Geomorphology, Stratigraphy, and Eruption Dynamics
The evolution of Popocatépetl volcano was determined through the definition of rock units following morphostratigraphic criteria and detailed geological sections. The primitive volcano, named Nexpayantla, probably contemporaneous with Pies volcano, a part of the Iztaccíhuatl volcanic complex to the north, grew beneath the site of today's cone. This volcano produced mainly andesitic to dacitic lava flows, presented flank activity in the form of several large dacitic lava domes, and was intruded by dacitic to rhyolitic dikes. The evolution of Nexpayantla volcano finished with a large collapse to the south that produced the Lower Tlayecac avalanche deposit. A new cone, Ventorrillo volcano, was built on the remains of Nexpayantla and was formed mostly by andesitic lava flows, but did not present any recognizable flank activity. Ventorrillo volcano collapsed in a large Bezymianny-type eruption toward the southwest, producing the Upper Tlayecac avalanche deposit and the Tochimilco pumice. The Calpan fan was derived from collapse and eruptions of Pies volcano.
The present-day cone grew through the emission of many andesitic to dacitic lava flows, which were grouped into eroded or covered lava slopes (Malpaís, Las Mesas, Metepec, and San Pedro Benito Juarez lava flows), and glaciated (Fraile lava flows) and nonglaciated (Las Cruces, Buenavista, Quimichule, Atlimiyaya, Chiquipixle, and Nealtican lava flows) lava slopes with marked features, both from the central vent and from flank eruptions, mainly to the northeast and southwest of the cone. The Ecatzingo and Ombligo-Xalipilcáyatl flank vents formed two well-defined lineaments. The relative ages of the lava flows were determined through morphology, stratigraphic relations, and tephra cover. Two stages of growth were separated by a large Plinian eruption, which emplaced the Black and White (B&W) and Pumice with Andesite (PWA) fall deposits, which were used as stratigraphic markers. Another twelve Plinian pumice deposits are interstratified with the lava flows.
Four large volcaniclastic fans and five valley fill deposits form the volcano's piedmont, and have resulted from the successive emplacement of pyroclastic flows, lahars, and fluvial deposits along several gullies that mark the lower slopes of the volcano. Glacier melting coincident with several of the Plinian eruptions could have been responsible for some of the extensive lahar deposits.