Modeling of gravity data along the ~1.1 Ga failed Mid-Continent Rift System shows systematic patterns in magma volume between and along the rift's two arms. The volume of magma increases towards the Lake Superior region, consistent with magma flowing away from a hotspot source there. The west arm experienced significantly more magmatism. These patterns are consistent with a model in which the two rift arms acted as boundaries of a microplate. The volume of magma along the west arm increases with distance from the Euler pole, indicating that it acted essentially as a spreading ridge, whereas the much smaller magma volumes along the east arm are consistent with its acting as a leaky transform. This view of the rift system's evolution is compatible with the rift being part of an evolving plate boundary system rather than an isolated episode of midplate volcanism.