Topography and Drainage
Two prominent mountain ranges--the Sierra Madre Occidental and the Sierra Madre Oriental--define northern Mexico. Both are extensions of ranges found in the United States. The Sierra Madre Occidental on the west is a continuation of California's Sierra Nevada (with a break in southeastern California and extreme northern Mexico), and the Sierra Madre Oriental on the east is a southward extension of the Rocky Mountains of New Mexico and Texas. Between these two ranges lies the Mexican altiplano (high plain), a southern continuation of the Great Basin and high deserts that spread over much of the western United States.
Beginning approximately fifty kilometers from the United States border, the Sierra Madre Occidental extends 1,250 kilometers 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 kilometers inland from the west coast of Mexico at its northern end but approaches to within fifty kilometers 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 meters in elevation, with peaks reaching 3,000 meters.
The Sierra Madre Oriental starts at the Big Bend region of the Texas-Mexico border and continues 1,350 kilometers until reaching Cofre de Perote, one of the major peaks of the Cordi-llera 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 seventy-five kilometers 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 meters, with some peaks at 3,000 meters.
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 meters 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 meters 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.
One other significant mountain range, the California system, 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 California system extends from the United States border to the southern tip of the Baja Peninsula, a distance of 1,430 kilometers. Peaks in the California system range in altitude from 2,200 meters in the north to only 250 meters near La Paz in the south. Narrow lowlands are found on the Pacific Ocean and the Gulf of California sides of the mountains.
The Cordillera Neovolcánica is a belt 900 kilometers long and 130 kilometers 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 meters: 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.
Several important mountain ranges dominate the landscape of southern and southeastern Mexico. The Sierra Madre del Sur extends 1,200 kilometers 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 meters in elevation. The range averages 100 kilometers in width, but widens to 150 kilometers in the state of Oaxaca. The narrow southwest coastal plain extends from the Sierra Madre del Sur to the Pacific Ocean. The Sierra Madre de Oaxaca begins at Pico de Orizaba and extends in a southeasterly direction for 300 kilometers until reaching the isthmus of Tehuantepec. Peaks in the Sierra Madre de Oaxaca average 2,500 meters in elevation, with some peaks exceeding 3,000 meters. South of the isthmus of Tehuantepec, the Sierra Madre de Chiapas runs 280 kilometers along the Pacific Coast from the Oaxaca-Chiapas border to Mexico's border with Guatemala. Although average elevation is only 1,500 meters, one peak--Volcán de Tacuma--exceeds 4,000 meters in elevation. Finally, the Meseta Central de Chiapas extends 250 kilometers through the central part of Chiapas to Guatemala. The average height of peaks of the Meseta Central de Chiapas is 2,000 meters. The Chiapas central valley separates the Meseta Central de Chiapas and the Sierra Madre de Chiapas.
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 percent of Mexico's average annual volume of surface water. All five rivers flow into the Gulf of Mexico; only the Río Pánuco is outside southeastern Mexico, which contains approximately 15 percent of national territory and 12 percent of the national population. In contrast, northern and central Mexico, with 47 percent of the national area and almost 60 percent of Mexico's population, have less than 10 percent of the country's water resources.
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 American 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 centered in the subduction zone off Acapulco killed more than 4,000 people in Mexico City, more than 300 kilometers away. Volcán de Colima, south of Guadalajara, erupted in 1994, and El Chichón, in southern Mexico, underwent a violent eruption in 1983. Paricutín in northwest Mexico began as puffs of smoke in a cornfield in 1943; a decade later the volcano was 2,700 meters 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.
Data as of June 1996