of these formation in the Basalt Columns Photo Gallery