Laccolith
A laccolith is a body of intrusive rock with a dome-shaped upper surface and a level base, fed by a conduit from below.
The pressure of the magma is high enough that the overlying strata are forced upward, giving the laccolith its dome-like form.
Over time, erosion can expose the solidified laccolith, which is typically more resistant to weathering than the host rock.
A laccolith is a type of igneous intrusion, formed when magma forces its way upwards through the Earth's crust but cools and solidifies before reaching the surface.
Laccoliths formed from sills only when they became large enough for the pressure of the magma to force the overlying strata to dome upwards.
[8] The periphery of a laccolith may be smooth, but it may also have fingerlike projections consistent with Rayleigh-Taylor instability of the magma pushing along the strata.
[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.
[14] 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.
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.
[18] Sheet intrusions tend to form perpendicular to the direction of least stress in the country rock they intrude.
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.
Dating of the intrusions has helped determine the point in geologic time when compression was replaced with extension.
In Big Bend Ranch State Park, at the southwesternmost visible extent of the Ouachita orogeny, lies the Solitario.
[22] 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.
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.
[25][26] 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.
