OCGC Seminar - Mantle convection with strong plates: multiple solutions, planetary evolution, and the dynamic state of a planet


Dr. John Crowley
Canadian Geodetic Survey, 
Natural Resources Canada 


Pressure-temperature graph


Thursday, March 10th
11:30 a.m.  

233 Advanced Research Complex (ARC) 
University of Ottawa  



Plate tectonics regulates the thermaland chemical state of a planet andmay be a necessary condition for astable surface environment and ahabitable planet. Predicting the rateat which tectonic plates move iscrucial for understanding both thethermal and chemical evolution of aplanet. Classic dynamic models forplate tectonics, such as the thermalboundary layer model of Turcotteand Oxburgh [1967], are based onthe properties and energetics of aplanets mantle. For simplicity, theyneglect the very plates they attemptto characterize, making theassumption that the plates move atthe 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. A simple new boundary layer model for thermal convection with finite-strength plates has been developed [Crowley and O’Connell, 2011]. 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.