A laccolith is a sheet intrusion (or concordant pluton) that has been injected between two layers of sedimentary rock. The pressure of the magma is high enough that the overlying strata are forced upward, giving the laccolith a dome or mushroom-like form with a generally planar base. As time passes, this will usually result in the formation of small hills and even mountains around the central peak. The development of laccoliths normally takes hundreds of years, and after a volcano has erupted, where the dome takes a very lengthy period for it to surface to the ground.
The term was first applied as laccolite by Grove Karl Gilbert after his study of intrusions of diorite in the Henry Mountains of Utah in about 1875. The word laccolith derived in 1875—1880, from Greek, lákko(s), meaning pond, plus -lith, meaning stone.
Where laccoliths formEdit
Laccoliths can only form after there is a fierce volcanic eruption. The areas where volcanic eruptions have occurred are usually acidic, making it naturally challenging to harbour any life Laccoliths tend to form at relatively shallow depths and are typically formed by relatively viscous magmas, such as those that crystallize to diorite, granodiorite, and granite. Cooling underground takes place slowly, giving time for larger crystals to form in the cooling magma. The surface rock above laccoliths often erodes away completely, leaving the core mound of igneous rock.
The filled and solidified magma chamber of Torres del Paine (Patagonia) is one of the best exposed laccoliths, built up incrementally by horizontal granitic and mafic magma intrusions over 162 ± 11 thousand years. Horizontal sheeted intrusions were fed by vertical intrusions.
The small Barber Hill syenite-stock laccolith in Charlotte, Vermont, has several volcanic trachyte dikes associated with it. Molybdenite is also visible in outcrops on this exposed laccolith. In Big Bend Ranch State Park, at the southwesternmost visible extent of the Ouachita orogeny, lies the Solitario. It consists of the eroded remains of a laccolith, presumably named for the sense of solitude that observers within the structure might have, due to the partial illusion of endless expanse in all directions.
Problems reconstructing shapes of intrusionsEdit
It is often difficult to reconstruct shapes of intrusions. For instance, Devils Tower in Wyoming was thought to be a volcanic neck, but study has suggested it may be an eroded laccolith. The rock would have had to cool very slowly so as to form the slender pencil-shaped columns of phonolite porphyry seen today. However, erosion has stripped away the overlying and surrounding rock, and so it is impossible to reconstruct the original shape of the igneous intrusion; that rock may not be the remnant of a laccolith. At other localities, such as in the Henry Mountains and other isolated mountain ranges of the Colorado Plateau, some intrusions demonstrably have shapes of laccoliths.
On Earth's moonEdit
Laccolith exposed by erosion of overlying strata in Montana
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- "Definitions for laccolith". definitions.net. Retrieved 29 December 2018.
- "laccolith". dictionary.com. Retrieved 29 December 2018.
- Leuthold, Julien; Müntener, Othmar; Baumgartner, Lukas; Putlitz, Benita; Ovtcharova, Maria; Schaltegger, Urs (2012). "Time resolved construction of a bimodal laccolith (Torres del Paine, Patagonia)". Earth and Planetary Science Letters. 325–326: 85–92. doi:10.1016/j.epsl.2012.01.032.
- Leuthold, Julien; Müntener, Othmar; Baumgartner, Lukas; Putlitz, Benita (2014). "Petrological constraints on the recycling of mafic crystal mushes and intrusion of braided sills in the Torres del Paine Mafic Complex (Patagonia)" (PDF). Journal of Petrology. 55 (5): 917–949. doi:10.1093/petrology/egu011.
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- Jules D. Friedman and Curtis Huffman, Jr., coordinators, "Laccolith Complexes of Southeastern Utah: Time of Emplacement and Tectonic Setting -- Workshop Proceedings", United States Geological Survey Bulletin 2158, 1998.
- The dictionary definition of laccolith at Wiktionary