EDITOR'S NOTE: This is a joint release of Brown University, Carnegie Institution of Washington D.C., National Science Foundation, Scripps Institution of Oceanography, University of Colorado, University of Oregon, University of Washington, and Woods Hole Oceanographic Institution. Visuals are available (see end of release).
Under the South Pacific, a new picture emerges of Earth's most volcanically active area.
For the first time, seismologists have captured detailed images of the deep underground processes that give birth to most of the planet's new surface along mid-ocean ridges where the seafloor pulls apart. Some of the underground patterns confirm predictions. Others are a surprise. Together, the new data represent a major step forward in understanding the formation of the crust, convection in the planet's interior, and the source of the most abundant volcanic activity on Earth.
In this week's issue of Science, geophysicists from Brown University and five other institutions report that melting rock flows up in a broad zone in the Earth's upper mantle rather than in a narrower plume that some researchers had predicted. Under the separating seafloor, magma starts forming nearly twice as deep as scientists had expected and, surprisingly, wells up slightly off center, not directly beneath the ridge where most of the magma eventually erupts and cools to form new oceanic crust.
"This is a really exciting experiment because it is the first time we've been able to image the structure beneath the ocean floor on this scale," said Anne Sheehan, an assistant professor of geological sciences at the University of Colorado at Â鶹¹ÙÍø who headed one of the data analysis teams.
Sheehan's specialty is analyzing the velocity of seismic waves, which tells scientists about the rock structures the waves are passing through. She worked on the project along with CU-Â鶹¹ÙÍø doctoral student Hersh Gilbert and research associate Ken Dueker.
Sheehan has done similar work with seismic waves to examine geologic structures and analyze earthquake hazards in Colorado and other parts of the western United States.
The early findings are reported in a special section in this week's Science. An overview article and seven research papers present first direct observations of the mantle from seafloor seismic recorders and from other geophysical observations. More details will be added Friday, May 29, at the American Geophysical Union meeting in Boston, when results from the second part of the experiment using electromagnetic imaging techniques will be reported for the first time.
"The seafloor spreading process is like a conveyor belt carrying away crust from mid-ocean ridges," says Donald Forsyth, geology professor at Brown University and coordinator of the imaging project. "We're seeing a suggestion that the upper mantle beneath the oceans is stirred up by small-scale convection so that melting would be pretty much the same no matter where a ridge opened up."
Funded by the National Science Foundation, the $7 million project is called the Mantle Electromagnetic and Tomography Experiment, also know as MELT. Believed to be one of the largest marine geophysical experiments ever conducted, its main goal is to find where melted rock, also known as magma or "melt," is formed and how it moves to the ridge crest to form new oceanic crust.
About two-thirds of the Earth's crust is created at the bottom of the ocean, submerged and hidden from curious geologists. Around the globe, huge tectonic plates cover the planet like a cracked egg shell. Where the plates pull apart, mantle rock wells up to fill the gap. Some of the upwelling mantle melts, generating magma that percolates up to the surface. The latest results suggest that the moving plates drive the upwelling and direct the flow to the gap.
The new data come from an experiment deep in the South Pacific a thousand miles off the west coast of South America, where two plates are pulling apart faster than just about anywhere else on Earth. Geologically speaking, the large Pacific plate rushes westward, where it dives under the Aleutian Islands, Japan, Guam and Fiji in earth-shaking thrusts. To the east, the smaller Nazca plate moves more slowly toward Peru and Chile, where it also ducks under the continent and causes huge earthquakes.
In Fall 1995, a research team representing six institutions sailed from San Diego to a site in the South Pacific where the jagged ridge between the continental plates straightens for about 440 miles. On board the Scripps Research Vessel Melville were 51 ocean-bottom seismometers able to eavesdrop on the planet's seismic heaves and shudders two miles under the sea. Similar to the way medical imaging reveals structures and activities in the human body, seismic measurements can reveal the active innards of the Earth below.
"This is a major advance in marine geophysics made possible by new instrumentation that can record geophysical data on the seafloor for as long as a year," says John Orcutt, director of Cecil H. and Ida M. Green Institute of Geophysics and Planetary Physics at Scripps.
In the South Pacific, researchers dropped the equipment overboard in two rows nearly perpendicular to the ridge. For six months, the seismometers continuously recorded earthquake waves that rumbled around the planet's surface and waves that took shortcuts through the mantle or bounced off the core.
The results show that melt is generated over a much larger region than many scientists had expected. Some of the surprising observations will likely lead to the development of a new generation of models of mantle flow and magma generation beneath mid-ocean ridges.
"These results raise many new questions," says Bob Detrick, senior scientist at the Woods Hole Oceanographic Institution. "One of the most puzzling is how to reconcile the presence of melt over such a broad zone inthe mantle with volcanic activity at the seafloor that mainly occurs over a very narrow zone only a mile or two wide."
Scientists had debated about the mantle activity, and it turns out that their predictions were at least partly wrong. "Before the MELT project, two models competed to explain how the undersea upwelling of mantle materials occurred," says Doug Toomey, associate professor of geology at the University of Oregon. One model described a broad, shallow region of passive upwelling and a second model predicted a narrow, shallow and active upwelling zone. Neither model predicted the asymmetry of the upwelling magma or the depth at which melting occurs.
"The experiment has set a new standard for the type and scale of experiments to be done in the oceans," says Dave Epp, program director in NSF's marine geology and geophysics program, which funded the research. "We expect this line of research to continue for the next decade or more."
Visuals:
-- Illustrations of the results can be found at
-- A color slide of a seismometer being hoisted over the ocean from the ship is available from Carol Cruzan Morton, (401) 863-1860, carol_morton@brown.edu
Further information:
-- Marine seismology & MELT overview, darkwing.uoregon.edu/~drt/overview.html
-- Pictures of seismometers and undersea hot vents at mid-ocean ridge (non-downloadable format), darkwing.uoregon.edu/~drt/photo.html