Geophys. Res. Lett.http://doi.org/mjg (2013)

The topographic depression that today forms Lake Superior in North America developed more than a billion years ago during a period of volcanic activity. Maps of the distribution and thickness of the lava flows show that this region once marked the boundary of a tectonic microplate.

Miguel Merino at Northwestern University, Illinois, and colleagues analysed the volumes of magmatic rocks in the crust around the Great Lakes using gravity data, which show thick and dense magmas as gravity anomalies. The lava flows form two distinct arms, one extending to the southwest into Kansas and the other extending southeast through Michigan. The western arm was more volcanically active compared with the eastern one, and the volume of magma increases towards Lake Superior. The patterns of magmatism are consistent with a scenario of eruptions above a mantle plume that upwells beneath Lake Superior. The plume could have caused the North American continent to begin to break apart. The western arm of volcanic activity therefore marked a nascent plate-spreading centre — the edge of a newly forming microplate.

The researchers suggest that the magmatic activity in the Great Lakes region marks the part of an evolving plate boundary that failed to develop into full continental break-up, rather than a short-lived episode of mid-plate volcanism as currently thought.