 During the Mesoproterozoic era of the Precambrian eon 1,109 million years ago, northwestern Ontario began to split apart to form the Midcontinent Rift System, also called the Keweenawan Rift.Lava flows created by the rift in the Lake Superior area were formed from basaltic magma.The upwelling of this magma was the result of a hotspot which produced a triple junction in the vicinity of Lake Superior.