There is a current debate within the geophysics community as to whether convection is likely to be 'layered' or 'whole'. Geochemical studies of mid ocean ridge basalts (MORB) and tectonically emplaced upper mantle material have found it to be depleted in 'incompatible' elements such as U and Th; whereas material thought to be derived from the lower mantle i.e. ocean island basalts (OIB) is found to be enriched in incompatible elements. This suggests that the upper and lower mantle are not well mixed so that convection cells are layered with the boundary being between the upper and lower mantle at about 670 km depth. However, geophysicists have employed seismic tomography and have detected features reminiscent of slabs at subduction zones penetrating through the 670 km and down to at least 1800 km depth, this supports the whole mantle convection hypothesis.