Someone argues: "A subducting slab is colder and denser than the surrounding mantle
PHYSICS · Subjective · Fermi University Physics · Earth, Climate & Energy Physics · Seismology, tectonics and geodynamics · Seismology, tectonics and geodynamics
The Solid Earth, Ch Plates, creep and convection: the mantle in motion
Problem
Someone argues: "A subducting slab is colder and denser than the surrounding mantle, so it is a mass excess. A mass excess always raises the geoid, the way a buried dense body raises it over a static column. Therefore a slab must always produce a geoid high, whatever the viscosity structure of the mantle." Explain what this argument leaves out, and state whether a dense slab sinking through a mantle of uniform viscosity produces a geoid high or a geoid low at long wavelength.