Black Mountain (Maricopa County, Arizona)
| Black Mountain | |
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Black Mountain as viewed from the south
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| Highest point | |
| Elevation | 3,403 ft (1,037 m) NAVD 88 |
| Prominence | 1,028 ft (313 m) |
| Coordinates | 33°48′55″N 111°56′29″W / 33.815243153°N 111.941358858°WCoordinates: 33°48′55″N 111°56′29″W / 33.815243153°N 111.941358858°W |
| Geography | |
 Black Mountain
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| Location | Maricopa County, Arizona, U.S. |
| Topo map | USGS Cave Creek, AZ |
Black Mountain is located in north-central Maricopa County, Arizona, between the towns of Carefree and Cave Creek.
The mountain is composed of two main rock types that are roughly divided between western and eastern halves of the mountain. The west side of the mountain is primarily composed of a ~250 m. bed of black meta-sedimentary rocks which are distinguishable as phyllite and argillite. Viewed from a distance the meta-sedimentary rock is often mistaken for basalt, which is common in the area; however basalt is not found on Black Mountain. The phyllite has a slaty cleavage and is foliated in all directions, the result of shortening due to pressure-solution volume-diffusion of as much as 50%. As a result of shortening, streambeds of nearby washes are filled with black alluvium.
The western face meta-sedimentary beds strike ~120, 45 and, when fully illuminated by the afternoon sun, resemble overlapping flatirons. The meta-sedimentary beds rise 830 feet above the local topography, with an actual elevation of 3398 feet above sea level.
The eastern side of the mountain is composed of a granitic intrusion into the meta-sedimentary rock. The granite is part of the granite formation that extends southward from Black Mountain and is expressed in outcrops in the McDowell Mountains, the Union Hills, and Camelback Mountain. Texturally, the granite varies from a large crystal with phenocrysts of up to 2.5 cm., to a finer grained crystal texture devoid of phenocrysts. The granite containing larger crystals has formed large joints as well, fractured both vertically and horizontally, and is weathered into spheroidal boulder columns and balanced rock formations. Jointing in the finer grained granite is smaller and proportional to grain size. The granitic half of Black Mountain tapers to a flat-topped peak that rises to a height just below that of the pointed, black phyllite peak of the western side. The two units meet as a saddle between the twin peaks of meta-argillite/phyllite and granite.
Along the contact on the eastern side of the mountain, clasts of pelitic schist can be found embayed in the large-grained granite. The granite at the contact is gray in color, having been discolored by the sedimentary rock during emplacement. Clasts of smaller grained granite are also found in the large-grained granite. The meta-sedimentary rock along with the granite batholith that forms Black Mountain are remnants of what laid hidden beneath a sea and underground, that are now exposed by headward erosion that continues northward today into Arizona’s Transition Zone. The highly shortened and foliated phyllite began as a mudstone which was metamorphosed to shale. The shale was then subjected to pressure-solution volume-diffusion; the rock was metamorphosed while deeply buried then subjected to heating during the emplacement of the batholith. The phyllite at the granite contact formed a weak horizon, which assisted the decollement process after the batholith was emplaced. These chloritic rocks acted as a lubricated sheet, effectively sliding off the batholith and tilting to the northwest during the Tertiary extension event. To the south, grabens that formed by normal faulting during crustal extension, are filled by the alluvium from the subsequent erosion.
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Wikipedia