First assumption is violated at the inner core boundary and outer-core boundary, the second assumption at the upper mantle mineral phase transitions at 400 km and 650 km depth, the third assumption in zones having more rapid conveciton. This relation is useful in deducing the mineralogy of the mantle. This problem, however, is very non-unique, and several mineralogies have been proposed for the mantle. Surface observations of gravity provide constarints on crustal thickness, lithospheric thickness, the viscosity and patterns of mantle convection, and plate forces. First discovered and described by surveyors who noticed smaller than excpected deflections of plumb - lines by mountains.
Rigid blocks float in a more fluid-like asthenosphere.
B. Corretlation of gravity and topography. This scale at which correlation begins has been proposed to be a measure or definition of lithospheric plate thickness. In some areas there are non-zero isostatic gravity anomalies. A time lag exists between the recovery of the lithosphere from the load of continental galciers. When loaded, the lithosphere buckles downwards.