The Black Basaltic Rock at Palouse Falls

Standing at the overlook of Palouse Falls, visitors can see that the basaltic rocks that make up the canyon have some sort of story to tell. These black rocks are part of the continental flood basalt province known as the Columbia River Basalt Group (CRBG), which covers an area of 234,000 square kilometers (Reidel et al., 2013; Vye-Brown et al., 2013). The thick package of basalt flows were the result of fissure eruptions between 16.7 and 5.5 million years ago (Reidel et al. 2013). Fissure eruptions are long cracks that allow magma to ooze out over large areas, which is currently more common in Hawaii than in the Inland Northwest. However, over a 1.1 million year period between 16.7 and 15.6 million years ago, 93% of the total volume of the CRBG is estimated to have been erupted (Reidel et al., 2013). Most of the rocks seen at Palouse Falls are part of this voluminous basaltic outpouring of magma which erupted approximately 15 million years ago.

Geologists estimate that the CRBG is the result of more than 350 continental flood basalt flows (Reidel et al., 2013). Geologists use groups, formations, and members to name rocks based on composition and age, similar to how biologists use genus and species to name different plants or animals. Swanson et al. (1979a) established the original naming system for the CRBG and its five formations: the Imnaha Basalt, the Grande Ronde Basalt, the Picture Gorge Basalt, the Wanapum Basalt, and the Saddle Mountains Basalt, from oldest to youngest. The Steens Basalt (Camp et al., 2013) and the Prineville Basalt (Tolan et al., 1989) were later added as distinct formations (Fig. 1 & 2), bringing the total to seven formal formations which make up the CBRG.

TOOLS FOR IDENTIFYING FORMATIONS
Geologists have identified formations, members, and flows within the CRBG by using paleomagnetic indicators and geochemistry (Reidel et al., 2013). CRBG flows include both normal and reverse polarities, which are indicators of changes in the Earth’s magnetic field that have aided geologists in identifying and correlating CRBG units (Reidel et al., 2013). Geologists have also used geochemistry to identify CBRG units by recognizing that different formations show different abundances of certain oxides or elements, such as TiO2, P2O5, Cr, MgO, CaO, Zr, and Ba (Reidel et al., 2013; Fig. 3). In the Palouse Falls area, geology students from Eastern Washington University (Hatch et al., 2014) have identified a series of Wanapum and Grande Ronde basalt formations, using geochemistry, which generally match previous work.

Images

Map