A gigantic Bezymianny-type event at the beginning of modern volcan Popocatepetl

Abstract

The history of volcan Popocatepetl can be divided into two main periods: the formation of a large primitive volcano - approximatively 30 km wide - on which is superimposed a modern cone (6-8 km in diameter and 1700m high). A major event of Bezymianny type marks the transition between these two dissimilar periods. The activity of the primitive volcano was essentially effusive and lasted several hundred thousands of years. The total volume of products ejected by the volcano is of the order of 500-600 km3. Its last differentiated magmas are dacitic. A gigantic debris flow (D.F.) spread on the southern side is related to the Bezymianny-type event which destroyed the summit area of the ancient edifice. An elliptical caldera ({reversed tilde equals} 6.5 × 11 km wide) was formed by the landslide. Its deposits, with a typical hummocky surface, cover 300 km2 for a volume of 28-30 km3. Numerous outcrops belonging to this debris flow show "slabs" of more or less fractured and dislocated rocks that come from the primitive volcano. These deposits are compared to two studied debris flows of similar extent and volume: the Mount Shasta and Colima's D.F. This eruption takes a major place in the volcanologic and magmatic history of Popocatepetl: pyroclastic products of surge-type with "laminites" and crude layers, ashflows, and pumiceous airfall layers are directly related to this event and begin the history of the modern volcano probably less than 50,000 years ago. In addition, a second andesitic and dacitic phase rose both from the central vent - forming the basis of modern Popo - and from lateral vents. The terminal cone is characterized by long periods of construction by lava flows alternating with phases of destruction, the duration of these episodes being 1000 to 2000 years. The cone is composed of two edifices: the first, volcan El Fraile, began with effusive activity and was partly destroyed by three periods of intense explosive activity. The first period occurred prior to 10.000 years B.P., the second from 10.000 to 8000 years B.P. and the third from {reversed tilde equals} 5000 to {reversed tilde equals} 3800 years B.P. Each period of destruction shows cycles producing collapsing pyroclastic flows or nuées of the St Vincent-type related to the opening of large craters, plinian air-fall deposits and minor lava flows. The second edifice, the summit Popo, produced lava flows until 1200 years B.P. and since that time, entered into an explosive period. Two cataclysmic episodes, each including major pyroclastic eruptions, occurred 1200 and 900-1000 years ago. During the Pre-Hispanic and historic times effusive activity was restricted entirely to the summit area alternating with plinian eruptions. Nevertheless, despite the quiet appearance of the volcano, the last period of pyroclastic activity which started 1200 years ago may not have ended and can be very dangerous for the nearby populations. © 1987.