Geological map of North America showing (in white) the Midcontinent Rift, here labeled Keweenawan Rift.

Lava flows created by the Midcontinent Rift System in the Lake Superior area were formed from basaltic magma. The upwelling of this magma may have been 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. Voluminous basaltic lava flows erupted from the central axis of the rift, similar to the rifting of the Afar Depression of the East African Rift system. The southwest and southeast extensions represent two arms of the triple junction while a third failed arm extends north into Ontario.[9][10] This failed arm now forms Lake Nipigon. It is also possible that the rift is the result of extensional forces behind the continental collision of the Grenville orogeny to the east which in part overlaps the timing of the rift development.[9]

It is likely that later compressive forces from the Grenville orogeny also played a major role in the rift's eventual failure and closure.[11][9] Had the rifting process continued, the eventual result would have been sundering of the North American craton and creation of a sea. The Midcontinent Rift appears to have progressed almost to the point where the ocean intruded.[12] But after about 10-20 million years the rift failed.[11] The Midcontinent Rift is the deepest closed or healed rift yet discovered; no deeper rift ever failed to become an ocean.

It's early, and I'm groggy, but it seems to me this excerpt, compliments of wikipedia, could shed some light on the recent discoveries in the Beardmore/Geraldton area.

GLTAL