Continents and Earth's rotational stability

Abstract:

Continents strongly constrain the planform of the mantle convection/plate tectonic system. They impose long wavelength structure on surface velocities and tend to collect subduction zones around their margins . Additionally, they may affect the ability of the underlying mantle to effectively cool, which would create persistent temperature gradients between subcontinental and suboceanic mantle. These effects will change the moment of inertia of the planet and may control its rotational dynamics. Configurations of the continents have been implicated for both Earth's apparent Cenozoic rotational stability as well as its potential for large scale true polar wander deeper in its past. Here we present investigations into how continents can affect Earth's long term rotational stability. We have developed a version of the mantle convection code Aspect with a free outer surface, allowing for isostatically compensated continents and dynamic topography due to plumes and slabs. This allows us to self-consistently calculate moment of inertia anomalies in mantle convection models with surface continents and lateral viscosity variations. We explore different surface fractions of continental material as well as different mantle viscosity structures to identify when continents have a controlling influence. Finally, we discuss implications for Earth history, during which both continental mass and mantle viscosity may have changed significantly.