At https://phys.org/print446447917.html … first seismic evidence for mantle exhumation at an ultra lsow spreading centre. Note the words ultra slow – which means barely negligible (or as we later find out, 2cm a year). The area referred to is the Canyon Trough off Grand Cayman island in the Caribbean. Exhumation is a reference to the regurgitation of mantle crust – a major plank of mainstream Plate Tectonics. Through gaps at plate boundaries material from the Earth's interior is thought to emerge, forming new sea floor. The material builds up as submarine mountains and spreads the plates apart – or that is what the Plate Terctonics theory claims (producing mountain chains in the process). All plate boundaries are thought to be smaller versions of the Mid Atlantic Ridge – where a lot of volcanice activity occurs and apparently creates new sea floor forcing the Americas further apart than Europe and Africa. Here the material disgorged is largely magmatic and this is also true of plate boundaries elsewhere. The extrusion of magma predominates.
The study in Nature Geoscience is written up by geologists from Kiel (Germany), Austin (Texas) and Durham (UK) universities and this is said to allow a detailed estimation on how much sea floor is formed by mantle rocks without magmatic processes being involved. . The sea floor on the Canyon Trough was investigated seismically. Signals sent through rocks and sediment layers were reflected and refracted in different ways. Rock that has been melted and then resolidified has a different signature from rock that has not been melted as it is derived from the mantle (although the mantle is thought to be very hot). However, the research was not as simple as that as sea water changes the signature. They used sheer waves to get round this issue – which occur in only solid materials. They found that 25 per cent of young ocean floor is not magmatic – and presumably this will be relayed to other spreading zones around the world. Expect global models to absorb this statistic and throw up all kinds of unlikely theories that will be accepted as reality as this paper seems to suggest occured in the Caribbbean.