Geography

The geography of Mexico entails the physical and human geography of Mexico, a country situated in the mid-latitudes of the Americas. It comprises much of southern North America, or also within Middle America. The country is bounded to the north by the United States (specifically, from west to east, by California, Arizona, New Mexico, and Texas), to the west and south by the Pacific Ocean, to the east by the Gulf of Mexico, and to the southeast by Belize, Guatemala, and the Caribbean Sea. A northern constituent of Latin America, it is the most populous Spanish-speaking country in the world.

Almost all of Mexico is on the North American Plate, with parts of the Baja California Peninsula in the northwest on the Pacific and Cocos Plates. Some geographers include the portion east of the Isthmus of Tehuantepec within Central America - namely, the five states of Campeche, Chiapas, Tabasco, Quintana Roo, and Yucatán, representing 12.1% of the country's total area. Alternatively, the Trans-Mexican Volcanic Belt delimits the region physiographically on the north. Geopolitically, Mexico is commonly not considered part of Central America and its southeastern border delimits the region.

Mexico's total area is 1,972,550 sq km, including approximately 6,000 sq km of islands in the Pacific Ocean (including the remote Guadalupe Island and the Islas Revillagigedo), Gulf of Mexico, Caribbean Sea and Gulf of California. On its north, Mexico shares a 3,141 km border with the United States. The meandering Río Bravo del Norte (known as the Rio Grande in the United States) defines the border from Ciudad Juárez east to the Gulf of Mexico. A series of natural and artificial markers delineate the United States-Mexican border west from Ciudad Juárez to the Pacific Ocean. On its south, Mexico shares an 871 km border with Guatemala and a 251 km border with Belize.

Mexico has a 10,143 kilometre coastline, of which 7,338 kilometres face the Pacific Ocean and the Gulf of California, and the remaining 2,805 kilometres front the Gulf of Mexico and the Caribbean Sea. Mexico's exclusive economic zone (EEZ), which extends 200 nautical miles (370 km) off each coast, covers approximately 2.7 billion square kilometres. The landmass of Mexico dramatically narrows as it moves in a south-easterly direction from the United States border and then abruptly curves northward before ending in the 500-kilometre-long Yucatán Peninsula. Indeed, the capital of Yucatán State, Mérida, is farther north than Mexico City or Guadalajara.

Physical Features

Sierra Madre Occidental

Beginning approximately 50 kilometres from the United States border, the Sierra Madre Occidental extends 5,000 kilometres south to the Río Santiago, where it merges with the Cordillera Neovolcánica range that runs east-west across central Mexico. The Sierra Madre Occidental lies approximately 300 kilometres inland from the west coast of Mexico at its northern end but approaches to within fifty kilometres of the coast near the Cordillera Neovolcánica. The northwest coastal plain is the name given the lowland area between the Sierra Madre Occidental and the Gulf of California. The Sierra Madre Occidental averages 2,250 metres in elevation, with peaks reaching 3,000 metres.

Sierra Madre Oriental

The Sierra Madre Oriental starts at the Big Bend region of the Texas-Mexico border and continues 1,350 kilometres until reaching Cofre de Perote, one of the major peaks of the Cordillera Neovolcánica. As is the case with the Sierra Madre Occidental, the Sierra Madre Oriental comes progressively closer to the coastline as it approaches its southern terminus, reaching to within 75 kilometres of the Gulf of Mexico. The northeast coastal plain extends from the eastern slope of the Sierra Madre Oriental to the Gulf of Mexico. The median elevation of the Sierra Madre Oriental is 2,200 metres, with some peaks at 3,000 metres.

Mexican Altiplano

The Mexican altiplano, stretching from the United States border to the Cordillera Neovolcánica, occupies the vast expanse of land between the eastern and western sierra madres. A low east-west range divides the altiplano into northern and southern sections. These two sections, previously called the Mesa del Norte and Mesa Central, are now regarded by geographers as sections of one altiplano.

The northern altiplano averages 1,100 metres in elevation and continues south from the Río Bravo del Norte through the states of Zacatecas and San Luis Potosí. Various narrow, isolated ridges cross the plateaus of the northern altiplano. Numerous depressions dot the region, the largest of which is the Bolsón de Mapimí.

The southern altiplano is higher than its northern counterpart, averaging 2,000 metres in elevation. The southern altiplano contains numerous valleys originally formed by ancient lakes. Several of Mexico's most prominent cities, including Mexico City and Guadalajara, are located in the valleys of the southern altiplano.

Peninsular Ranges

One other significant mountain range, the Peninsular Ranges, cuts across the landscape of the northern half of Mexico. A southern extension of the California coastal ranges that parallel California's coast, the Mexican portion of the Peninsular Ranges extends from the United States border to the southern tip of the Baja Peninsula, a distance of 1,430 kilometres. Peaks in the California system range in altitude from 2,200 metres in the north to only 250 metres near La Paz in the south. Narrow lowlands are found on the Pacific Ocean and the Gulf of California sides of the mountains.

Cordillera Neovolcánica

The Cordillera Neovolcánica is a belt 900 kilometres long and 130 kilometres wide, extending from the Pacific Ocean to the Gulf of Mexico. The Cordillera Neovolcánica begins at the Río Grande de Santiago and continues south to Colima, where it turns east along the nineteenth parallel to the central portion of the state of Veracruz. The region is distinguished by considerable seismic activity and contains Mexico's highest volcanic peaks. This range contains three peaks exceeding 5,000 metres: Pico de Orizaba (Citlaltépetl) - the third highest mountain in North America - and Popocatépetl and Iztaccíhuatl near Mexico City. The Cordillera Neovolcánica is regarded as the geological dividing line between North America and Central America.

Sierra Madre del Sur

Several important mountain ranges dominate the landscape of southern and southeastern Mexico. The Sierra Madre del Sur extends 1,200 kilometres along Mexico's southern coast from the southwestern part of the Cordillera Neovolcánica to the nearly flat isthmus of Tehuantepec. Mountains in this range average 2,000 metres in elevation. The range averages 100 kilometres in width, but widens to 150 kilometres in the state of Oaxaca. The narrow southwest coastal plain extends from the Sierra Madre del Sur to the Pacific Ocean.

Sierra Madre de Oaxaca

The Sierra Madre de Oaxaca begins at Pico de Orizaba and extends in a south-easterly direction for 300 kilometres until reaching the isthmus of Tehuantepec. Peaks in the Sierra Madre de Oaxaca average 2,500 metres in elevation, with some peaks exceeding 3,000 metres.

Sierra Madre de Chiapas

South of the isthmus of Tehuantepec, the Sierra Madre de Chiapas runs 280 kilometres along the Pacific Coast from the Oaxaca-Chiapas border to Mexico's border with Guatemala. Although average elevation is only 1,500 metres, one peak (Volcán de Tacuma) exceeds 4,000 metres in elevation.

Meseta Central de Chiapas

The Meseta Central de Chiapas extends 250 kilometres through the central part of Chiapas to Guatemala. The average height of peaks of the Meseta Central de Chiapas is 2,000 metres. The Chiapas central valley separates the Meseta Central de Chiapas and the Sierra Madre de Chiapas.

Rivers

Mexico has nearly 150 rivers, two-thirds of which empty into the Pacific Ocean and the remainder of which flow into the Gulf of Mexico or the Caribbean Sea. Despite this apparent abundance of water, water volume is unevenly distributed throughout the country. Indeed, five rivers (the Usumacinta, Grijalva, Papaloapán, Coatzacoalcos and Pánuco) account for 52% of Mexico's average annual volume of surface water. All five rivers flow into the Gulf of Mexico; only the Río Pánuco is outside south-eastern Mexico, which contains approximately 15% of national territory and 12% of the national population. In contrast, northern and central Mexico, with 47% of the national area and almost 60% of Mexico's population, have less than 10% of the country's water resources.

Seismic Activity

Situated atop three of the large tectonic plates that constitute the earth's surface, Mexico is one of the most seismologically active regions on earth. The motion of these plates causes earthquakes and volcanic activity.

Most of the Mexican landmass rests on the westward moving North American plate. The Pacific Ocean floor off southern Mexico, however, is being carried northeast by the underlying motion of the Cocos Plate. Ocean floor material is relatively dense; when it strikes the lighter granite of the Mexican landmass, the ocean floor is forced under the landmass, creating the deep Middle America Trench that lies off Mexico's southern coast. The westward moving land atop the North American plate is slowed and crumpled where it meets the Cocos plate, creating the mountain ranges of southern Mexico. The subduction of the Cocos plate accounts for the frequency of earthquakes near Mexico's southern coast. As the rocks constituting the ocean floor are forced down, they melt, and the molten material is forced up through weaknesses in the surface rock, creating the volcanoes in the Cordillera Neovolcánica across central Mexico.

Areas off Mexico's coastline on the Gulf of California, including the Baja California Peninsula, are riding northwestward on the Pacific plate. Rather than one plate subducting, the Pacific and North American plates grind past each other, creating a slip fault that is the southern extension of the San Andreas fault in California. Motion along this fault in the past pulled Baja California away from the coast, creating the Gulf of California. Continued motion along this fault is the source of earthquakes in western Mexico.

Mexico has a long history of destructive earthquakes and volcanic eruptions. In September 1985, an earthquake measuring 8.1 on the Richter scale and centred in the subduction zone off Acapulco killed more than 4,000 people in Mexico City, more than 300 kilometres away. Volcán de Colima, south of Guadalajara, erupted in 1994, and El Chichón, in southern Mexico, underwent a violent eruption in 1983. Parícutin in northwest Mexico began as puffs of smoke in a cornfield in 1943; a decade later the volcano was 2,700 metres high. Although dormant for decades, Popocatépetl and Ixtaccíhuatl ('smoking warrior' and 'white lady,' respectively, in Náhuatl) occasionally send out puffs of smoke clearly visible in Mexico City, a reminder to the capital's inhabitants that volcanic activity is near. Popocatépetl showed renewed activity in 1995 and 1996, forcing the evacuation of several nearby villages and causing concern by seismologists and government officials about the effect that a large-scale eruption might have on the heavily populated region nearby.

Environmental Conditions

Mexico faces significant environmental challenges affecting almost every section of the country.

Deforestation

Vast expanses of southern and south-eastern tropical forests have been denuded for cattle-raising and agriculture. For example, tropical forests covered almost half of the state of Tabasco in 1940 but less than 10% by the late 1980s. During the same period, pastureland increased from 20 to 60% of the state's total area. Analysts reported similar conditions in other tropical sections of Mexico. Deforestation has contributed to serious levels of soil erosion nationwide. In 1985 the government classified almost 17% of all land as totally eroded, 31% in an accelerated state of erosion, and 38% demonstrating signs of incipient erosion. Mexico has developed a Biodiversity Action Plan to address issues of endangered species and habitats that merit protection.

Soil destruction is particularly pronounced in the north and northwest, with more than 60% of land considered in a total or accelerated state of erosion. Fragile because of its semiarid and arid character, the soil of the region has become increasingly damaged through excessive cattle-raising and irrigation with waters containing high levels of salinity. The result is a mounting problem of desertification throughout the region.

Coastline

Mexico's vast coastline faces a different, but no less difficult, series of environmental problems. For example, inadequately regulated petroleum exploitation in the Coatzacoalcos-Minatitlán zone in the Gulf of Mexico has caused serious damage to the waters and fisheries of Río Coatzacoalcos. The deadly explosion that racked a working-class neighbourhood in Guadalajara in April 1992 serves as an appropriate symbol of environmental damage in Mexico. More than 1,000 barrels of gasoline seeped from a corroded Mexican Petroleum (Petróleos Mexicanos--Pemex) pipeline into the municipal sewer system, where it combined with gases and industrial residuals to produce a massive explosion that killed 190 persons and injured nearly 1,500 others.

Mexico City

Mexico City confronts authorities with perhaps their most daunting environmental challenge. Geography and extreme population levels have combined to produce one of the world's most polluted urban areas. Mexico City sits in a valley surrounded on three sides by mountains, which serve to trap contaminants produced by the metropolitan area's 15 million residents. One government study in the late 1980s determined that nearly 5 million tons of contaminants were emitted annually in the atmosphere, a tenfold increase over the previous decade. Carbons and hydrocarbons from the region's more than 3 million vehicles account for approximately 80% of these contaminants, with another 15%, primarily of sulphur and nitrogen, coming from industrial plants.

The resulting dangerous mix is responsible for a wide range of respiratory illnesses. One study of twelve urban areas worldwide in the mid-1980s concluded that the residents of Mexico City had the highest levels of lead and cadmium in their blood. The volume of pollutants from Mexico City has damaged the surrounding ecosystem as well. For example, wastewater from Mexico City that flows north and is used for irrigation in the state of Hidalgo has been linked to congenital birth defects and high levels of gastrointestinal diseases in that state.

Beginning in the mid-1980s, the government enacted numerous antipollution policies in Mexico City with varied degrees of success. Measures such as vehicle emissions inspections, the introduction of unleaded gasoline and the installation of catalytic converters on new vehicles helped reduce pollution generated by trucks and buses. In contrast, one of the government's most prominent actions, the No Driving Day program, may have inadvertently contributed to higher pollution levels. Under the program, metropolitan area residents were prohibited from driving their vehicles one day each work week based on the last number of their license plate. However, those with the resources to do so purchased additional automobiles to use on the day their principal vehicle was prohibited from driving, thus adding to the region's vehicle stock. Thermal inversions reached such dangerous levels at various times in the mid-1990s that the government declared pollution emergencies, necessitating sharp temporary cutbacks in vehicle use and industrial production.