 | The examples and perspective in this articledeal primarily with the United States and do not represent aworldwide view of the subject. You mayimprove this article, discuss the issue on thetalk page, orcreate a new article, as appropriate.(October 2015) (Learn how and when to remove this message) |
.jpg)
Exfoliation joints orsheet joints are surface-parallelfracture systems in rock, often leading to the 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 geomorphologistGrove Karl Gilbert in 1904. The basis of this theory is thaterosion ofoverburden 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:[6][10][12]
One possible extension of this theory to match with thecompressive stress theory (outlined below) is as follows[3] (Goodman, 1989): The exhumation of deeply buried rocks relieves verticalstress, 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 ofthermal 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).[12] Largediurnal or fire-induced temperature fluctuations have been observed to create thin lamination and flaking at the surface of rocks, sometimes labeled exfoliation.[13] However, since diurnal temperature fluctuations only reach a few centimeters depth in rock (due to rock's lowthermal conductivity), this theory cannot account for the observed depth of exfoliation jointing that may reach 100 meters.[1][3][6][10]
Mineralweathering by penetrating water can cause flaking of thin shells of rock since the volume of some minerals increases uponhydration.[10] However, not all mineral hydration results in increased volume, while field observations of exfoliation joints show that the joint surfaces have not experienced significant chemical alteration, so this theory can be rejected as an explanation for the origin of large-scale, deeper exfoliation joints.
Large compressivetectonicstresses parallel to the land (or a free) surface can create tensile modefractures in rock, where the direction of fracture propagation is parallel to the greatest principle compressive stress and the direction of fracture opening is perpendicular to the free surface.[3][6][7][8][9][10][14] This type of fracturing has been observed in the laboratory since at least 1900 (in both uniaxial and biaxial unconfined compressive loading; see Gramberg, 1989).[15] Tensile cracks can form in a compressive stress field due to the influence of pervasivemicrocracks in the rock lattice and extension of so-calledwing cracks from near the tips of preferentially oriented microcracks, which then curve and align with the direction of the principle compressive stress.[16][17] Fractures formed in this way are sometimes called axial cleavage, longitudinal splitting, or extensional fractures, and are commonly observed in the laboratory during uniaxial compression tests. High horizontal or surface-parallel compressive stress can result from regionaltectonic or topographic stresses, or by erosion or excavation of overburden.
With consideration of the field evidence and observations of occurrence, fracture mode, and secondary forms, high surface-parallel compressive stresses and extensional fracturing (axial cleavage) seems to be the most plausible theory explaining the formation of exfoliation joints.
Recognizing the presence of exfoliation joints can have important implications ingeological engineering. Most notable may be their influence on slope stability. Exfoliation joints following the topography of inclined valley walls, bedrock hill slopes, and cliffs can create rock blocks that are particularly prone to sliding. Especially when the toe of the slope is undercut (naturally or by human activity), sliding along exfoliation joint planes is likely if the joint dip exceeds the joint's frictional angle. Foundation work may also be affected by the presence of exfoliation joints, for example in the case ofdams.[18] Exfoliation joints underlying adam foundation can create a significant leakagehazard, while increased water pressure in joints may result in lifting or sliding of the dam. Finally, exfoliation joints can exert strong directional control ongroundwater flow and contaminant transport.
Media related toDesquamation (geology) at Wikimedia Commons