The Columbia River Basalt Group

Click on a formation or member in the table below to see the mapped flow extent, area, and volume.

Interactive diagram showing volume and area of all of the formations and the major members.

The Columbia River Basalt Group (CRBs) is a series of flood basalts. Flood basalts are large volcanic eruptions, similar to Hawaii, where a great deal of basalt is erupted and cover an extensive area. In this case, the CRBs are Miocene in age (17 million to 6 million years old), and originated from a wide area near the border of Washington, Idaho, and Oregon. The basalt mostly came from fissures in the ground, perhaps sourced from a hot spot that is now beneath the Yellowstone Caldera.

The CRBs cover an area over 87,000 miles², with an estimated volume of 50,000 miles³, and cover about 36 percent of the state. It reaches a thickness estimated at 16,000 feet in places.

The flows are comprised of seven major groupings (called Formations), with a total of over 350 individual lava flows. The formations are (oldes to youngest): the Steens Basalt, Imnaha Basalt, Grande Ronde Basalt, Picture Gorge Basalt, Prineville Basalt, Wanapum Basalt, and the Saddle Mountains Basalt. By far, the largest of the flows is the Grande Ronde Basalt, which boasts an estimated area of 58,500 miles² and a volume of 35,500 miles³. This flow erupted between 16 and 15.6 million years ago, but 72 percent of it erupted over about a 400,000 year period. The older flows have a much higher volume and area than the younger flows in general.

CRB Interbeds

Sedimentary interbeds lie in between CRB lava flows. Thin beds of non-basaltic rock between the different units show that a great deal of time passed between the flows. Enough time that sediment was able to accumulate on the ground surface, and covered by the next flow. Volcanic deposits from ancestral Cascade arc volcanoes, similar to the stratovolcanoes in the Cascades today also left behind swaths of ash and lahars in between CRB flows.

Anatomy of a lava flow

Many of the stacked flows are very similar looking to one another. Subtle differences in appearance and chemistry can tell you a lot about each flow's history.

Diagram of a typical Columbia Basalt Group flow.

The base of many of the flows, when they can be seen, are often characterized by pillow lava. This forms when lava is erupted underwater, forming rocks with smooth, rounded shapes. When hot lava meets cool water, the crust cools quickly and is inflated additional molten lava as it continues to move.

Just above the flow base lies what is called the colonnade. The colonnade cools over a longer period of time than the bottom and top of a flow because it is more insulated. The slower cooling causes five- to seven-sided columns to form. As the flow cools, it contracts. Cracks form in a uniform manner, creating the columns. These columns often look so perfect as to be man-made.

The top of the flow is closer to the surface and cools proportionally faster. This area is called the entablature, forming in a similar fashion to the colonnade, but often more disorganized. The columns are often more slender, curved, and are full of cracks going every which way. In most basalt flows, the entablature is overlain by an upper colonnade. Yet, in the Columbia River Basalt Group flows, the upper colonnade is usually absent.

View from the top of Dry Falls, in Dry Falls State Park, Washington. Photo credit: Dave Norman.

Glaciers

During the Pleistocene, an ice sheet covered northern Washington. This process repeated every 100,000 years or so for the last million years, and the latest was about 14,000 years ago. The ice was miles thick in places. Portions of the glacier repeatedly blocked large river drainages, including the Columbia and Clark Fork Rivers. Lakes formed behind the ice dams, and then small inland seas rivaling the size of the Great Lakes.

Regional map of Pleistocene glaciers, glacial lakes, and outburst floods.

Glacial Lake Missoula

The largest of the glacial lakes was Glacial Lake Missoula, formed in the Clark Fork River basin, in Montana. The lake was approximately 2,500 feet deep in places, and extended over 3,000 square miles. It held as much water as about half of lake Michigan. When the ice dam burst, all of the water drained in the matter of days and crashed over eastern Washington. The amount of water moving over a very short period of time carved deeply into the Columbia River Basalts, forming much of the landscape today.

The features it created are on a colossal scale. The water plucked giant columns out of the basalt, created smoothed mesas and giant potholes, formed ripple marks between 15 and 30 feet tall, creating the landscape we call the Channeled Scablands. The floods left behind many other distinctive features, such as mesas, buttes, coulees, potholes, giant ripple marks, and flood bars.At the time of the floods, Dry Falls was an active waterfall. It was 400 feet high and 3.5 miles wide, making it four times the length of Niagara Falls.

Potholes created from flood waters.

The massive volume of water over a very short period of time overwhelmed the ability of the landscape to conduct it to the ocean as quickly as it appeared. Wallula and Rowena Gaps were among the places that constricted the flow. Floodwaters ponded behind the gap and created a huge temporary lake, called Lake Lewis. The water took days to empty out of Glacial Lake Missoula, but took weeks to drain out of the Lake Lewis and into the ocean. Wallula Gap forced the water to back flood into the Yakima, Walla Walla, and Tucannon valleys.

In the aftermath of the flood, the process of river blockage, ice-dam lake formation, and catastrophic release occurred repeatedly. Evidence suggests that there could have been as many as 100 separate floods at intervals of about every 50 years.

Photo of giant ripple marks left behind by glacial floods. Photo courtesy of brewbooks, Flickr (Creative Commons)

Touchet Formation

The Touchet Formation is a thick rhythmic sequence of glacial flood deposits that are hundreds of feet thick in places. Sometimes informally called the Touchet beds, they consist of horizontal beds of sediment stacked on top of each other and record about 40 individual flood events. Beds are usually graded, meaning the grain size decreases upwards within each bed, giving the sequence its distinct appearance.

As the water from the glacial floods ripped over the landscape, it plucked boulders out of the Columbia River Basalt, scoured Palouse loess sediment, and transported both along with the floodwaters. It deposited boulders downstream, creating large boulder fields. In the relatively calm areas of the backwater valleys, the water left behind huge amounts of suspended sediment and loess it collected upstream.

Touchet beds in eastern Washington. Each color change from tan to brown represents a different flood event.

Lake Bonneville

There was an even larger flood from Lake Bonneville. Lake Bonneville was the predecessor of the Great Salt Lake in Utah. It was roughly the same size as Lake Michigan, and covered most of Utah in water during the same period as the Missoula floods. Lake Bonneville lacked natural outlets to the ocean and a natural dam of alluvial deposits at Red Rock Pass blocked the only easy path for the water. Eventually, the lake was able to breach the dam and overflowed into the Snake River. From there, it followed the Columbia River into the ocean. Though the volume of water was much greater than the other glacial floods, it happened over a longer period of time. Instead of days, it occurred over weeks. This allowed the water to remain in the river canyons through their entire length. It did not create a slackwater lake like the Missoula floods did, but the water scoured the area in and around the canyons heavily.

Washington's Ice Age Floods Story Map

Click on the image to open our Story Map about Washington's Ice Age Floods. This Story Map is best viewed on a desktop or laptop computer. Mobile devices will not show all of the content.

Get the Posters

	Ice Age Floods National Geologic Trail, Washington Section Daniel E. Coe and Ashley A. Cabibbo DNR Earth Science Week 2016
	The Cheney-Palouse Tract of Washington's Channeled Scablands Daniel E. Coe

Folds and faults

In the western portion of the Columbia Basin Province lies the Yakima Fold and Thrust Belt. The belt is a series of giant folds and faults created by regional compression of Washington that trend southeast to northwest. Ahtanum, Umtanum, Manastash, Toppenish and Yakima ridges were created from this deformation, as were the Horse Heaven Hills, Rattlesnake Hills, and Saddle Mountains. These structures deform the Columbia River Basalts. Generally anticlines form the ridges, and synclines lie in the valleys.

North to south compression and clockwise rotation of the western margin of North America created this stacked sequence of folds and faults. As the rocks fold, they crack and break, creating a series of faults to relieve the strain. Ongoing research suggest that many of these structures are still active today, creating an earthquake hazard for this region.

Quaternary-age faults in the Yakima Fold and Thrust Belt showing their estimated age, or last movement. Click on the map to view larger image.

The rolling grasslands of the Palouse Hills are the result of Pleistocene wind.

The Palouse Hills lie on the east side of the Columbia Basin Province, near Pullman. The area is known for picturesque rolling silt dunes and great farmland.

The Palouse Hills are covered in what is called loess. Loess is formed when wind accumulates very fine particles of silt and clay. Because the silt isn't deposited directly by a river or glacier, the loess forms smaller rolling hills rather than long ridges. Much of the sediment was deposited during the last ice age. The glacial floods also influenced the Palouse Hills, scouring the loess and depositing it along the Columbia Basin Province as the Touchet Formation.