To refine this estimate, we need to go back and take additional samples of peridotite from the exposed lithospheric section so that we can achieve a higher resolution in the curve describing temporal variations of degree of melting of the mantle.
Why is the Mid-Atlantic Ridge north of the equator becoming gradually hotter? We can only speculate. Perhaps a wave of plume-derived hot mantle has been flowing southward toward the equator since a few tens of million years ago. We have hints that major oscillations in the intensity of mid-ocean ridge activity occurred in the distant past.
For example, studies by Roger Larson of the University of Rhode Island suggest that a mantle "superplume" roughly 100 million years ago caused swelling of mid-ocean ridges, faster seafloor spreading, rising sea levels, and warming of the climate as a result of larger quantities of carbon dioxide, methane and other greenhouse gases released from the mantle [see "The Mid-Cretaceous Superplume Episode," on page 22].
Much remains to be done before geologists develop a complete picture of mantle dynamics and its influence on surface geology. Debate persists as to the origins of mantle convection and whether it extends into the lower mantle. Indeed, symposia that include theoreticians, geophysicists, geochemists and petrologists invariably yield heated discussions and much dissent. On one point there is unanimity: Earths mantle is very much alive and is an exciting region to study.
ENRICO BONATTI holds degrees in geology from the University of Pisa and the Scuola Normale Superiore in Pisa, Italy. After coming to the U.S. in 1959, he spent several years as a research scientist in marine geology at the University of Californias Scripps Institution of Oceanography and as a professor at the University of Miami's Rosenstiel School of Marine Sciences. Since 1975 he has been with Columbia University's Lamont-Doherty Earth Observatory. Recently he has been teaching and researching in his native country. He has led or participated in expeditions in all the major oceans and in some remote but geologically intriguing lands, from the polar Ural region of Russia to the desert island of Zabargad in the Red Sea. Bonatti wishes to thank Daniele Brunelli, Anna Cipriani and Marco Ligi, who have collaborated with him in his research during the past decade.