The Sierra Madre Occidental ('Western Mother/Grand Mountain Range') is a major mountain range system which runs northwest–southeast through northwestern and western Mexico, and along the Gulf of California. The Sierra Madre is part of the American Cordillera, an almost continuous chain of mountain ranges that forms the western "backbone" of the Americas.
Etymology The Spanish name sierra madre means "mother mountain range" in English, and occidental means "western", these thus being the "Western mother mountain range". To the east, from the Spanish oriental meaning "eastern" in English, the Sierra Madre Oriental range or "Eastern mother mountain range" runs generally parallel to the Sierra Madre Occidental along eastern Mexico and the Gulf of Mexico.
The range extends from northern Sonora, a state near the Mexico–U.S. border at Arizona, southeast towards to the Trans-Mexican Volcanic Belt and Sierra Madre del Sur ranges. The high plateau that is formed by the range is cut by deep river valleys. This plateau is formed from volcanic rock overlying a basement of metamorphic rock.
This uplift has caused changes in weather patterns; increased rainfall occurring in the mountains has provided areas where ecosystems can form in wetter areas than surrounding land. This water source forms watersheds that provide the arid surroundings with water that makes it possible to irrigate and farm crops. The wet ecosystems are islands of biodiversity, differing significantly from what would otherwise be a desert landscape. Oak forests are the predominant plant life, and extend into the lowland deserts.
This forest and canyon land provided a place for a variety of indigenous people to live, until Spanish settlers with associated mestizos came into the area to found towns for the silver mines in the area. The major industries in the area now are agriculture and forestry, which have become contentious because of land degradation and the native population's opposition to these practices.
Geography
The range trends from the north to southeast. Canyons cut by the rivers of the wet western slopes exist in addition to those of the northeast slopes, notably the Copper Canyon.
Location The range runs parallel to the Pacific coast of Mexico, from just south of the Arizona-Sonora border southeast through eastern Sonora, western Chihuahua, Sinaloa, Durango, Zacatecas, Nayarit, Jalisco, Aguascalientes to Guanajuato, where it joins with the Sierra Madre del Sur and the Eje Volcánico Transversal (Transverse Volcanic Axis) of central Mexico after crossing . The mountains range from from the Gulf of California in the north, but begin to approach within of the Pacific in the south. These mountains are generally considered to be part of the much larger American cordillera, the mountains extending from Alaska down to these across western North America.
Regions Sierra Tarahumara or Tarahumara is the name for the region of the Sierra Madre beginning at the Durango border and extending north. This name comes from the Tarahumara natives. This is a dramatic landscape of steep mountains formed by a high plateau that has been cut through with canyons including Copper Canyon, larger and, in places, deeper than the Grand Canyon. This plateau has an average elevation of with most of the more eroded canyons on the western slope, due to the higher moisture content.
The highest elevations occur in the Tarahumara range. The exact elevations of the highest peaks are not known within accurate enough ranges to determine their relative elevation. The highest point is probably Cerro Mohinora, located at . Estimates for the height of the mountain start around and go up to . However, Cerro Barajas, at , may be as high as although other sources give as the elevation. Cerro Gordo, at , may have an elevation between and .
The southern end of the mountains may be referred to as the Sierra Huichola. In this area, the Sierra Madre begins to give way to the Basin and Range Province. Subranges of this area include the Sierra de Alica and the Sierra Pajaritos, both in Nayarit, and the Sierra los Huicholes, Sierra de Morones, Sierra Nochistlán, and Sierra Fría extending eastwards into Jalisco, Zacatecas, and Aguascalientes.
Water
The mountains act as a source of water, in an otherwise arid environment from the increased precipitation from the mountain range. As such, rivers that have headwaters in the mountains provide water for irrigation in the surrounding lands. The need for water to irrigate prompted the construction of dams, which has been the source of several environmental concerns in the area.
The northern end of the range is more arid; hence, the Yaqui River, which drains that area, is an important source of irrigation in the state of Sonora. The Yaqui drains into the Gulf of California, as do the Fuerte River in Sinaloa and the Humaya River further south. The Río Grande de Santiago drains from the southern slopes of the Sierra Madre. Along the more arid eastern slopes of the mountains, the Nazas River and Aguanaval River drain the mountains into a closed basin. These mountains supply 90% of the water used for irrigation within the watershed. North of this system is the Conchos River which drains into the Rio Grande.
Along many rivers the arid conditions have caused courses to be dammed to provide water for irrigation. These dams have caused concerns along with those caused by other activities. The Yaqui has been dammed with three large reservoirs along its course. The Río Grande de Santiago has also been dammed, including the Aguamilpa Dam begun in 1991 and the El Cajón Dam upstream from it. To provide irrigation water for farming the dry basin, the Nazas river was dammed in the 1930s and 1940s. This has led to the former drainage lakes in the area drying up, and soil depletion from the elimination of the river as a source of soil. Due to the continued logging in the area, erosion on the slopes of hillsides has increased.
Human Numerous small populations live in Copper Canyon, some of them around old missions. Many settlements consist of single families or small family groups near agricultural fields. There are also a few towns of note, including Creel, Chihuahua, with 5,000 inhabitants; it serves as a stop on the train line. South of Copper Canyon is the town of El Salto, Durango, situated along highway 40. To the west is Copala, Sinaloa.
To the south live the indigenous Cora people in the towns of Jesús María and La Mesa del Nayar. In addition, 1,000 people live in the village of Huajimic in Nayarit. The Bolaños River valley of Jalisco is populated with mining communities, such as San Martín de Bolaños, which has a population of around 3,000.
Few paved roads cross the mountains, meaning that travel times can be long, even between cities that are relatively close. In the north, Mexico Federal Highway 16 connects Hermosillo, Sonora and La Junta, Chihuahua. In the mountains, the La Entrada al Pacifico Corridor is an incomplete road intended to connect Los Mochis, Sinaloa and Chihuahua. Durango and Mazatlán are connected by highway 40, around 24 degrees north latitude.
Geology The Sierra Madre Occidental is a high plateau of volcanic rock that is eroded in areas to reveal a basement composed of plutonic and sedimentary rocks underlying the two groups of extrusive volcanics. The lower of these groups is a series of volcanic rocks formed by lava flows. The surface of the plateau is made up of a second series of volcanic ignimbrites, covered mostly by clay rich organic soils. Some intrusions in the area have deposited ore veins that are commercially viable.
The landscape was shaped after rock placement. Rivers cut valleys into the plateau, following folds in the rocks. Rocks have broken off of the slopes, forming rocky sides. The bedrock is covered by the clay rich soils, except in eroded areas, where water carries away deposits leaving a stony surface.
Geological history The basement is thought to be Older North American basement, especially in the North, and a number of different accreted terranes, which are mostly covered by Cenozoic volcanism. The northern extent of the Sierra Madre is the Cortés Terrane, although the northern extreme of this contains a small portion of the Carborca Terrane. Some of the northern end may also be underlain by a Precambrian basement placed around the same time as the Grenville Orogeny. Some of this area has a series of Paleozoic sedimentary sequences from a deep marine environment underlying it, considered by some to be part of the Cotes Terrane and by others to be transported. In some areas, distinct sedimentary rocks can be identified by shallow-water Silurian and Mississippian sedimentary rocks underlying deep-water Pennsylvanian and Permian, which are divided by Mississippian rhyolite. The Permian clastics contain some detrital rock that sit alongside low-grade metamorphosed sedimentary and volcanic rocks with serpentinite.
South of the Cortes and Carborca Terranes is the San José de García Terrane, which is a combination of Cretaceous arc volcanics and volcaniclastics, which may be thrusting over the Cortes terrane. The southern part of the range is made up of the Guerrero Composite Terrane, a large body, which is actually a complex of five different subterranes, not all of them within the main body of the Sierra Madre. However, the terrane is mostly covered by volcanics and sedimentary deposits, so it is only visible where erosion has revealed it. The main subterranes of the Guerrero Composite Terrane that are within the Sierra Madre are the Tahue and Zihuatanejo terranes. Dividing the Guerrero Terrane from the rest of the Sierra Madre terranes is a boundary that is thought to represent the Early Cretaceous Arperos Basin, a marine basin which separated the island arc that came to form the Guerrero terrane from the accretions that came to form the Sierra Madre terrane. It contains a lower formation made up of pillow basalts with pillow breccias, tuffs, and shales underlying a group of pelagic limestones, oozes, and turbidites.
Covering the southern basement are sedimentary and metamorphosed sedimentary rocks containing argillite, limestone, volcanic schist, slate, and greywacke which were exposed in the canyon of the Santiago River. Near these sediments late Oligocene to early Miocene granite and granodiorite intrusive bodies occur. A possible cause of this is that these bodies were roof pendants which were uplifted by plutons. Mesozoic limestone also occurs in northern portion of the mountains, mostly from the Cretaceous.
Towards the end of the Cretaceous, the Laramide orogeny increased the activity of magmatism in the area, forming the first major igneous series in the area. The igneous series are made up of formations of plutonic and volcanic rock, which would later be exposed. Interbedded with these rocks are sedimentary deposit rocks. In the center of the range, some of these rocks have been deformed by tectonic forcing that occurred at the same time. The southern part of the range contains none of the volcanism that is apparent in the northern range. These formations ended in the Paleocene. Eocene volcanism formed a series of andesitic and rhyolitic formations in the area, with spatial and temporal variations throughout. Most of the gold and silver deposits are also in these rocks.