Thermal Cracking- A Driving Factor in Plate Tectonics?

Sherwin Yu | sherwin.yu@yale.edu December 11, 2007

Plate tectonics on Earth has long been regarded as a somewhat puzzling phenomenon. Why is Earth’s lithosphere – the outer crust and the cold part of the upper mantle – divided into several massive plates that drift over the globe? No other terrestrial planets such as Venus and Mars show evidence of such a process.

While modern science does not yet have a complete answer, Assistant Professor of Geology and Geophysics Jun Korenaga has developed a new theory based on thermal contraction deep within the oceanic lithosphere. Conventional thinking predicts that, due to the low temperature of the lithosphere, the viscosity of the rock should create a stiff, singular shell that does not allow for any movement.

Viscosity is the tendency of a substance to resist flow, and, in the case of Earth materials, is strongly temperature-dependent; colder rock will resist flowing much more strongly than hotter rock will.

According to this logic, plate tectonics should not be taking place. Korenaga proposed, however, that this intuitively formidable property may actually help to explain plate tectonics. As rocks cool rapidly near the surface, their viscosity increases. Unable to flow and simply contract, the rocks undergo “thermal cracking.”

By considering the known physical properties of these rocks, Korenaga calculated that this crack system could extend down to depths of thirty to fifty kilometers beneath the ocean floor. The cracks allow water to seep deep into the lower part of the lithosphere, near the hot mantle. At this depth, serpentinization, or the hydration of mantle rocks to form serpentite, can occur.

According to Korenaga, “The deep cracking of oceanic lithosphere has an inevitable chemical consequence, i.e. serpentinization, and its likely outcome is to create the localized zones of weakness in the stiffest part of lithosphere as well as self-sustained subduction, thereby facilitating the generation of plate tectonics.”

Korenaga’s study is the first of its kind to provide a physically plausible mechanism to support a commonly-held speculation that water might be an important factor in plate tectonics. The development of this theory provides new insight that may improve scientists’ understanding of the forces that have shaped the world as we know it.