Many of the Columbia River flood basalts that erupted in the states of Oregon, Washington, and Idaho during the Miocene are remarkably iron rich (12–17 wt% total FeO) and under volatile-free conditions would have been too dense to erupt through continental crust. Their eruption through >20 km of sialic crust sets limits on their minimum, preeruptive total volatile (H2O + CO2) concentrations. These limits are obtained by quantitatively comparing the average density of the overlying crustal column with the calculated effect of dissolved H2O or exsolved CO2 on the density of various Columbia River tholeiitic basalts as a function of depth. Minimum total volatile (H2O + CO2) concentrations range to >4 wt% and are therefore considerably higher than those found in tholeiitic basalts at oceanic islands, like Hawaii. These conclusions are consistent with the strong enrichments in other incompatible elements (e.g., K, P, Ba) observed in the Columbia River flood basalts. Such high concentrations of H2O + CO2 would have had a marked effect on the solidus temperature of the source region, which may explain the remarkably large individual flow volumes (∼103 km3) that characterize this flood-basalt province.