Fig. 1. Oceanic-continental convergence (from Pidwirny, 2001) Popocate'petl is a 5,465-meter andesitic stratovolcano that is located 19.02 degrees north and 98.62 degrees west. It is approximately 55 km east of Mexico City and 45 km east of Puebla (Fig. 2).
Fig. 2. Major Volcanoes of Mexico (from USGS/CVO, 1998) The volcano covers 500 square kilometers and has a 900-meter crater that is 200 meters deep. Popocate'petl means "Smoking Mountain," in the Aztec language of Nahuatl (Dunlap, 1996). It is the second highest peak in Mexico and is one of the country's most volatile volcanoes (Dunlap, 1996). It has had approximately 30 eruptions in historical time, mostly mild to moderate Vulcanian eruptions. Popocate'petl has also produced several Plinian type eruptions, and there are records of at least three enormous ones in the past 5,000 years (Ray, 2001).
Subduction of Cocos & Rivera Plates Popocate'petl is part of the Trans-Mexico Volcano Belt (TMVB) (fig. 3). "Volcanic activity related to the Trans-Mexican Volcanic Belt started during Early stratovolcano Miocene" (Capral, L. et al., 2000). "The TMVB is 1200 kilometers long and goes from the Gulf of California all the way through Central Mexico and meets the Central American Volcano belt near El Chichon, a volcano in Chiapas, Mexico" (Johnson, 1991). This line of volcanoes is caused by the subduction of the Cocos and Rivera plates beneath the North American plate. These two plates "spread away" from the East Pacific Rise, and are separated from each other by a transform fault known as the Rivera fracture zone (Johnson, 1991). This fracture zone results from the different rates of motion of the two plates-6 cm per year for the Cocos plate and 1 cm per year for the Rivera plate (Johnson, 1991).
Fig. 3. A very generalized map of central Mexico showing the Trans Mexican Volcanic Belt, the Michoacán-Guanajuato Volcanic Field, the Chichinautzin Formation and various volcanoes located in the Trans-Mexico Volcano Belt (from Newton, 1998) The volcanoes in the Popocate'petl area are the nearest of all the volcanoes to the Middle American Trench. The Middle American Trench occurs where the Coccos and the Rivera plates are subducting. The reason the distance from the trench to the volcanoes in chain (the arc trench gap) varies so much (from 80 to 400 km), is due to the fact that the "depth and inclination of the subduction zone varies" (Johnson, 1991). Popocate'petl is in the middle of the TMVB implying that the crust dips approximately "15 degrees" at that point (Flueh et al., 1996). "Near the trench [the Coccos and Rivera plates dip] with 5 degrees, 60 km landward the dip is 15 degrees and 100 km from the trench it reaches 23 degrees (Flueh et al., 1996)." Geological History Little is known about the early geologic history of Popo. The oldest rocks found so far at Popo have not been dated, but they are stratigraphically younger than rocks from Iztaccihuatl volcano, immediately to the north. "This suggests that the locus of magma production has migrated southwards during the course of time" (Rogers, 1995). Popo's present cone is not the first huge volcanic structure that evolved at this site, as evidenced by at least three debris avalanche deposits that fan out towards the south, from the same site of origin (Siebe et al., 1995). "The present cone consists of interlayered andesitic...