The Columbia River Basalt Group (CRBG) is the youngest, smallest and one of the best-preserved continental flood basalt province on Earth, covering over 210,000 km2 of mainly eastern Oregon and Washington, western Idaho, and part of northern Nevada.
The thick, layered lava flows of the CRBG erupted as flood basalts, which originate as some of the most highly effusive eruptions in the world. The CRBG sequence a classic example of flood basalt activity that erupted more than 350 lava flows from about 16.7 Ma to 5.5 Ma. The eruptions originated from a series of generally north-northwest-trending linear fissures, ranging from tens to hundreds of kilometers in length, located along the Washington/Oregon/Idaho border. The magma that fed these massive eruptions may have come from a plume-like upwelling from the mantle called a hot spot. Since the time of the CRBG eruptions, the North American plate has moved in a west-southwestwardly motion, and that hot spot is now believed to reside beneath Yellowstone volcano in northwest Wyoming.
Most (93%) of the flood basaltvolume erupted in a time span of about 1.1 million years (around 16.7 to 15.6 Ma). During that time, many flows erupted as extraordinary volumes, commonly exceeding 1000 km3 (240 mi3) and traveling many hundreds of kilometers from their vent systems. As the flood basalt flows moved across the ground surface, they crossed the Miocene Cascade volcanic arc through an east-northeast–trending lowland gap, where they spread across much of the northern Willamette Valley region. From there flows progressed through the Coast Range and eventually reached the Pacific Ocean where they continued to advance onto the continental shelf.
The Columbia River Basalt Group consists of seven formations: The Steens Basalt, Imnaha Basalt, Grande Ronde Basalt, Picture Gorge Basalt, Prineville Basalt, Wanapum Basalt, and Saddle Mountains Basalt. Many of these formations are subdivided into formal and informal members and flows. The formations, members, and many flows of the CRBG can be identified by looking at and measuring the rocks' chemical composition, physical characteristics, magnetic polarity, and by observing the position of lava flows in relation to each other (stratigraphy).
CRBG flows have interesting textural characteristics. Units often have a flow top, a dense interior and a flow bottom. The flow interiors can form regular patterns or styles during cooling including columnar-blocky jointing. This texture is often called "columnar basalt" and consists of mostly vertically oriented polygonal columns that can range from about 0.5 m (1.5 ft) to greater than 3 m (10 ft) in diameter. If columnar basalt is viewed from the top, the pattern looks like a bee's hive. The flow interiors can also include vesicle pipes and cylinders (essentially vacant spaces) as gas from the lava rose upward toward the top of the flow. Lava tubes are rarely observed in Columbia River Basalt Group flows except near their terminal margins. This is because these flows were emplaced as sheet flows and were not tube fed as are Hawaiian compound flows.
Read the comprehensive field-trip guide to the vents, dikes, stratigraphy, and structure of the CRBG in Oregon and Washington.