New, Surprising Picture Emerges of Sub-Seafloor Magma Formation
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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. 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.
Funded by the National Science Foundation, the $7 million project is called the Mantle Electromagnetic and Tomography Experiment, also known as MELT. 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.
"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."
In this week's issue of Science, geophysicists from Brown University and six other institutions report that melting rock flows up in a broad zone in the Earth's upper mantle, rather than in the 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.
"The seafloor spreading process is like a conveyor belt carrying away crust from mid-ocean ridges," says Donald Forsyth, a geologist 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, such that melting would be pretty much the same no matter where a ridge opened up."
The early findings are reported in a special section in this week's Science. An overview article and seven research papers present 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 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. Scientists had debated about mantle activity, and it turns out that their predictions were at least partly wrong. 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.
Other institutions involved in MELT are: the Carnegie Institution of Washington, the Scripps Institution of Oceanography in San Diego, the University of Colorado, the University of Oregon, the University of Washington in Seattle, and the Woods Hole Oceanographic Institution in Massachusetts.
Editors: visuals are available.
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