Dr. John Crowley : Mantle convection... (13-10-2016)

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Dr. John Crowley (Ressources naturelles Canada)

Mantle convection with strong plates: Multiple solutions, planetary evolution, ans the dynamic state of a planet

Jeudi 13 octobre 2016 à 13h30/ Thursday, October 13, 2016, 1:30pm

Local PK-7605, 201 ave. Président-Kennedy, UQAM


Plate tectonics regulate the thermal and chemical state of a planet and may be a necessary condition for a stable surface environment and a habitable planet. Predicting the rate at which tectonic plates move is crucial for understanding both the thermal and chemical evolution of a planet.
Classic dynamic models for plate tectonics, such as the thermal boundary layer model of Turcotte and Oxburgh [1967], are based on the properties and energetics of a planets mantle. For simplicity, they neglect the very plates they attempt to characterize, making the assumption that the plates move at the same rate as the convecting mantle. Decades of numerical simulations have shown that this assumption does not hold when the strength of the plates exceed that of the convecting mantle beneath (due to temperature dependent viscosity). The numerical simulations also displayed a wide variety of solutions for the coupled plate-mantle system that have been difficult to characterize and classify due to significant differences in the formulation of models that exist today (2D vs 3D, thermal vs thermochemical, layered mantle vs homogeneous, different grids, different solvers, etc.).
A simple new boundary layer model for thermal mantle convection with finite-strength plates has been developed [Crowley and O’Connell, 2012]. Multiple solutions are found with three solution branches for the plate velocity representing three distinct modes of thermal convection. All three modes emerge naturally from the simple energy balance model and have varying degrees of plate-mantle coupling, as well as different dominant plate driving forces. One branch of solutions reproduces the classic convective scaling laws [Turcotte and Oxburgh, 1967], while two new branches of solutions with slower plate velocities represent sluggish-lid convection cells where the strength of the plate determines the dynamics of the system.
Together with numerical simulations, the analytic model is used to explore and gain physical insight into several key issues including: plate tectonics as a dynamic process, plate tectonics throughout the thermal evolution of the Earth, why similar planets like Earth and Venus are so tectonically distinct, and the likelihood of plate tectonics on other planets.