[Magazine] Scientific American. Vol. 269. No 4

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Tarduno and his collaborators at the Scripps Institution of Oceanography, in conjunction with Mahoney, conclude that the Ontong Java province was constructed in less than three million years. Our analysis of rock dating, conducted by Hugh Davies, formerly of the Australian Geological Survey Organization, and Hubert Whitechurch, formerly of the Uni46 of the most voluminous igneous provinces were so huge that they must have originated at least in part in the lower mantle, most likely at the D′′. The spheres on the right depict the minimum (white) and maximum (dark orange) inferred diameters of the plumes associated with Þve major igneous provinces.

McKenzie; SCIENTIFIC AMERICAN, July 1989]. Under certain circumstances, the plume may penetrate the thick central regions of continental blocks and give birth to a continental ßood basalt. If the upwelling mantle plume surfaces underneath the seaßoor, it may give rise to an oceanic plateau. Laboratory experiments suggest that a drawn-out tail of hot material should lag behind the spherical head of the plume, yielding a long-lived, focused source of magma. Over millions of years, plate motions cause the ocean ßoor to migrate over the site of the hot spot.

The eruptions that built up the Ontong Java province unleashed between 12 and 15 cubic kilometers of igneous rock each year; Deccan volcanism produced between two and eight cubic kilometers annually. Assuming that the creation of igneous provinces, like other eruptive processes, occurs in Þts and starts, the pace of crustal production may have been far greater in some years. For reference, Roger L. Larson of the University of Rhode Island estimates that the global network of mid-ocean ridges has yielded between 16 and 26 cubic kilo- SCIENTIFIC AMERICAN October 1993 meters of ocean crust a year over the past 150 million years.

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