The North Shore Volcanic Group, a thick sequence of plateau lavas of Keweenawan (Proterozoic, 1,100 m.y.) age, was erupted during the development of the Midcontinent Rift of North America. It contains many well-exposed physical features that imply a range of eruptive styles and flow mechanisms. These include (a) high-volume and high-effusion-rate floods of basalt and basaltic andesite, which ponded and crystallized largely after flow had ceased; (b) moderate- to small-volume and -effusion-rate flows and small floods, some probably building low shields; and (c) small flows to very large flows and ignimbrites of icelandite and rhyolite lacking visible vent areas. Viscosities calculated from 183 chemical analyses correlate well with the physical features. The style of volcanism combined characteristics of the Columbia Plateau and the Snake River Plain ("plains volcanism") and was very similar to that of the Tertiary of eastern Iceland.