While temperature drop across the mantle’s basal thermal boundary layer (TBL) is likely $>$1000 K, the temperature anomaly of plumes believed to rise from that TBL is only up to a few hundred Kelvins. Reasons for that discrepancy are still poorly understood and a number of causes have been proposed. Here we use the ASPECT software to model plumes from the lowermost mantle and study their excess temperatures. We use a mantle viscosity that depends on temperature and depth with a strong viscosity increase from below the lithosphere towards the lower mantle, reaching about $10^{23}$ Pas above the basal TBL, consistent with geoid modelling and slow motion of mantle plumes. With a mineral physics-derived pyrolite material model, the difference between a plume adiabat and an ambient mantle adiabat just below the lithosphere is about two thirds of that at the base of the mantle, e.g. 1280 K vs.\ 835 K. 3-D models of isolated plumes become nearly steady-state >10-20 Myr after the plume head has reached the surface, with excess temperature drop compared to an adiabat for material directly from the CMB usually less than 100 K. In the Earth, plumes are likely triggered by slabs and probably rise preferrably above the margins of chemically distinct piles. This could lead to reduced excess temperatures, if plumes are more sheet-like, similar to 2-D models, or temperature at their source depth is less than at the CMB. Excess temperatures are further reduced when averaged over the plume conduit or melting region.

The Indian Ocean Geoid Low (IOGL) appears as a prominent feature if the geoid is, as usual, shown with respect to the Earth’s reference shape. However, if it is shown relative to hydrostatic equilibrium, i.e. including excess flattening, it appears as merely a regional low on a north-south trending belt of low geoid. For a mantle viscosity structure with an increase of 2-3 orders of magnitude from asthenosphere to lower mantle, which is suitable to explain the long-wavelength geoid, a geoid low can result from both positive density anomalies in the upper mantle and negative anomalies in the lower mantle. Here we propose that the IOGL can be explained due to a linear, approximately north-south-trending high-density anomaly in the lower mantle, which is crossed by a linear, approximately West-Southwest-East-Northeast trending anomaly low-density anomaly in the upper mantle. While the former can be explained due to its location in a region of former subduction and inbetween the two Large Low Shear Velocity Provinces (LLSVPs), we propose here that the latter is due to an eastward outflow from the Kenya plume rising above the eastern edge of the African LLSVP. We show that, with realistic assumptions we can approximately match the size, shape and magnitude of the geoid low.