Ph.D. thesis, Harvard University, 1996.

Motion of Hotspots and Changes of the Earth's Rotation Axis Caused by a Convecting Mantle

Bernhard Steinberger


Abstract

Hotspots are used as reference frame for plate motions; the rotation axis is another reference point. This thesis investigates motion of hotspots and changes of the rotation axis caused by mantle convection.

An analytical model showing motion of hotspots opposite to plate motion is presented, followed by numerical calculations of advection of plumes in realistic mantle flow. The abrupt change in direction of the Hawaiian-Emperor chain implies an upper mantle viscosity under the Pacific of ~ 1.5* 10**20 Pas or less. Slow relative motion of hotspots requires high lower mantle viscosity, unless hotspots are located at large scale stationary upwellings that are currently unresolved by seismic tomography. For our preferred model, we obtain coherent motion of Pacific hotspots in the mean lithospheric reference frame, as well as relative to African hotspots, caused by return flow in the lower mantle antiparallel to plate motion. Coherent motion can largely explain relative motion of Pacific and African hotspots during the last 43 million years. Before that, it is still necessary to invoke additional Pacific-Antarctic motion at an unknown plate boundary. Mean lithospheric rotation can be reduced, but not eliminated.

Changes in the rotation axis caused by emplacement of non-hydrostatic excess masses were calculated. Results of eigenmode and time-domain approach were compared, with little difference found. Although the number of viscoelastic relaxation eigenmodes is infinite, for an adiabatic Earth mantle only two eigenmodes must be considered for approximately correct description. Results for viscous and viscoelastic Earth models are very similar. Neither a phase nor a chemical boundary within the mantle has a large effect on the results. The maximum rate of change of the rotation axis depends mostly on lower mantle viscosity. For realistic gradual emplacement of non-hydrostatic excess masses and realistic lower mantle viscosity, any significant change of the rotation axis requires several million years. The observed rate of polar motion suggests a lower mantle viscosity of not more than ~ 3.6* 10**24 Pas. Advection of mantle density heterogeneities was used to infer changes of the non-hydrostatic moment of inertia tensor and corresponding changes of the rotation axis. Results were compared with paleomagnetic results, and good agreement was found.