Fracture is perhaps the most important topic in rock mechanics today, primarily because it is the dominant mechanism of rock failure at the relatively low pressures and temperatures at shallow depths in the earth''s crust and because existing fractures strongly influence the physical and mechanical behavior of the rock mass.
Robert Zimmerman, Ian Main, in International Geophysics, 2004. 7.2.1 Geometry of Rock Fractures. An idealized rock fracture or joint consists of two nominally planar, rough surfaces. The surfaces are typically in contact with each other at some locations, but separated at others. The distance of separation, usually measured perpendicular to the nominal fracture plane, is known as the aperture.