Scientists have long held the belief that the fracturing of the Earth’s brittle outer shell into faults along the deep ocean’s mountainous landscape occurs only during long periods when no magma has intruded. Challenging this predominant theory, findings from a completed study show how differences in mid-ocean ridge magma-induced activity produce distinctly different types of ocean floor faulting. W. Roger Buck, Doherty Senior Research Scientist at the Lamont-Doherty Earth Observatory (LDEO), is one of a trio of scientists who developed these new models for faults seen at mid-ocean ridges where the Earth’s tectonic plates split apart and basaltic magma rises to form the oceanic crust that today covers two-thirds of the planet. The scientists’ work has culminated in the publishing of their findings in the April 7, 2005 issue of Nature.
Unlike faults on land, those formed along mid-ocean ridges are practically a dime a dozen. "The rate of fault generation across these ridges is a hundred times greater than on land," explains Buck. "And while land faults are easily eroded and often cut older faults in complex, hard-to-untangle ways, submarine faults break into newly formed crust and lithosphere and are little obscured by erosion. Recent observations show a huge range of fault types and sizes at ridges." These combined factors make mid-ocean ridges "the place to learn about how faults form and grow."
The team’s findings challenge the standard view that all faults at these ridges result from tectonic stretching of thin near-ridge lithosphere (the Earth’s brittle outer shell, where earthquakes are concentrated) in the absence of magma, hot molten rock from deep within the Earth. Among several recent observations that do not fit this standard model, two stand out: the first concerns where the faults form and the second deals with how far the faults slip. Faults formed at fast-spreading centers, like the East Pacific Rise, are tiny in comparison to faults that bound deep ocean hills at slow-spreading centers like the Mid-Atlantic Ridge. All ridge faults start off growing close to the ridge. Mid-Atlantic faults die only a short distance from where they are formed. In comparison, faults along the East Pacific Rise continue growing--although very slowly--much farther from the ridge axis. The new models show that these faults may form due to bending, not stretching, of the lithosphere.
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