It is postulated that a mantle superswell (convective upwelling thousands of kilometres in diameter) developed beneath a stationary supercontinent aggregated in the late Early Proterozoic, giving rise to Middle Proterozoic anorogenic magmatism across Laurentia. Magmatism is attributed to invasion and ponding of mantle melts in the crust, causing uplift and anhydrous partial melting of the lower crust. Products include vast sheets of rhyolite, syenogranite, anorthosite, and gabbro, extensive mafic dike swarms, and rifts containing up to 20 km of basalt. Rifting occurred as a consequence rather than the cause of mantle upwelling. Thermal dissipation of lithospheric mantle may have provided rheological conditions favorable for whole-crustal imbrication observed in the parautochthonous Grenville orogen. The Middle Proterozoic supercontinent engendered more pronounced anorogenic magmatism than subsequent supercontinents because of secular cooling of the mantle. Before the Middle Proterozoic, crustal mass may have been insufficient to produce supercontinents large enough to effectively insulate the mantle and promote superswells. The theory of supercontinental episodicity makes testable predictions concerning paleolatitudes, relative sea levels, and globally synchronous episodes of Proterozoic orogenic and anorogenic activity.