Mexico

Geologic origins

Mexico is located in one of the Earth’s most dynamic tectonic areas. It is a part of the circum-Pacific “Ring of Fire”—a region of active volcanism and frequent seismic activity. Among its towering volcanic peaks are Citlaltépetl (also called Orizaba), which forms the highest point in the country at 18,406 feet (5,610 metres), and the active volcano Popocatépetl, which rises to 17,930 feet (5,465 metres) to the southeast of Mexico City. These and other Mexican volcanoes are young in geologic terms, from the Paleogene and Neogene periods (about 65 to 2.6 million years ago), and are examples of the volcanic forces that built much of the central and southern parts of the country. Mexico is situated on the western, or leading, edge of the huge North American Plate, whose interaction with the Pacific, Cocos, and Caribbean plates has given rise to numerous and severe earthquakes as well as the earth-building processes that produce southern Mexico’s rugged landscape. It is in this dynamic and often unstable physical environment that the Mexican people have built their country.

Physiographic regions

Mexico can be divided into nine major physiographic regions: Baja California, the Pacific Coastal Lowlands, the Mexican Plateau, the Sierra Madre Oriental, the Sierra Madre Occidental, the Cordillera Neo-Volcánica, the Gulf Coastal Plain, the Southern Highlands, and the Yucatán Peninsula.

The Baja California peninsula in northwestern Mexico is an isolated strip of extremely arid land extending between the Pacific Ocean and the Gulf of California (Sea of Cortez). Unevenly divided between the states of Baja California and Baja California Sur, the peninsula is nearly 800 miles (1,300 km) long but seldom more than 100 miles (160 km) wide. The central core of the peninsula is a granitic fault block with peaks of more than 9,000 feet (2,700 metres) above sea level in the Sierra San Pedro Mártir and Sierra de Juárez. The gently sloping western side of these mountain ranges is in contrast to the steep eastern escarpment, which makes access from the Gulf of California extremely difficult. The Sonoran Desert extends onto the peninsula along the northern end of the gulf.

The Pacific Coastal Lowlands begin near Mexicali and the Colorado River delta in the north and terminate near Tepic, some 900 miles (1,450 km) to the south. For most of that distance, they face the Gulf of California while traversing the states of Sonora, Sinaloa, and Nayarit. Bounded on the east by the steep-sided Sierra Madre Occidental, the lowlands are a series of coastal terraces, mesas, and small basins interspersed with riverine deltas and restricted coastal strips. Although the vast Sonoran Desert dominates their northern section, parts of the lowlands have been irrigated and transformed into highly productive farmland.

The largest and most densely populated region is the inland Mexican Plateau, which is flanked by the Sierra Madre Occidental and Sierra Madre Oriental. The plateau consists of the vast Mesa del Norte (Northern Plateau) and the smaller but heavily populated Mesa Central (Mesa de Anáhuac). The Mesa del Norte begins near the U.S. border; covers great stretches of the states of Chihuahua, Coahuila, Durango, Zacatecas, Jalisco, and Aguascalientes; and ends near San Luis Potosí city. From there the Mesa Central stretches to a point just south of Mexico City. The plateau tilts gently upward from the north toward the south; at its northern end, the Mesa del Norte is about 4,000 feet (1,200 metres) above sea level. Throughout the region, relatively flat intermontane basins and bolsones (ephemeral interior drainage basins) are interrupted by mountainous outcrops. In the north the Chihuahuan Desert covers a section of the plateau that is more extensive than the U.S. state of California.

The Mesa Central covers large parts of Michoacán, Guanajuato, Querétaro, Hidalgo, and México states and the Federal District (Mexico City). Its southern end rises 7,000–9,000 feet (2,100–2,700 metres) in the vicinity of Mexico City. The Mesa Central, moister and generally flatter than the Mesa del Norte, is divided into a series of fairly level intermontane basins separated by eroded volcanic peaks. The largest valleys rarely exceed 100 square miles (260 square km) in area, and many others are quite small. Among the generally fertile basins is the Bajío (El Bajío, or the Basin of Guanajuato), the traditional breadbasket of the country, which is located in the northern part of the Mesa Central. Many of the basins were once sites of major lakes that were drained to facilitate European and mestizo settlement. Around Mexico City the weak, structurally unstable soils that remain have caused the colonial-era Metropolitan Cathedral and other buildings to shift on their foundations and, over many years, to list or sink unevenly into the ground.

The largely volcanic Sierra Madre Occidental, which forms the western border of the Mexican Plateau, has an average elevation of 8,000–9,000 feet (2,400–2,700 metres) and extends roughly 700 miles (1,100 km) from north to south. It has been highly incised by westward-flowing streams that have formed a series of gorges, or barrancas, the most spectacular of which is the complex known as Copper Canyon (Barranca del Cobre) in southwestern Chihuahua state.

The Sierra Madre Oriental, a range of folded mountains formed of shales and limestones, is situated on the eastern side of the Mexican Plateau. Often considered an extension of the Rocky Mountains (which are cut by the Rio Grande but continue in New Mexico and western Texas), it runs roughly 700 miles (1,100 km) from north to south before merging with the Cordillera Neo-Volcánica. Its average elevations are similar to those of the Sierra Madre Occidental, but some peaks rise above 12,000 feet (3,650 metres). The mountains have major deposits of copper, lead, and zinc.

The Cordillera Neo-Volcánica, also called the Neo-Volcanic Axis or Trans-Volcanic Axis, is a geologically active mountain range whose smoldering cinder cones link the Sierra Madre Occidental with the Sierra Madre Oriental at the southern edge of the Mesa Central. As it crosses Mexico from Cape Corrientes on the west coast to Xalapa and Veracruz on the eastern coast, it forms a mountainous backdrop to the states of Jalisco, Michoacán, Guerrero, México, Morelos, and Puebla as well as the Federal District. This volcanic range includes the spectacular peaks Citlaltépetl, Popocatépetl, and Iztaccíhuatl (Ixtacihuatl), among others. One of the world’s youngest volcanoes, Parícutin emerged violently from the fields of Michoacán between 1943 and 1952. The region is rich in silver, lead, zinc, copper, and tin deposits. The hot, dry Balsas Depression, which takes its name from the major river draining the region, is immediately south of the Cordillera Neo-Volcánica. The depression is formed of small, irregular basins interrupted by hilly outcrops, which give the area a distinctive physical landscape.

The Gulf Coastal Plain, which is much wider than its Pacific coast counterpart, extends some 900 miles (1,450 km) along the Gulf of Mexico from Tamaulipas state (on the Texas border) through Veracruz and Tabasco states to the Yucatán Peninsula; it includes the Tabasco Plain in its southeastern section. The triangular northern portion of the plain, which is characterized by lagoons and low-lying swampy areas, reaches a width of more than 100 miles (160 km) near the U.S. border but tapers toward the south. North of the port of Tampico, an outlier of the Sierra Madre Oriental reaches the sea and interrupts the continuity of the Gulf Coastal Plain. South from there the plain is narrow and irregular, widening at the northern end of the Isthmus of Tehuantepec.

The Southern Highlands are a series of highly dissected mountain ranges and plateaus, including the Sierra Madre del Sur, Mesa del Sur, and the Chiapas Highlands, also called the Sierra Madre de Chiapas. On their southwestern side, approximately from Puerto Vallarta to the Gulf of Tehuantepec, are a series of relatively low ranges known collectively as the Sierra Madre del Sur. The crystalline mountains, which achieve elevations of 7,000–8,000 feet (2,100–2,400 metres), often reach the sea to create a rugged coastal margin, part of which is known as the Mexican Riviera. Several coastal sites, such as Ixtapa-Zihuatanejo, Acapulco, and Puerto Escondido, have become alluring tourist destinations. However, the less-hospitable inland basins provide a difficult environment for traditional peasant farmers. Farther northeast is the Mesa del Sur, with numerous stream-eroded ridges and small isolated valleys some 4,000–5,000 feet (1,200–1,500 metres) above sea level. The picturesque Oaxaca Valley is the largest and most densely settled of these, with a predominantly indigenous population. It is one of the poorest areas of Mexico.

Bisecting the Southern Highlands is the Isthmus of Tehuantepec, a low-lying, narrow constriction of land that reaches an elevation of less than 900 feet (275 metres). Its hilly central area descends to narrow coastal plains on the south and to the Tabasco Plain on the north.

The Chiapas Highlands are an extension of the mountain ranges of Central America. Within the highlands the low, crystalline Sierra de Soconusco range lies along the Pacific coast. To the northwest and paralleling the coast is the Grijalva River valley. A group of highly dissected, folded, and faulted mountains is located between the valley and the Tabasco Plain, a southeastern extension of the Gulf Coastal Plain. Among the active volcanic peaks of the region is El Chichón, which destroyed several villages in 1982.

The Yucatán Peninsula lies to the northeast of the Tabasco Plain and extends northward, forming a divider between the Gulf of Mexico and the Caribbean Sea. The peninsula’s limestone (karst) terrain is generally pockmarked and uneven but seldom exceeds 500 feet (150 metres) in elevation. There is little surface drainage, and subterranean erosion has produced caverns and sinkholes (cenotes), the latter being formed when cavern roofs collapse. The islands of Cozumel and Mujeres lie off the peninsula’s northeastern tip, near the resort boomtown of Cancún.