THE HEAT-PIPE MODEL OF EARLY EARTH

Early in its history, the Earth's mantle experienced far greater internal heating due to radioactive decay, tidal heating, and loss of accretional heat. While there has been considerable debate on the viability of plate tectonics over Earth's history, little attention has been paid to the role of volcanic heat transport. A wide range of geological evidence shows that the Earth experienced a heat-pipe phase for roughly a billion years prior to the initiation of plate tectonics. Diverse studies show that, prior to about 3.2 Ga, Earth's crust consisted of very thick (10s of km) and stable (spanning hundreds of Myr) sequences of mafic and ultramafic volcanic rocks with depressed geotherms, deep burial of surface materials, remelting within the mafic stack, and internal deformation driven by granitoid emplacement. These observations match the predictions for a planet in the heat-pipe mode, in which near-global volcanism results in a downgoing crustal conveyor belt moving cold surface materials to great depth producing a cold and thick lithosphere that does not participate in the flow of the mantle. Although there are plate tectonic explanations for these observations, distinctive plate tectonic features with high preservation potential such as paired metamorphic belts and passive margins are absent from the geologic record. Numerical models of heat transport in the heat-pipe mode reveal that heat pipes reduce stress in the lithosphere by removing buoyancy from the mantle, but as internal heating wanes, stresses increase leading naturally to a plate-tectonic regime. Heat-pipe Earth provides a coherent dynamic framework for the understanding of planetary evolution from initial crystallization of the lithosphere atop the magma ocean to the onset of plate tectonics.