The Granite Mountains of Arizona is a

mountain range A mountain range or hill range is a series of mountains or hills arranged in a line and connected by high ground. A mountain system or mountain belt is a group of mountain ranges with similarity in form, structure, and alignment that have aris ...

in the

Sonoran Desert The Sonoran Desert () is a hot desert and ecoregion in North America that covers the northwestern Mexican states of Sonora, Baja California, and Baja California Sur, as well as part of the Southwestern United States (in Arizona and California). It ...

of southern Arizona. It is located in extreme western

Pima County Pima County ( ) is a County (United States), county in the south central region of the U.S. state of Arizona, one of 15 List of counties in Arizona, counties in the state. As of the 2020 United States census, 2020 census, the population was 1 ...

, Arizona, bordering southeast Yuma County. The Granite Mountains are long. This range trends mostly northwest-southeast and it turns midrange northerly to align with the Aguila Mountains of Yuma County north. The Granite Mountains range in width from about in width.USGS, 1965. ''Granite Mountains Quadrangle'', ''15 Minute Series (Topographic), AMS 3348 IV —Series V798'', scale 1:62,500, Reston, Virginia, United States Geological Survey.USGS, 1965. ''Aguila Mountains Quadrangle'', ''15 Minute Series (Topographic), AMS 3349 III —Series V798'', scale 1:62,500, Reston, Virginia, United States Geological Survey. The Granite Mountains' highest point is unnamed at . The west and south end of the mountains lie at the southeastern beginning of the

San Cristobal Valley The San Cristobal Valley is a valley in the lower regions of the western Gila River Valley in southwestern Arizona in the western Sonoran Desert. The San Cristobal Wash drains northwest, but exists only in the southern half of the valley. The Sa ...

flowing northwest and north to the

Gila River Valley The Gila River Valley is a multi-sectioned valley of the Gila River, located primarily in Arizona. The Gila River forms in western New Mexico and flows west across southeastern, south-central, and southwestern Arizona; it changes directions as i ...

. The east side of the range borders the north-flowing Growler Valley. The southern end of the mountains are adjacent to a

water divide A drainage divide, water divide, ridgeline, watershed, water parting or height of land is elevated terrain that separates neighboring drainage basins. On rugged land, the divide lies along topographical ridges, and may be in the form of a single ...

where south-flowing drainages enter into portions of northern

Sonora Sonora (), officially Estado Libre y Soberano de Sonora (), is one of the 31 states which, along with Mexico City, comprise the Administrative divisions of Mexico, Federal Entities of Mexico. The state is divided into Municipalities of Sonora, 72 ...

, Mexico.USGS, 1965. ''Aguila Mountains Quadrangle'', ''15 Minute Series (Topographic), AMS 3349 III —Series V798'', scale 1:62,500, Reston, Virginia, United States Geological Survey.

Physiography

The Granite Mountains are located in the

within the

Basin and Range Province The Basin and Range Province is a vast United States physiographic region, physiographic region covering much of the inland Western United States and Northern Mexico, northwestern Mexico. It is defined by unique basin and range topography, charac ...

. It is a

physiographic province physiographic province is a geographic region with a characteristic geomorphology, and often specific subsurface rock type or structural elements. The continents are subdivided into various physiographic provinces, each having a specific characte ...

characterize by alternating parallel alluvial basins and intervening mountain ranges known as ''

basin and range topography Basin and range topography is characterized by alternating parallel mountain ranges and valleys. It is a result of crustal extension due to mantle upwelling, gravitational collapse, crustal thickening, or relaxation of confining stresses. The e ...

''. This topography is the result of asymmetric

tilted block faulting Tilted block faulting, also called rotational block faulting, is a mode of structural evolution in extensional tectonic events, a result of tectonic plates stretching apart. When the upper lithospheric crust experiences extensional pressures, t ...

reflecting crustal extension due to mantle upwelling, gravitational collapse, crustal thickening, or relaxation of confining stresses during more or less the past 17 million years. The Basin and Range Province covers southern, central, and western Arizona and all of Nevada. It also covers parts of California, New Mexico, Oregon, Texas, and Utah, and much of northwestern Mexico.Pearthree, P.A., Freeman, A.K.L, and Demsey, K.A., 2001. ''Surficial geology and geoarchaeology of San Cristobal and Growler Valleys, Barry M. Goldwater Air Force Range, southwestern Arizona''. ''Arizona Geological Survey Open File Report'', OFR-01-01, 3 map sheets, map scales 1:100,000 and 1:40,000, 40 pp.Shafiqullah, M., Damon, P.E., Lynch, D.J., Reynolds, S.J., Rehrig, W.A. and Raymond, R.H., 1980. ''K-Ar geochronology and geologic history of southwestern Arizona and adjacent areas''. ''Arizona Geological Society Digest'', 12, pp.201-260. The Granite Mountains are part of the not very high, north- to northwest-trending mountain ranges and broad, minimally dissected basins that characterize the Sonoran Desert. They are narrow, rugged, but not very lofty. The axis of the Granite Mountains may be quite linear, but their mountain fronts are very embayed, sinuous, and exhibit considerable erosional retreat from the faults that formed them. Very wide

piedmont Piedmont ( ; ; ) is one of the 20 regions of Italy, located in the northwest Italy, Northwest of the country. It borders the Liguria region to the south, the Lombardy and Emilia-Romagna regions to the east, and the Aosta Valley region to the ...

s with minimal topographic relief and gentle slopes extend from the mountains to the axial washes that flow down the central axis of the alluvial valleys. Scattered, outlying

bedrock In geology, bedrock is solid rock that lies under loose material ( regolith) within the crust of Earth or another terrestrial planet. Definition Bedrock is the solid rock that underlies looser surface material. An exposed portion of bed ...

hills, called ''

inselberg An inselberg or monadnock ( ) is an isolated rock hill, knob, ridge, or small mountain that rises abruptly from a gently sloping or virtually level surrounding plain. In Southern Africa, a similar formation of granite is known as a koppie, an ...

s'' commonly rise above the piedmonts. The pediments continue right up to the sharply defined mountain edges. This, together with the highly sinuous and deeply eroded mountain fronts other geomorphic features indicate that faulting has not occurred for millions of years in this area.Tucker Jr, W.C., 1980. ''Tectonic geomorphology of the Luke Air Force Range, Arizona''. ''Arizona Geological Society Digest'', 12, pp.63-87. A minimum thickness of

colluvium Colluvium (also colluvial material or colluvial soil) is a general name for loose, unconsolidated sediments that have been deposited at the base of hillslopes by either rainwash, Sheet erosion , sheetwash, slow continuous downslope creep, or a va ...

covers these mountains slopes. This reflects the dominance of

erosion Erosion is the action of surface processes (such as Surface runoff, water flow or wind) that removes soil, Rock (geology), rock, or dissolved material from one location on the Earth's crust#Crust, Earth's crust and then sediment transport, tran ...

that is facilitated by uncommon, but intense, rainfall and runoff, steep slopes, and sparse vegetative cover.Bryan, K., 1925. ''The Papago Country, Arizona: A geographic, geologic, and hydrologic reconnaissance with a guide to desert watering places''. ''United States Geological Survey Water Supply Paper'', 499. 436 pp. During

glacial A glacier (; or ) is a persistent body of dense ice, a form of rock, that is constantly moving downhill under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires ...

and regionally pluvial intervals of the

Quaternary The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS), as well as the current and most recent of the twelve periods of the ...

, significantly more hillslope colluvium covered bedrock slopes in the mountains. This especially true of mountain ranges composed of

granitic A granitoid is a broad term referring to a diverse group of coarse-grained igneous rocks that are widely distributed across the globe, covering a significant portion of the Earth's exposed surface and constituting a large part of the continental ...

metamorphic rock Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock ( protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, caus ...

s.Bull, W.B., 1991. ''Geomorphic Responses to Climate Change''. New York, New York, Oxford University Press. 326 pp.

Geology

The bedrock geology of the Granite Mountains has been mapped on a reconnaissance level. According to this mapping,Gray, Floyd, Miller, R.J., Grubensky, M.J., Tosdal, R.M., Haxel, G.B., Peterson, D.W., May, D.J., and Silver, L.T., 1988. ''Geologic map of the Ajo and Lukeville 1 degree by 2 degree quadrangle, southwest Arizona'', 'United States Geological Survey, Open-File Report'', OF-87-347, scale: 1:250,000. the bedrock consists of granitic rock of late

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

age intruded into orthogneiss of middle

Proterozoic The Proterozoic ( ) is the third of the four geologic eons of Earth's history, spanning the time interval from 2500 to 538.8 Mya, and is the longest eon of Earth's geologic time scale. It is preceded by the Archean and followed by the Phanerozo ...

and (or) early Proterozoic age. It is described as light to medium gray or pink, distinctively white-weathering, highly leucocratic, fine-grained to coarse grained,

muscovite Muscovite (also known as common mica, isinglass, or potash mica) is a hydrated phyllosilicate mineral of aluminium and potassium with formula KAl2(Al Si3 O10)( F,O H)2, or ( KF)2( Al2O3)3( SiO2)6( H2O). It has a highly perfect basal cleavage y ...

biotite Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron- endmember annite, and the magnesium-endmember phlogopite; more al ...

-bearing

granite Granite ( ) is a coarse-grained (phanerite, phaneritic) intrusive rock, intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly coo ...

and

granodiorite Granodiorite ( ) is a coarse-grained (phaneritic) intrusive igneous rock similar to granite, but containing more plagioclase feldspar than orthoclase feldspar. The term banatite is sometimes used informally for various rocks ranging from gra ...

. The orthogneiss consists of variety of

gneiss Gneiss (pronounced ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. This rock is formed under p ...

es. They include, from oldest to youngest: 1. biotite-

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

feldspar Feldspar ( ; sometimes spelled felspar) is a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagiocl ...

schist Schist ( ) is a medium-grained metamorphic rock generally derived from fine-grained sedimentary rock, like shale. It shows pronounced ''schistosity'' (named for the rock). This means that the rock is composed of mineral grains easily seen with a l ...

s and gneisses 2.

hornblende Hornblende is a complex silicate minerals#Inosilicates, inosilicate series of minerals. It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole. Hornblende minerals are common ...

-biotite augen gneiss; 3. granitic, typically highly leucocratic, gneiss; 4.

amphibolite Amphibolite () is a metamorphic rock that contains amphibole, especially hornblende and actinolite, as well as plagioclase feldspar, but with little or no quartz. It is typically dark-colored and dense, with a weakly foliated or schistose ...

gneiss; and 5. biotite granitoid gneiss. Three nearby, prominent inselbergs that lie east of the Granite Mountains consist of

Oligocene The Oligocene ( ) is a geologic epoch (geology), epoch of the Paleogene Geologic time scale, Period that extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that defin ...

to middle

Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...

age,

basaltic Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron ( mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90% ...

volcanic rock Volcanic rocks (often shortened to volcanics in scientific contexts) are rocks formed from lava erupted from a volcano. Like all rock types, the concept of volcanic rock is artificial, and in nature volcanic rocks grade into hypabyssal and me ...

s, all of which are coated with dark rock varnish. Study of the economic geology of the of Granite Mountains in early 1980s found it lacking in any interesting mineral potential.Peirce, H.W., 1983. ''Earth Materials Evaluation-Arizona Rare II Areas''. ''Open-File Report,'' 83-13. Arizona Bureau of Geology and Mineral Technology, Tucson, Arizona.

References

{{Mountains of Arizona Mountain ranges of the Sonoran Desert Mountain ranges of Pima County, Arizona Mountain ranges of Arizona

Physiography

Geology

See also

References