Reflections on the physics of slow dynamics in rocks

Slow Dynamics is a peculiar property exhibited by many sedimentary rocks and was first reported in the mid 1990s. Slow dynamics refers to the process of changing the elastic state of a rock, softening its macroscopic stiffness with application of an AC acoustic drive. What makes slow dynamics unique is that the elastic state of the rock recovers as log(time) once the acoustic drive is turned off. Many creep-like mechanisms have been proposed to explain the effect, notably it resembles recoverable/reversible DC creep. The most common explanation is due to the effects of water, or a form of it, in the spaces between grains. However, it has been exceedingly difficult to pin down the real physics. One experiment with Berea sandstone at high vacuum and temperatures showed that slow dynamics still persists even at these conditions. Followup experiments are in progress, using a pure quartz Fontainebleau sandstone to avoid any effects of clays. Another experiment is planned which will mimic conditions on Mars where water in any form is of great interest to planetary geologists. [Work supported by the Office of Basic Sciences, DOE and by Laboratory Directed Research and Development funding.]