A laccolith is a sheet-like intrusion (or concordant pluton) that has been injected within or between layers of sedimentary rock (when the host rock is volcanic, the laccolith is referred to as a cryptodome). The pressure of the magma is high enough that the overlying strata are forced upward and folded, giving the laccolith a dome or mushroom-like form (or possibly conical or wedge-shape) with a generally planar base. Over time, erosion can form small hills and even mountains around a central peak since the magma rock is likely more resistant to weathering than the host rock. The growth of laccoliths can take as little as a few months when associated with a single magma injection event, 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.
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 form
Laccoliths tend to form at relatively shallow depths and in some cases are formed by relatively viscous magmas, such as those that crystallize to diorite, granodiorite, and granite. In those cases cooling underground may take place slowly, giving time for larger crystals to form in the cooling magma. In other cases less viscous magma such as shonkinite may form phenocrysts of augite at depth, then inject through a vertical feeder dike that ends in a laccolith. The surface rock above laccoliths often erodes away completely, leaving the core mound of igneous rock.
Tabular 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 Ortiz porphyry belt in New Mexico likely formed during Laramide compression of the region 33 to 36 million years ago.
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 intrusions
The original shape of intrusions can be difficult to reconstruct. For instance, Devils Tower in Wyoming and Needle Rock in Colorado were both thought to be volcanic necks, but further study has suggested they are eroded laccoliths. At Devils Tower, intrusion 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, so it is impossible to reconstruct the original shape of the igneous intrusion, which may or 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 the classic shapes of laccoliths.
On Earth's moon
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