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Exfoliation joint

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

Exfoliation joints wrapping around Half Dome in Yosemite National Park, California.
Exfoliation joints in granite at Enchanted Rock State Natural Area, Texas, US. Detached blocks have slid along the steeply-dipping joint plane.

General characteristics

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Formation

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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.

Removal of overburden and rebound

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Exfoliation joints exposed in a road cut in Yosemite National Park, California.
 
Exfoliation of granite at Enchanted Rock State Natural Area, Texas, USA

This theory was originally proposed by the pioneering geomorphologist Grove Karl Gilbert in 1904. 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:[6][10][12]

  • Exfoliation joints can be found in rocks that have never been deeply buried.
  • Laboratory studies show that simple compression and relaxation of rock samples under realistic conditions does not cause fracturing.
  • Exfoliation joints are most commonly found in regions of surface-parallel compressive stress, whereas this theory calls for them to occur in zones of extension.

One possible extension of this theory to match with the compressive stress theory (outlined below) is as follows[3] (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.

Thermoelastic strain

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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).[12] Large diurnal 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 low thermal conductivity), this theory cannot account for the observed depth of exfoliation jointing that may reach 100 meters.[1][3][6][10]

Chemical weathering

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Mineral weathering by penetrating water can cause flaking of thin shells of rock since the volume of some minerals increases upon hydration.[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.

Compressive stress and extensional fracture

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Exfoliation joints have modified the near-surface portions of massive granitic rocks in Yosemite National Park, helping create the many spectacular domes, including Half Dome shown here.

Large compressive tectonic stresses parallel to the land (or a free) surface can create tensile mode fractures 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 pervasive microcracks in the rock lattice and extension of so-called wing 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 regional tectonic 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.

Engineering geology significance

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Recognizing the presence of exfoliation joints can have important implications in geological 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 of dams.[18] Exfoliation joints underlying a dam foundation can create a significant leakage hazard, while increased water pressure in joints may result in lifting or sliding of the dam. Finally, exfoliation joints can exert strong directional control on groundwater flow and contaminant transport.

See also

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References

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  1. ^ a b c d e Gilbert, G.K. (1904). "Domes and dome structures of the high Sierra". Bulletin of the Geological Society of America. 15 (1): 29–36. Bibcode:1904GSAB...15...29G. doi:10.1130/GSAB-15-29.
  2. ^ a b c Matthes, F.E. (1930). "Geologic history of the Yosemite Valley". U.S. Geological Survey Professional. 160.
  3. ^ a b c d e f g h i Goodman, R.E. (1993). Engineering Geology. New York: John Wiley and Sons.
  4. ^ a b c Dale, T.N. (1923). "The commercial granites of New England". United States Geological Survey Bulletin. 738.
  5. ^ a b c d Jahns, R.H. (1943). "Sheet structures in granites". Journal of Geology. 51 (2): 71–98. Bibcode:1943JG.....51...71J. doi:10.1086/625130. S2CID 129646638.
  6. ^ a b c d Holzhausen, G.R. (1989). "Origin of sheet structure, 1. Morphology and boundary conditions". Engineering Geology. 27 (1–4): 225–278. Bibcode:1989EngGe..27..225H. doi:10.1016/0013-7952(89)90035-5.
  7. ^ a b Bahat, D.; Grossenbacher, K.; Karasaki, K. (January 1999). "Mechanism of exfoliation joint formation in granitic rocks, Yosemite National Park". Journal of Structural Geology. 21 (1): 85–96. Bibcode:1999JSG....21...85B. doi:10.1016/s0191-8141(98)00069-8. ISSN 0191-8141.
  8. ^ a b Mandl, G. (2005). Rock Joints. Berlin: Springer-Verlag. ISBN 9783642063916.
  9. ^ a b Bradley, W.C. (1963). "Large-scale exfoliation in massive sandstones of the Colorado Plateau". Geological Society of America Bulletin. 74 (5): 519–527. doi:10.1130/0016-7606(1963)74[519:LEIMSO]2.0.CO;2.
  10. ^ a b c d e f Twidale, C.R. (1973). "On the origin of sheet jointing". Rock Mechanics and Rock Engineering. 5 (3): 163–187. Bibcode:1973RMFMR...5..163T. doi:10.1007/BF01238046. S2CID 129852438.
  11. ^ a b Romani, J.R.; Twidale, C.R. (1999). "Sheet fractures, other stress forms and some engineering implications". Geomorphology. 31 (1–4): 13–27. Bibcode:1999Geomo..31...13V. doi:10.1016/S0169-555X(99)00070-7.
  12. ^ a b Wolters, R. (1969). "Zur Ursache der Entstehung oberflächenparalleler Klüfte". Rock Mechanics and Rock Engineering. 1 (1): 53–70. Bibcode:1969RMFMR...1...53W. doi:10.1007/BF01247357. S2CID 129174541.
  13. ^ Blackwelder, E. (1927). "Fire as an agent in rock weathering". Journal of Geology. 35 (2): 134–140. Bibcode:1927JG.....35..134B. doi:10.1086/623392. S2CID 129709077.
  14. ^ Brunner, F.K.; Scheidegger, A.E. (1973). "Exfoliation". Rock Mechanics. 5 (1): 43–62. Bibcode:1973RMFMR...5...43B. doi:10.1007/bf01246756. ISSN 1434-453X.
  15. ^ Gramberg, J. (1989). A non-conventional view on rock mechanics and fracture mechanics. A.A.Balkema. ISBN 9061918065.
  16. ^ Hoek, E.; Bieniawski, Z.T. (1965). "Brittle fracture propagation in rock under compression". International Journal of Fracture Mechanics. 1 (3): 137–155. doi:10.1007/BF00186851. S2CID 198179354.
  17. ^ Fairhurst, C.; Cook, N.G.W. (1966). "The phenomenon of rock splitting parallel to the direction of maximum compression in the neighbourhood of a surface". Proceedings 1st Congress, International Society of Rock Mechanics: 687–692.
  18. ^ Terzaghi, K. (1962). "Dam foundation on sheeted granite". Geotechnique. 12 (3): 199–208. Bibcode:1962Getq...12..199T. doi:10.1680/geot.1962.12.3.199. ISSN 0016-8505.
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