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Oozing Magma of Ocean Ridge Tells About Mantle Below
Santa Barbara - March 5, 2001 A recent article in Nature reports new information about the movement of the upper mantle immediately underneath the Earth's crust. Plate tectonics is the surface manifestation of this movement. The plate including India is crashing into Asia, pushing up toward the Himalayan Mountains. The recent large earthquake in India is part of this movement. By analyzing the magma that oozes up from a long crack in the Earth's crust under the Indian Ocean, the researchers studied the movement of the mantle, according Frank Spera, professor of geological sciences at the University of California, Santa Barbara, and co-author of the paper. The scientists analyzed properties of rocks dredged from 5,800 kilometers along the Southeast Indian Ridge. Samples from 6 different voyages, including both American and French expeditions, were included. This project represents the first time that scientists have related the geochemical variability of the Earth's mantle to its movement, or geophysics -- on an intermediate length scale of approximately 400 kilometers below the crust. The connection of geochemistry to geophysics is a difficult bridge to cross, according to Spera. By using chemical tracers, principally the isotope composition of helium, the researchers discovered that small scale convection occurs in the upper 400 kilometers of the Earth's mantle. The upper mantle moves in small rotating gyres, with a three-dimensional helical flow. Fault lines sometimes correlate with these secondary flows. It has already been established that larger gyres on the scale of 2700 kilometers occur at a deeper level in the mantle. The mantle is the area between the Earth's core and its crust. Ocean ridges are excellent areas for the study of the mantle because the Earth's crust is thinner there than it is on the continents. The oceanic crust is created by spreading centers where magma comes up in sheets through ridges and then separates laterally. The magma wells up and then spreads apart. About 30 cubic kilometers of magma per year oozes up under all the Earth's oceans, according to Spera. The researchers are developing a detailed map of the composition of the Earth's upper mantle, using chemical tracers as a key. "You have to look at the surface to build a picture of what the mantle looks like underneath," said Spera.
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