April GEOLOGY and GSA TODAY media highlights

Hartz and Podladchikov show that frictional heating, the heat produced when rocks deform, is a major component of Earth’s heat budget, which in turn controls many Earth processes. It has long been accepted that the temperature of Earth’s stiff outer shell (the lithosphere, also called “the jelly sandwich”) controls its strength, and thus how it deforms. Hartz and Podladchikov suggest that the opposite is also. Frictional heating is not a new concept in Earth science, but it has not previously been recognized that deformation locally can compete with radiogenic processes in being the main heat producer on a lithospheric scale. Furthermore, the calculations applied here are simple and easy to reproduce: Earth’s strength (“forces”) is multiplied by its rate of deformation. Both factors are given in lithospheric-scale strength diagrams. The simplicity of the new method allows the results to be checked against the strength of Earth and its surface heat flow (both are model-independent parameters that can be directly measured), and thus we can predict weakening due to shear heating without any additional assumptions. This new quantification of shear heating provides simple explanations for many grand-scale Earth processes, including the remarkable weakness of active mountain belts, their high heat flow, abundant magmatism, and deep earthquakes.

GSA TODAY Science Article

Temporal evolution of continental lithospheric strength in actively deforming regions

Wayne Thatcher and Fred F. Pollitz, U.S. Geological Survey, Mail Stop 977, 345 Middlefield Road, Menlo Park, California 94025-3591, USA

Earth has a strong, brittle outer shell that is broken into a series of plates. It is the interaction of these brittle plates that gives us mountain ranges, earthquakes, tsunamis, and volcanoes. But what drives plate tectonics, and how did the plates form" To answer these questions, we need to first determine the depth to which the plates extend and to understand what makes the plates brittle and strong. In an article published in the April-May 2008 GSA Today, Wayne Thatcher and Fred Pollitz, geophysicists with the U.S. Geological Survey, report on their studies of how plates deform in various settings, ranging from the shaking resulting from major earthquakes to the slow bending of plates beneath the weight of glaciers. Their results may require a major re-think of plate tectonics, for it appears that on geological time scales, only the upper 10 to 15 km of plates have any strength at all. This unexpected result contrasts markedly with the prevailing view of plates consisting of 100-km-thick beams, and may indicate that our understanding of the processes that give rise to mountains and that control plate movements is in need of reevaluation.

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