The volcanic rocks associated with the Midcontinent Rift System accumulated in at least eight basins in the Lake Superior area as the intensity and character of rifting changed in time and place. Each basin sank centrally, and the edges of some were eroded prior to the effusion of overlapping plateaus. Dike sets aligned parallel to the rift axis, the wide extent of many flows, and physical features indicating low viscosity show the volcanism to be dominated by fissure-fed flood eruptions, though a few central volcanoes were present. Al-rich olivine tholeiite is the dominant rock type, followed by transitional to weakly alkaline olivine basalt and a large proportion of high Fe tholeiite that grades into tholeiitic (basaltic) andesite. A few icelandites and more rhyolites are also present. The high A1 olivine tholeiites include rather primitive members that could have been derived with only slight loss of olivine from partial melts of mantle lherzolite that had not previously been depleted in incompatible elements. Some rhyolites show Sr isotopic evidence of crustal contamination, but others do not. Specific fractional crystallization models involving major and trace elements have been unsuccessful, and the petrogenetic relations between the flows and the associated and underlying Duluth Complex remain obscure.