The Columbia River Basalt Group (CRBG) is one of the largest continental flood basalt provinces on Earth. This feature is defined by thick layers of solidified lava that blanket significant portions of the Pacific Northwest, covering parts of eastern Washington, Oregon, western Idaho, and northern Nevada. The CRBG is considered the youngest and best-preserved example of this type of large-scale volcanism, fundamentally reshaping the region’s geology.
Timing and Scale of the Eruptions
The lava flows that formed the CRBG occurred during the Miocene Epoch, beginning approximately 17 million years ago and continuing until about 6 million years ago. Although the activity spanned millions of years, over 95% of the total volume was extruded in an intense pulse lasting less than one million years, roughly between 16.7 and 15.9 million years ago.
The total estimated volume of basalt erupted ranges between 174,000 and 210,000 cubic kilometers. This material covered an area exceeding 210,000 square kilometers, creating a vast, high plateau. Individual flows were highly fluid and voluminous, with some traveling over 600 kilometers to reach the Pacific Ocean.
The Grande Ronde Basalt is the most voluminous single unit, constituting over 70% of the CRBG’s total volume. This concentration of material in a short timeframe highlights the intensity of the main eruptive phase, placing these events among the most significant in Earth’s history.
The Magma Source and Eruption Dynamics
The source of the CRBG’s magma is attributed to the initial arrival of the Yellowstone Hotspot, which represents a deep mantle plume. A mantle plume is an upwelling of abnormally hot rock from deep within the Earth. This rock melts as it nears the surface due to decreased pressure, generating the vast quantity of basaltic magma required for the flood basalts.
The magma erupted not from a single volcano, but through long, linear cracks in the Earth’s crust known as fissures. These fissure eruptions allowed the magma to flood the surface. The feeder dikes, which are solidified magma within these fissures, are concentrated in dike swarms, such as the Chief Joseph dike swarm in the eastern province.
The great flow distance was possible because the basaltic magma was very low in silica content. This low-viscosity, highly fluid composition allowed the lava to spread rapidly and thinly across the landscape. Eruption temperatures, typically ranging from 1000 to 1200 degrees Celsius, further contributed to this fluidity.
As the North American tectonic plate moved southwest over the stationary mantle plume, the volcanism migrated eastward. The main phase began in the Steens Mountain area in eastern Oregon and moved progressively northward along the fissure systems. This movement over the hotspot created the age-progressive volcanic track that extends today to the Yellowstone Caldera.
Defining Characteristics of the Basalt Flows
The solidified lava of the CRBG is characterized by distinctive physical structures visible across the Columbia Plateau. The most recognizable feature is columnar jointing, where flows cooled and contracted to form vertical columns. This jointing appears in two distinct styles: the colonnade and the entablature.
Colonnade and Entablature
The colonnade typically forms the base of a flow, consisting of thick, well-formed polygonal columns. These structures result from slower, uniform cooling from the base upward. Above the colonnade lies the entablature, characterized by smaller, more irregular, and often randomly oriented columns. The entablature structure results from accelerated cooling, likely caused by surface water seeping into the newly emplaced lava. This rapid cooling from the top downward created the complex jointing pattern.
Interflow Zones
The CRBG consists of hundreds of individual lava flows stacked on top of each other. The boundaries between these separate flows are known as interflow zones. These zones often contain weathered horizons, including ancient soils or sedimentary layers called paleosols. The presence of these layers indicates periods of quiescence between major eruptions. These gaps allowed for erosion, sediment accumulation, and the development of ecosystems before the next flood of lava buried the landscape.