No. 61: Jan-Feb 1989
"In a dive on the submersible Alvin just west of the Mariana trench, scientists discovered a cache of unusual features, including chimneys spewing out mineral-laden cold water on top of submerged mountains that rise 2,500 meters from the seafloor. While volcanic eruptions form most sea-mounts, these mountains consist of a nonvolcanic rock called serpentinite, and oceanographers are not entirely sure how the serpentinite mountains formed."
The theory of plate tectonics has the Pacific plate diving under the Philippine plate along the Mariana trench. It may be that water trapped in the downgoing crust leaks out, rises, and serpentinizes the crust above. This altered rock, being lighter than that surrounding it, may slowly rise through it, eventually forming undersea mountains.
(Monastersky, Richard; "Novel Mountains and Chimneys in the Sea," Science News, 134:333, 1988.)
Comment. This all sounds pretty speculative, but those mountains had to come from somewhere.
Perhaps the serpentinite mountains are just one manifestation of a larger phenomenon: the chaotic slithering and popping up and down of crustal material. The following is from New Scientist:
"Geophysicists in California and Illinois say that they have found the Earth's "missing" crust by analyzing shock waves from earthquakes to determine the chemical composition of the Earth's interior. If the researchers are correct, then the view of the interior of the Earth that scientists have previously accepted is wrong.
"The geophysicists say that they have found minerals like those in the Earth's crust in a layer of crustal material, 250 kilometres thick, which starts about 400 kilometres below the surface and extends to a depth of 650 kilometres. There is enough crustal material at this level, according to geophysicists to form a crust 200 kilometres thick - the average thickness of the Earth's crust is only 20 kilometres.
"The material is not trapped at this depth: the layer acts like a conveyor belt which returns the crustal material to the surface by a process of convection. At the surface, the material cools and sinks along the subduction zones. Below the surface, it reheats and rises to join the crust again, along one of the Earth's midocean ridges."
(Anderson, Ian; "Seismic Waves Reveal Earth's Other Crust,: New Scientist, p. 28, November 26, 1988.)
Comment. An obvious question is: What does this repeated circulation of crustal material do to radiometric and indexfossil dating of the crustal material we can access at the surface? Large sections of the stratigraphic record are missing on our planet; maybe they have now been found.