No. 118: Jul-Aug 1998
According to plate tectonics, the earth's continents are being transported -- very slowly -- on a subterranean conveyor belt of sorts. Although plate tectonics, nee continental drift, was ridiculed 50 years ago, it has been very successful in accounting for many geological phenomena. The theory is rarely challenged these days, but there are anomalies out there.
"According to the simple plate-tectonic theory, the age of oceanic lithosphere is zero at a spreading oceanic ridge and increases with distance from the ridge. Thus the lithosphere of the central Atlantic, which current palaeogeographical reconstructions assure us began to open no earlier than 120 million years ago, has zero age at the Mid-Atlantic Ridge and, supposedly an age of about 120 million years close to the land masses of Africa and South America at the appropriate latitude. Yet, Bonatti and others (Nature, v. 380, p. 518, 1996) have now recovered samples of 140-million-year-old pelagic limestones not even from the edges of the Atlantic but right in the middle of the ocean, close to the ridge. How can this be possible?"
The only explanation (?) seems to be that this errant chunk of crust got "trapped" in the middle of the Atlantic -- like a misdirected suitcase on an airport conveyor belt.
(Anonymous; "Old Rocks near the MidAtlantic Ridge," Geology Today, 13:17, 1997.)
Background. Ocean crust is presently being formed by upwelling molten rock at the Mid-Atlantic Ridge and, consequently, has zero age. The oldest ocean crust is adjacent to the continents.