Laccolith
A laccolith is a body of
Over time,
Description
A laccolith is a type of
A laccolith forms after an initial sheet-like intrusion has been injected between layers of sedimentary rock. If the intrusion remains limited in size, it forms a sill, in which the strata above and below the intrusion remain parallel to each other and the intrusion remains sheetlike. The intrusion begins to lift and dome the overlying strata only if the radius of the intrusion exceeds a critical radius, which is roughly:[6]
where is the pressure of the magma, is the
More recent study of laccoliths has confirmed Gilbert's basic conclusions, while refining the details. Both sills and laccoliths have blunt rather than wedgelike edges, and sills of the Henry Mountains are typically up to 10 meters (33 ft) thick while laccoliths are up to 200 meters (660 ft) thick.
where is the height of the laccolith roof, is the acceleration of gravity, is the elastic modulus of the host rock, is the horizontal distance from the center of the laccolith, and is the outer radius of the laccolith.[4] Because of their greater thickness, which slows the cooling rate, the rock of laccoliths is usually coarser-grained than the rock of sills.[5]
The growth of laccoliths can take as little as a few months when associated with a single magma injection event,[10][11] or up to hundreds or thousands of years by multiple magmatic pulses stacking sills on top of each other and deforming the host rock incrementally.[12]
Over time, erosion can form small hills and even mountains around a central peak since the intrusive rock is usually more resistant to weathering than the host rock.
Etymology
The term was first applied as laccolite by Gilbert after his study of intrusions of diorite in the Henry Mountains of Utah in about 1875.[15][16] The word laccolith was derived in 1875–1880, from Greek lákko(s) 'pond' plus -lith 'stone'.[17]
Where laccoliths form
Laccoliths tend to form at relatively shallow depths and in some cases are formed by relatively
Sheet intrusions tend to form perpendicular to the direction of least stress in the country rock they intrude. Thus laccoliths are characteristic of regions where the crust is being compressed and the direction of least stress is vertical, while areas where the crust is in tension are more likely to form dikes, since the direction of least stress is then horizontal. For example, the laccoliths of the
Examples
In addition to the Henry Mountains, laccolith mountains are found on the nearby Colorado Plateau in the La Sal Mountains and Abajo Mountains.[14]
The filled and solidified
The small
One of the largest laccoliths in the United States is Pine Valley Mountain in the Pine Valley Mountain Wilderness area near St. George, Utah.[23]
A system of laccoliths is exposed on the Italian island of Elba, which form a "Christmas tree" laccolith system in which a single igneous plumbing system has produced multiple laccoliths at different levels in the crust.[24]
Problems reconstructing shapes of intrusions
The original shape of intrusions can be difficult to reconstruct. For instance,
Extraterrestrial laccoliths
There are many examples of possible laccoliths on the surface of the Moon. Some are centered in impact craters and may form as part of the post-impact evolution of the crater.[28] Others are located along possible faults or fissures.[29] Laccoliths on the Moon are much wider but less thick than those on Earth, due to the Moon's lower gravity and more fluid magmatism.[30]
Possible laccoliths have also been identified on Mars, in western Arcadia Planitia.[31]
Gallery
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Dolerite intrusion laccolith in Sydney, Australia
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Laccolith exposed by erosion of overlying strata in Montana
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Pink monzonite laccolith intrudes within the grey Cambrian and Ordovician strata near Notch Peak, Utah[32]
See also
- Batholith – Large igneous rock intrusion
- Lopolith – Lenticular igneous intrusion with a depressed central region
- Stock – Smaller igneous intrusion
References
- ^ ISBN 9780521880060.
- ISBN 978-0-19-562816-6.
- ISSN 2169-9313.
- ^ a b c Philpotts & Ague 2009, p. 95.
- ^ a b Philpotts & Ague 2009, p. 94.
- ^ Philpotts & Ague 2009, equation 4.4, p.94.
- ^ Philpotts & Ague 2009, pp. 79–80.
- .
- .
- PMID 27876800.
- ISSN 2296-6463.
- .
- ISBN 9781607810049.
- ^ a b Fillmore 2010, pp. 292–293.
- doi:10.3133/70038096.)
{{cite book}}
: CS1 maint: location missing publisher (link - ^ Aber, James S. "Grove Karl Gilbert". academic.emporia.edu. Retrieved 29 December 2018.
- ^ "laccolith". dictionary.com. Retrieved 29 December 2018.
- ^ "Beall, Joseph J." "Pseudo-Rhythmic Layering in the Square Butte Alkali-Gabbro Laccolith." American Mineralogist. 57:7-8 (July–August 1972).
- ^ Maynard, Steven R. (February 2005). "Laccoliths of the Ortiz porphyry belt, Santa Fe County, New Mexico" (PDF). New Mexico Geology. 27 (1). Retrieved 8 June 2020.
- .
- .
- ^ ISBN 978-0-87842-265-4.
- ^ Cook, E.F. (1957). "Geology of the Pine Valley Mountains Utah". Utah Geological and Mineralogical Studies (58): 1–111.
- .
- ISBN 978-1-86239-216-8
- ^ "Noe, D. C., and Alexander T. Klink. OF-15-06 Geologic Map of the Crawford Quadrangle, Delta and Montrose Counties, Colorado. 1:24,000". Colorado Geological Survey. Retrieved 26 July 2019.
- .
- .
- .
- .
- .
- .
Further reading
- Blatt, Harvey; Tracy, Robert J. (1996). Petrology : igneous, sedimentary, and metamorphic (2nd ed.). New York: W.H. Freeman. pp. 13–15. ISBN 0-7167-2438-3.
- Friedman, Jules D.; Huffman, Curtis Jr., eds. (1998). "Laccolith Complexes of Southeastern Utah: Time of Emplacement and Tectonic Setting — Workshop Proceedings". United States Geological Survey Bulletin. 2158. doi:10.3133/B2158.
- Hyndman, D. W.; Alt, D. (November 1987). "Radial Dikes, Laccoliths, and Gelatin Models". The Journal of Geology. 95 (6): 763–774. S2CID 128562770.
External links
- The dictionary definition of laccolith at Wiktionary