Exfoliation (geology)

Exfoliation joints or sheet joints are surface-parallel fracture systems in rock often leading to erosion of concentric slabs.

Despite their common occurrence in many different landscapes, geologists have yet to reach an agreement on a general theory of exfoliation joint formation. Many different theories have been suggested, below is a short overview of the most common.

This theory was originally proposed by the pioneering geomorphologist Grove Karl Gilbert in 1904 and is widely found in introductory geology texts. The basis of this theory is that erosion of overburden and exhumation of deeply buried rock to the ground surface allows previously compressed rock to expand radially, creating tensile stress and fracturing the rock in layers parallel to the ground surface. The description of this mechanism has led to alternate terms for exfoliation joints, including pressure release or offloading joints. Though the logic of this theory is appealing, there are many inconsistencies with field and laboratory observations suggesting that it may be incomplete, such as:

One possible extension of this theory to match with the compressive stress theory (outlined below) is as follows (Goodman, 1989): The exhumation of deeply buried rocks relieves vertical stress, but horizontal stresses can remain in a competent rock mass since the medium is laterally confined. Horizontal stresses become aligned with the current ground surface as the vertical stress drops to zero at this boundary. Thus large surface-parallel compressive stresses can be generated through exhumation that may lead to tensile rock fracture as described below.

Rock expands upon heating and contracts upon cooling and different rock-forming minerals have variable rates of thermal expansion / contraction. Daily rock surface temperature variations can be quite large, and many have suggested that stresses created during heating cause the near-surface zone of rock to expand and detach in thin slabs (e.g. Wolters, 1969). Large diurnal or fire-induced temperature fluctuations have been observed to create thin lamination and flaking at the surface of rocks, sometimes labeled exfoliation. However, since diurnal temperature fluctuations only reach a few centimeters depth in rock (due to rock’s low thermal conductivity), this theory cannot account for the observed depth of exfoliation jointing that may reach 100 meters.

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