Southward from its 1,500 mile long border with the United States lies the Estados Unidos Mexicanos. A country with slightly more than 750,000 square miles in area, Mexico has a vast array of mineral resources, limited agricultural land, and a rapidly growing population. These factors are the basis for many of the country\'s present problems as well as opportunities for future development. The nation is struggling to modernize its economy. With more than 80 million people in the mid-1980s, Mexico\'s overall population density exceeds 110 per square mile. More than half of its inhabitants live in the country\'s central core, while the arid north and the tropical south are sparsely settled.
The stereotype of Mexico is that it is a country with a population consisting mainly of subsistence farmers has little validity. Petroleum and tourism dominate the economy, and industrialization is increasing in many parts of the nation. Internal migration from the countryside has caused urban centers to grow dramatically: more than two thirds of all Mexicans now live in cities. Mexico City, with a metropolitan area population of approximately 16 million people, is the largest city in the world. While still low by United States standards, the nation\'s gross national product per capita rose significantly during the 1970s. Despite impressive social and economic gains, since 1981 Mexico has been wracked by severe inflation and an enormous foreign debt brought on in large part by precipitous declines in the value of petroleum products.
Geologically, Mexico is located in one of the Earth\'s most dynamic areas. It is a part of the "Ring of Fire," a region around the Pacific Ocean highlighted by active volcanism and frequent seismic activity. Within the context of plate tectonics, a theory developed to explain the creation of major landform features around the world, Mexico is situated on the western, or leading, edge of the huge North American Plate. Its interaction with the Pacific, Cocos, and Caribbean plates has given rise over geologic time to the Earth-building processes that created most of Mexico. Towering peaks, like Citlaltepetl at some 18,000 feet, are extremely young in geologic terms and are examples of the volcanic forces that built much of central Mexico. The spectacular eruption of the volcano Chinchon in 1981 was more powerful than that of Mount St. Helens in the United States a year earlier and led to widespread devastation.
Much of the complexity found in southern Mexico\'s physiography is related to the interaction of three tectonic plates. Such interaction creates regions that are often highly unstable, producing numerous and severe Earth movements. A 1985 quake, with an epicenter off the coast of Acapulco, caused billions of dollars in damage nationwide, destroyed hundreds of buildings in Mexico City, and killed several thousand people. It is on this often unstable and dynamically active physical environment that the Mexican people must build their nation.
The plateau can be subdivided into two major sections. The Mesa del Norte begins near the international border and ends around San Luis Potosi. In this arid lower part of the plateau, interior drainage predominates with few permanent streams. On its west side the mesa is flanked by the largely volcanic Sierra Madre Occidental, with an average height of 8,000 to 9,000 feet (2,400 to 2,700 meters). It has been highly dissected by westward-flowing streams that eroded a series of deep barrancas, or canyons. The most spectacular of these is the Barranca del Cobre, Mexico\'s equivalent of the Grand Canyon. The Sierra Madre Oriental, a range of folded mountains formed of shale and limestone, is on the east side of the mesa. With average elevations similar to those of the Sierra Madre Occidental, these dissected highlands have peaks that reach 13,000 feet.
The Mesa Central stretches from San Luis Potosi to the volcanic axis south of Mexico City. Formed largely by volcanic action, the general plateau surface of this mesa is higher, moister, and generally flatter than the Mesa del Norte. The Mesa Central is divided into a series of fairly flat intermountain basins separated by eroded volcanic peaks. These basins are generally quite fertile and have been the most densely populated portions of Mexico for several hundred years. The largest valleys such as those of Mexico City, Puebla, and Guadalajara rarely exceed 100 square miles in area, while many others