Plate Tectonic theory asserts that the tectonic plates slide over a substrate of ultramafic rock known as serpentinite that corresponds with the MOHO, a thin seismic boundary interpreted as a physical phase change due to temperature, aka an isotherm.
But again we have the usual outcome of geophysical theorising – the drilling of a hole to test the interpretation which, in this case, falsified the geophysical modelling. Even when plausible rock types are factored into the modelling, drilling always seems to cause geophysical embarrassment. Mind you some geophysical techniques do work such as the use of electromagnetics involving rock conductivities to locate metal sulphides underground but it’s the techniques based on gravity theory that seem to be problematical.
Extract from NCGT Newsletter Issue 35, page 33.
WHERE IS THE MOHO?
KERR, Richard, 2005. Pursued for 40 years, the Moho evades ocean drilling once again. Science, v. 307, p. 1707, 18 March. (Excerpt and summary of the article)
….IOPD (Integrated Ocean Drilling Program) drilled in mid-North Atlantic onboard the JOIDES Resolution failed to find the Moho after drilling through 1,415 m of solid rock. The seismic probing had put it at a depth of 1 km or less just off the Mid-Atlantic Ridge at about 30oN (at the intersection with Atlantis Fracture Zone), but drilling cores never showed any sign of the predicted fresh mantle rock. Seismologist onboard lamented “Identifying deep rock is a hard call to make based on seismic velocity alone. Rocks of different compositions can have the same seismic velocity. It’s a problem that plagues seismology”. We had the similar experience in the Kola Peninsula super deep hole. –Geology comes first, geophysics follows—Indeed! (D.R. Choi)