During the past few decades, most marine EM explorations were attempted to study electrical conductivity distributions in the upper mantle near
mid-oceanic ridges, hot-spot regions or subduction zones, where distinct lateral heterogeneities can be expected due to the vertical movement of mantle materials.
After that, MAR-ECO expanded the scientific cruises to the South Atlantic
Mid-Oceanic Ridge, from 0[degrees] to 34[degrees]S, in the Walvis Ridge Sector (WRS).
At
mid-oceanic ridges, rocks are hot and their density is low, making them buoyant or more able to float.
Thayer, T.P., 1969, Peridotite-gabbro complexes as keys to petrology of
mid-oceanic ridges: Geological Society of America Bulletin v.
The trough associated with the
mid-oceanic ridges is deeply depressed, and on occasion extends into the lower mantle.
Each year, Congress has reinstated funding for this program, which includes, among other activities, expeditions with the submersible ALVIN to
mid-oceanic ridges. Last year's fight between Congress and the administration over whether to keep funding two failing remote-sensing satellites (SN: 3/18/89, p.172) may also repeat itself.
Just as the scars and wrinkles on a person's skin can tell the story of a life, the faults and topography of the seafloor may reveal a history of the ocean crust as it is churned out at
mid-oceanic ridges. In particular, scientists have been studying the geometry of ocean ridges for clues to how the process of seafloor spreading works.
For researchers who have used multibeam systems in their studies, "the data have been like allowing us to remove several layers of opaque glass and get closer to an accurate definition of the seafloor," says Jeff Fox at the University of Rhode Island at Narragansett, who uses academic Seabeam systems in his research on the magmatic and tectonic processes at
mid-oceanic ridges. Fox says he and his colleagues think of the swath systems as "the American Express card: You don't leave port without it."
When scientists were outlining the scientific objectives of the Ocean Drilling Program (ODP) five years ago, they made drilling at
mid-oceanic ridges their number one priority.
Because these larger features have been well documented, Marsh's group focused on detecting smaller features such as the shape of the water above fracture zones that slice through
mid-oceanic ridges. "The computations are good enough to reliably see a feature that has an amplitude as small as 10 centimeters,' a precision two to three times better than that of previous maps, according to Marsh.
The seafloor has been likened to a giant tape recorder, because as it is churned out conveyor-belt-style at
mid-oceanic ridges, it becomes imprinted with the earth's changing magnetic field.
Peter Vogt and John Brozena, both marine geophysicists at the Naval Research Laboratory in Washington, D.C., studied the rates at which the six major
mid-oceanic ridges churn out new lithosphere (oceanic crust overlying tectonic plates).
