Just south of Mexico City, a 560-mile chain of volcanoes crosses the country from the Pacific Ocean to the Gulf of Mexico. Composed of more than 20 peaks and many more cinder cones, this Trans-Mexican Volcanic Belt has been developing since the Miocene, about 12 million years ago; over that time, volcanoes have risen, erupted and, eventually, eroded to the surface and today's landscape is but a snapshot in geologic time (most of the "modern" features have appeared over the past 1-2 million years). Indeed, satellite photos demonstrate many old calderas (remnants of past volcanoes) while Paricutin, in southwest Mexico, has risen in just the past 66 years.
Encompassing Mexico's three highest peaks, Pico de Orizaba (18,490 feet), Popocatepetl (17,802) and Ixtaccihuatl (17,160), this volcanic belt has formed (and is forming) as a consequence of subduction. The Cocos Plate, like the Juan de Fuca Plate of the Pacific Northwest and the Nazca Plate off the west coast of South America, is a remnant of the Farallon Plate which has been subducting beneath the American Plates since the Atlantic began to open, some 160 million years ago. To the south of Mexico, the Cocos Plate is also subducting beneath the western edge of the Caribbean Plate, producing a volcanic chain along the west coast of Central America.
Volcanism and earthquakes will continue in these subduction zones until the remnant plates are fully consumed. Popocatepetl, just 45 miles southeast of Mexico City, poses a significant threat to that metropolis, just as Mt. Ranier does to Seattle. The last major eruption along the Trans-Mexican Volcanic Belt was of El Chichon, in 1982, but the massive, 1985 earthquake in Mexico City (which killed 10,000) was also a reminder that subduction and tectonic activity persist in the region.
