We examined Grande Ronde Basalt lava flows from surface sections and boreholes throughout Washington, Oregon, and Idaho to determine chemical and physical properties that would allow the recognition and mapping of these flows on a regional scale. We estimate there are ~100 flows covering nearly 170,000 km², with a total volume of ~150,400 km³, that were erupted over four polarity intervals (reverse 1, normal 1, reverse 2, and normal 2) in ~0.42 m.y. These flows are the largest known on Earth, with individual volumes ranging from ~100 km³ to greater than 10,000 km³. Although all known Grande Ronde Basalt flows erupted in the eastern part of the Columbia River flood basalt province, the thickest and most complete sections (>3 km) occur in the central Columbia Basin. From the center of the basin, the number of flows decreases outward, resulting in a nearly complete stratigraphy in the interior and an abbreviated and variable stratigraphy along the margins. The areal extent of many flows suggests that the Chief Joseph dike swarm greatly expanded after Imnaha Basalt time, and now many dikes are buried beneath younger flows in the eastern part of the province.

The Grande Ronde Basalt has a relatively uniform lithology with only a few distinctive flows. However, when compositions are combined with paleomagnetic polarity, lithology, and stratigraphic position, the Grande Ronde Basalt can be subdivided into at least 25 mappable units. Grande Ronde Basalt flows are siliceous, with typically SiO₂ >54 wt%, MgO contents ranging from ~2.5 to 6.5 wt%, and TiO₂ ranging from 1.6 to 2.8 wt%, with an enrichment in iron and incompatible elements relative to mid-ocean-ridge basalt. Although most Grande Ronde Basalt flows have homogeneous compositions, some are heterogeneous. Dikes that fed the heterogeneous flows show that the first composition erupted was not typical of the flow, but as the eruption progressed, the compositions gradually evolved to the bulk composition of flow. The average effusion rate was ~0.3 km³/yr, with basalt volume peaking during the R2 polarity with the eruption of the Wapshilla Ridge Member. Eruption and emplacement rates for the flows are controversial, but available data collected from the field suggest that many of the flows could have been emplaced in a few years to perhaps a decade.