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 (Orizaba) at some 18,000 feet (5,500 meters), are extremely young in geologic terms (late Tertiary) 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. Land Regions Mexico can be divided into eight major physiographic regions. The largest and most significant in terms of human habitation is the Mexican Plateau. Extending from the isthmus of Tehuantepec northward to the United States border, the region consists of a central plateau and its dissected borders. The central plateau tilts gently from north to south. At its northern end it is about 4,000 feet (1,200 meters) above sea level and rises to more than 8...