Although
the ridge has bathymetric expression, the sea floor morphology relates to the
age of the oceanic lithosphere (as discussed in the ridge
part of this site). Furthermore, upwelling is likely to produce domal or
elongate domal uplift. The abrupt termination and offset of ridge morphology
at transform faults is not consistent with active upwelling. Note that it can
happen where mantle plumes coincide with ridges - such as at Iceland.
An image showing subtle variations in P-wave velocity with depth around the Tonga trench. This is an example of seismic tomography. Here the Pacific plate can be traced, crossing the 670km layer and penetrating the deep mantle. This image comes from studies by Rob van der Hilst in the mid 1990s. Images like this (while are increasingly being created from new tomographic studies) are strongly supportive of single layer convection operating through almost all the mantle.
