Geodynamics

Geodynamics is a subfield of geophysics dealing with dynamics of the Earth. It applies physics, chemistry and mathematics to the understanding of how mantle convection leads to plate tectonics and geologic phenomena such as seafloor spreading, mountain building, volcanoes, earthquakes, faulting and so on. It also attempts to probe the internal activity by measuring magnetic fields, gravity, and seismic waves, as well as the mineralogy of rocks and their isotopic composition. Methods of geodynamics are also applied to exploration of other planets.

Overview

Geodynamics is generally concerned with processes that move materials throughout the Earth. In the Earth’s interior, movement happens when rocks (or melts) deform and flow in response to a stress field. This deformation may be brittle, elastic, or plastic, depending on the magnitude of the stress and the material’s physical properties, especially the stress relaxation time scale. Rocks and other geomaterials are structurally and compositionally heterogeneous and are subjected to variable stresses, so it is common to see different types of deformation in close spacial and temporal proximity. When working with geological timescales and lengths, it is convenient to use the continuous medium approximation and equilibrium stress fields to consider the average response to average stress.

Experts in geodynamics commonly use data from geodetic GPS, InSAR, and seismology, along with numerical models, to study the evolution of the Earth's lithosphere, mantle and core.

Work performed by geodynamicists may include:

• Modeling brittle and ductile deformation of geologic materials
• Predicting patterns of continental accretion and breakup of continents and supercontinents
• Observing surface deformation and relaxation due to ice sheets and post-glacial rebound, and making related conjectures about the viscosity of the mantle
• Finding and understanding the driving mechanisms behind plate tectonics.

Physical concepts

Elasticity

A lot of geodynamics deals with the deformation of rocks in response to stresses. Stress is defined as the average force per unit area exerted on each part of the rock. The deformation can be measured as strain, a change in length normalized by the total length of the body. If the deformation is elastic, the rock can spring back to its original shape after the stress is released. In elastic solids, the strain is proportional to the stress.

Pressure is the part of stress that changes the volume of a solid; shear stress changes the shape. If there is no shear, the fluid is in hydrostatic equilibrium. Since, over long periods, rocks readily deform under pressure, the Earth is in hydrostatic equilibrium to a good approximation. The pressure on rock depends only on the weight of the rock above, and this depends on gravity and the density of the rock. In a body like the Moon, the density is almost constant, so a pressure profile is readily calculated. In the Earth, the compression of rocks with depth is significant, and an equation of state is needed to calculate changes in density of rock even when it is of uniform composition. Since it is not of uniform composition, information from seismology is also needed to determine the elastic properties of deep rock (see also Adams–Williamson equation).

As long as the deformation is small, the equations of elasticity can be used to describe how a solid deforms under stress. Equations for bending are used to calculate the effect on the lithosphere of adding or removing loads. Examples include bending of the lithosphere under volcanic islands or sedimentary basins, and bending at oceanic trenches.

See also

• Computational Infrastructure for Geodynamics

References

• Ismail-Zadeh, Alik; Tackley, Paul J. (2010). Computational methods for geodynamics. Cambridge University Press. ISBN 9780521867672. {{cite book}}: Invalid |ref=harv (help)
• Jolivet, Laurent; Nataf, Henri-Claude; Aubouin, Jean (1998). Geodynamics. Taylor & Francis. ISBN 9789058092205. {{cite book}}: Invalid |ref=harv (help)
• Turcotte, D.; Schubert, G. (2002). Geodynamics (2nd ed.). New York: Cambridge University Press. ISBN 0-521-66186-2. {{cite book}}: Invalid |ref=harv (help)

External links

• Geological Survey of Canada - Geodynamics Program
• Geodynamics Homepage - JPL/NASA
• NASA Planetary geodynamics
• Los Alamos National Laboratory–Geodynamics & National Security
• Computational Infrastructure for Geodynamics