Introduction

Fractures result from mechanical breaks in intact geologic media such as rocks or compacted glacial tills. Although a fracture that is completely filled by minerals is still considered a fracture in the geologic sense, within the context of subsurface fluid flow we think of a fracture as a mechanical break that results in void space between the fracture walls. This void space is more or less planar – one of its dimensions (the aperture or distance between fracture walls) is much smaller than the other two (the extension of the fracture plane). When interconnected, fractures provide pathways for fluid flow through geologic media that would be significantly less permeable if the media were unfractured.

The geometry of subsurface fractures varies greatly, with fracture lengths ranging from less than a millimeter (e.g., a microcrack in a rock grain) to thousands of kilometers (e.g., a fault along a tectonic-plate boundary). Fracture apertures vary from minute “hairline” cracks, nearly imperceptible to the naked eye, to solution-enlarged channels wide enough for human exploration. Fractures can be highly interconnected in a densely fractured rock, or isolated and poorly connected in a sparsely fractured rock. Some fracture networks exhibit a nested pattern, with smaller fractures bounded by larger ones (Barton and Hsieh, 1989). Studies of the processes that create fractures over this broad range of scales constitute an active area of research in the earth sciences.

In a study of fragmentation, Turcotte (1986) found that the fragmentation process often results in a power-law or fractal distribution of fragment sizes.