Grande Ronde Basalts at the Grande Ronde

The southeast corner of Washington State provides an opportunity to see the source area of some of the great flood basalt flows that cover much of eastern Washington. While there are plenty of impressive basalt cliffs and canyons in Washington State, the canyon land topography of the Hells Canyon and Grande Ronde River Canyons as well as tributaries gives an even better impression of  thickness of the basalt flows.

View down Rattlesnake Creek to the Grande Ronde River.

Basalt lined slopes of Rattlesnake Creek

A feeder dike can be seen just west of the Highway 129 bridge across the Grande Ronde River. Schuster's 1993 map of the area marks the dike as Grande Ronde basalt. 

Feeder dike of basalt cutting through older basalt flows in Grande Ronde canyon

The Grande Ronde basalts are the most extensive flows by area and by volume These flows extend all the way to the Spokane area and all the way to the ocean in southwest Washington. The Grand Ronde flood basalts make up somewhere on the order of 85 percent of the volume of the Columbia River Basalt Group.

Camp and Ross provide one take on the massive out pourings of basalt from the mantle plume camp.   The Grande Ronde basalt came out of some of the earlier dikes in the Chief Joseph dike swarm in northeast Oregon and southeast Washington. The name Chief Joseph is derived from the fact the area of the basalt feeder dikes is within Chief Joseph's beloved homeland before he and his group of Nez Perce were forced off their land.

From http://www.mantleplumes.org/RadVolcMigrations.html