observations from many disciplines. In the study of active faulting, for example, geologists map faults, geomorphologists date fault motion, seismologists locate earthquakes, geodesists measure deformations, and rock mechanists investigate the frictional properties of fault materials. Numerical simulations of active fault systems attempt to bring together these various types of observations in the context of a self-consistent model. The success of such a model in reconciling diverse types of information can thus be used to confirm the compatibility of the data from different disciplines and ferret out inconsistencies, in addition to giving researchers confidence in their underlying assumptions and hypotheses.

The problems of relating observations and simulations are particularly difficult in the research fields sponsored by EAR, because the solid Earth is characterized by physical and chemical processes that generally have shorter ranges and longer durations than the fluid systems investigated by meteorologists and oceanographers. Two representative examples illustrate this point. First, the global circulation time is on the order of a month for the troposphere and about a thousand years for the deep ocean, but it exceeds 100 million years for mantle convection. Second, chemical diffusion is an important process both above and below the Earth’s solid surface, but the diffusivities of the common cations are 10 to 15 orders of magnitude lower in solid rock than in liquid water.

Fluid-bearing geosystems in the Critical Zone and upper crust—rock bodies containing magmas, petroleum, or water—present special challenges in this regard because the relevant processes range from the atomic level (i.e., sorption-desorption on mineral surfaces) to tens of kilometers or more. Their elucidation requires systematic, coordinated observations involving multiple disciplinary techniques that are spatially dense and extend over long time intervals. Field studies of this type are often most efficiently accomplished through the joint efforts of several groups of investigators in carefully chosen localities. Measurements and experiments within such “natural laboratories” may have to continue for many years. This mode of research is becoming more common as the trend toward the quantification of geological processes and system-level behaviors accelerates. Hence, the demand for basic research funds to invest in natural laboratories can be expected to increase.

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