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.[40] Lava flows created by the rift in the Lake Superior area were formed from basaltic magma.[40] The upwelling of this magma was the result of a hotspot which produced a triple junction in the vicinity of Lake Superior. The hotspot made a dome that covered the Lake Superior area.[40] Voluminous basaltic lava flows erupted from the central axis of the rift, similar to the rifting that formed the Atlantic Ocean.[40] A failed arm extends 150 kilometres (93 mi) north into mainland Ontario where it forms a geological formation known as the Nipigon Embayment.[41] This failed arm includes Lake Nipigon, the largest lake entirely within the boundaries of Ontario.[41]