Understanding the softness in Earth’s lithosphere

June 15, 2015

Yale researchers have proposed a new model to explain the drop in elastic stiffness in the middle of the Earth’s continental lithosphere.

Lithosphere is the stiff layer of rock that lies atop the slow-motion convection of Earth’s solid, yet ductile, interior. It is the “plate” of plate tectonics, the system of interlocking fragments that explains earthquakes, volcanoes, and even the long-term variation of carbon dioxide in the atmosphere.

A softening in the middle of the continental lithosphere was discovered when the seismologists studied the structure of the United States using high-resolution techniques. This same softening was observed in other continents, as well, at a depth of 80 to 150 km. Researchers found the phenomenon puzzling, because softening detected by seismology is usually linked to softening that occurs over millions of years. If that were the case, the continental lithosphere would have a weak layer in it, and it would be difficult to explain the continents’ long-term stability.

Yale geophysicists Shun-ichiro Karato, Tolulope Olugboji (a former Yale student, now at the University of Maryland), and Jeffrey Park may have found the answer.

In a study published June 15 in the journal Nature Geoscience, they present a new model to describe the phenomenon. They say the softening is a natural consequence of the way rocks deform as temperatures rise below the melting point. The key is that this softening occurs in a second or less (the seismic time scale), rather than over millions of years.

Rocks are composed of strong mineral crystals separated by

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