THE STRANGE STORY OF THE INDIAN OCEAN GEOID LOW (IOGL)

In the early nineteen sixties, satellite tracking was able to provide spherical harmonics coefficients that define Earth’s gravity field. One of the early results was that just south of India, the geoid was at its lowest value. A geoid low can be expressed equally well as a gravity low which can be inverted to discover the source of IOGL. Considering spherical harmonics coefficients up to degree and order 9, which correspond to a wavelength of about 4000 km, the surface expression of the gravity low is circular which lends itself to iterative forward modelling. The inversion yields several alternative source bodies for the gravity low. A cylindrical body that extends up from the core mantle boundary and which plausibly represents an undiscovered mantle plume appears to be the most satisfactory solution. Seismic experiments could verify the existence of this plume.