## Geoid Anomalies and Dynamic Topography from Time-dependent, Spherical Axisymmetric Mantle Convection

Walter S. Kiefer (Lunar and Planetary Institute, Houston TX)

Louise H. Kellogg (Dept. of Geology, University of California, Davis CA)

Physics of the Earth and Planetary Interiors, 106, 237-256, 1998.

Abstract:Geoid anomalies and dynamic topography are two important diagnostics of mantle convection. We present geoid and topography results for several time-dependent convection models in spherical axisymmetric geometry for Rayleigh numbers between 10^{6}and 10^{7}with depth-dependent viscosity and mixtures of bottom and internal heating. The models are strongly chaotic, with boundary layer instabilities erupting out of both thermal boundary layers. In some instances, instabilities from one boundary layer influence the development of instabilities in the other boundary layer. Such coupling between events at the top and bottom of the mantle has been suggested to play a role in a mid-Cretaceous episode of enhanced volcanism in the Pacific. These boundary layer instabilities produce large temporal variations in the geoid anomalies and dynamic topography associated with the convection. The amplitudes of these fluctuations depend on the detailed model parameters, but fluctuations of 30-50% relative to the time-averaged geoid and topography are common. The convective planform is strongly sensitive to the specific initial conditions. Convection cells with larger aspect ratio tend to have larger fractional fluctuations in their geoid and topography amplitudes, because boundary layer instabilities have more time to develop in long cells. In some instances, we observe low-amplitude topographic highs adjacent to the topographic lows produced by cold downwellings. We discuss applications of these results to several situations, including the temporal variability of hotspots such as Hawaii, the topography of subduction zone outer rises, and the topography of coronae on Venus.Text of article (on Elsevier ScienceDirect website)

Color Versions of Selected Figures

Figure 1. The spherically axisymmetric convection model geometry. The model coordinates are the radius, r, and the colatitude, theta. The radius of the planet's surface is a and the radius of the core is c. The model is rotationally symmetric about the azimuthal direction. Click on the image for an enlarged view.

Figure 1 is copyright © 1998 by Elsevier Science B.V.

Figure 2. Representative thermal fields for Model 1 (see paper for detailed description of model parameters). The model geometry is the same as in Figure 1, with the axisymmetry pole running vertically through the center of each image. Temperature is shown by the color bar, with non-dimensional values between 0 and 1. The dynamic surface topography produced by the mantle flow at each timestep is superimposed in gray on the top of each model, with substantial vertical exaggeration.

- Top Left: Figure 2a, non-dimensional time 0.03154.
- Top Center: Figure 2b, non-dimensional time 0.03310.
- Top Right: Figure 2c, non-dimensional time 0.03697.
- Bottom Left: Figure 2d, non-dimensional time 0.03838
- Bottom Center: Figure 2e, non-dimensional time 0.04026
- Bottom Right: Figure 2f, non-dimensional time 0.04155

Figure 5. Representative thermal fields for Model 2 (see paper for detailed description of model parameters).

- Left: Figure 5a, non-dimensional time 0.01805.
- Right: Figure 5b, non-dimensional time 0.02928.

Figure 8. Representative thermal fields for Model 3 (see paper for detailed description of model parameters). Note that the temperature color scale is different than used for Figures 2 and 5.

- Left: Figure 8a, non-dimensional time 0.02539.
- Right: Figure 8b, non-dimensional time 0.03582.

Figures 2, 5, and 8 appeared as black and white line drawings when published in Physics of the Earth and Planetary Interiors. The color images shown here as Figures 2, 5, and 8 are copyright © 1998 by Walter S. Kiefer. All rights reserved.Additional figures (includes a less technical description of this work)

Figure acknowledgements: Figure 1 was drafted by Janice Fong. Figures 2, 5, and 8 were prepared using software developed by Amanda Kubala and Scott Lee.