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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. 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 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 Torres del Paine (Patagonia) magma chamber is one of the best exposed laccolith, built up incrementally by horizontal granitic and mafic magma intrusions over 162 ± 11 thousand years.〔Julien Leuthold, Othmar Müntener, Lukas Baumgartner, Benita Putlitz, Maria Ovtcharova and Urs Schaltegger, Time resolved construction of a bimodal laccolith (Torres del Paine, Patagonia), Earth and Planetary Science Letters, 325-326:85-92, 2012〕 Horizontal sheeted intrusions were fed by vertical intrusions.〔Julien Leuthold, Othmar Müntener, Lukas Baumgartner, Benita Putlitz, Petrological constraints on the recycling of mafic crystal mushes and intrusion of braided sills in the Torres del Paine Mafic Complex (Patagonia), Journal of Petrology, 55(5):917-949, 2014〕 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 revealed it to 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. 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.〔 One of the largest laccoliths in the United States is Pine Valley Mountain in the Pine Valley Mountain Wilderness area near St. George, Utah There are many examples of possible laccoliths on the surface of the Moon.〔 These igneous features may be confused with impact cratering. ==See also== *Batholith *Dike *Hydrolaccolith *Lopolith *Sill *Stock 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「laccolith」の詳細全文を読む スポンサード リンク
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