THE GRANDE RONDE BASALT AFTER 30 YEARS OF STUDY

The Grande Ronde Basalt comprises nearly 150,000 cubic kilometers and represents the peak period of volcanism of the Columbia River Basalt Group. Research over the past several decades has improved our understanding of the size of flows, where they erupted, and their characteristics. Recent age dating now places the Grande Ronde volcanic episode as less than 1 million years and perhaps as little as 750,000 years. Originally four magnetostratigraphic groups of flows were recognized in the Grande Ronde Basalt but more recent work suggests that there may be more magnetostratigraphic units that are represented by smaller volume flows. At least two magnetic excursions have now been recognized as well. The Grande Ronde Basalt has been subdivided into 17 groups of flows based on composition but more detailed studies indicate that this is probably a lower limit. We define the size of an eruption as flows or flow units with the same composition in stratigraphically adjacent lava flows. The largest eruption known, the Wapshilla Ridge Member, contains more than 25,000 cubic kilometers of lava. Several compositions, including the Wapshilla Ridge, are repeated through time and may represent cycles of recharge, differentiation, mixing, and eruption. Dike and vent areas that have been identified for most of the flows show that they erupted in western Idaho, eastern Washington and northern Oregon. Although the extent of flows has been used to suggest an increasing shift in volcanism northward with time, the dikes for all the eruptions, from the earliest ones to the last, do not appear to support this. Mapping has shown that dikes for flows throughout the sequence occur in the same area and suggest that location of Grande Ronde Basalt eruptions did not change over time.