Terrestrial planets are almost spherical because of gravity and the weakness of rock in large masses. The largest departures from sphericity are due to rotation and variations in buoyancy of the surface and interior shells. Otherwise, the overall shape of the Earth and its heat flow are manifestations of convection in the interior and conductive cooling of the outer layers. The style of convection is uncertain. There are various hypotheses in this field that parallel those in petrology and geochemistry. The end-members are whole-mantle convection in a chemically uniform mantle, layered convection with interchange and overturns, and irreversible chemical stratification with little or no interchange of material between layers. Layered schemes have several variants involving a primitive lower mantle or a depleted (in U and Th) lower mantle. In a convecting Earth we lose all of our reference systems. The mantle is heated from within, cooled from above and experiences secular cooling. Global topography and gravity provide constraints on mantle dynamics.

Topography

Although the Earth is not flat or egg-shaped, as previously believed at various times, neither is it precisely a sphere or even an ellipsoid of revolution. Although mountains, ocean basins and variations in crustal thickness contribute to the observed irregular shape and gravity field of the Earth, they cannot explain the long-wavelength departures from a hydrostatic figure.