Mechanics of Fluid-Rock Systems

Abstract

Most of the Earth's mass is solid. The crust and mantle of the Earth extend down almost 3000 km and support seismic shear waves, a standard test for solidity in the earth sciences. The inner core, with a radius of 1200 km, is believed to be solid for the same reason. It is less often noted that in large regions, this solid mass is actually a two-phase or multiphase medium, containing varying amounts of fluid (usually liquid) interspersed throughout the granular solid. Processes in fluid-rock media play an essential role in many important geological phenomena. It is percolation through such a medium (a partial melt) that permits melts to escape prior to volcanic eruptions and other igneous emplacements. Nearer the Earth's surface, the migration of aqueous or hydrocarbon fluids in the crust is of interest in metamorphic and sedimentary geology as well as in hydrology and hydrocarbon exploration. Fluid migration at all levels within the Earth can profoundly influence heat flow and material transport. The layering of the Earth, including the existence and nature of the core and the stratification of the mantle and crust, is likely to be due in part to the behavior of fluid-rock systems. There are many different fluid-rock systems in geology, and this review considers only a subset of these. The primary emphasis is on two geologic environments where the coupling between fluid flow and rock deformation is important: high-temperature systems where a melt coexists with a solid matrix that can deform by creep as the melt migrates; and low-temperature systems where a fluid, usually aqueous, saturates a matrix that has varying degrees of cohesion and fracturing. We do not consider suspensions (e.g. crystals in a convecting magma chamber or rock fragments carried in the plume of a volcanic eruption). At the opposite extreme, we exclude solidlike systems where fluid is present but little or no fluid movement occurs (e.g. fluid inclusions in crystals). Minor consideration is given to systems where the fluid moves but the solid is essentially rigid (e.g. the flow of oil, water, and gas in hydrocarbon reservoirs). We also limit ourselves to processes where inertia is negligible: Both the microscopic motion of fluid through cracks and pores and the macroscopic motion of the solid matrix are assumed to be characterized by a small Reynolds number. For example, we do not consider seismic-wave propagation in poroelastic media. Even with these restrictions, more important geodynamic phenomena are covered than can be effectively reviewed in detail. Our goal is rather to identify common ground between studies of different geological environments, and to demonstrate to practitioners of fluid mechanics the interesting range of phenomena that fluid-rock systems exhibit. We have divided the main body of this review into three sections: theory, material properties, and modeling and applications. The latter section is necessarily eclectic but encompasses studies ranging from laboratory experiments on analogue systems to computer simulations of large-scale processes in the crust and mantle.

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