Short-term Potential Volcanic Hazards at Popocatepetl, Mexico

Popocatépetl (smoking mountain in náhuatl) stands at the southern end of an 80 km-long chain that trends north-south and divides the basin of Mexico to the west from the basin of Puebla to the east. Popo is only 60 km southeast of Mexico City and 40 km west of the city of Puebla. The combined population of these two metropolitan areas exceeds 30 million inhabitants. The December 2000, activity of Popocatepetl has justifiably received intense public scrutiny.

The current dome growth and explosive activity present volcanologists and civil protection authorities with vexing problems that stem from our lack of a full understanding of the manner in which eruptions evolve at Popo. Large plinian eruptions have occurred at Popo with intervals ranging from 1000 to 3000 years and their ages are fairly well known. In contrast, the historic and geologic record of more frequent small to moderate eruptions is not as clear, and it may be difficult to forecast their occurrence and characteristics with accuracy. Important questions facing public safety officials at the moment revolve around prudent decisions based on the perceived effects of expected short-term events. What is the prognosis for the next few months and years?

Eruption History

The most recent Mt. St. Helens-type collapse of the cone occurred ca 23,000 years ago and the resulting debris avalanche traveled more than 100 km to the south. Since then, the present cone started to grow. Activity during the past 20,000 years included at least 7 large Plinian eruptions that produced extensive pumice-and-ash fallout, pyroclastic flows, and lahars. Each of these eruptions produced ca. 5-10 km3 of fragmental material. The most recent of these explosive eruptions occurred within the period of human occupation about 5000, 2100, and 1100 yr. BP (Siebe et al., 1996) with devastating effects, as evidenced by numerous archaeological remains buried by pumice-and-ash as well as lahars. People have repeatedly repopulated the area because of the volcano's long repeat time and the availability of water and agriculturally productive soil. Recurrence of such a cataclysmic Plinian eruption in the near future would certainly represent a volcanic disaster of unprecedented dimensions in human history.

Volcanic activity between cataclysmic eruptions is poorly preserved in the stratigraphic record and/or difficult to date radiometrically. Since the Spanish conquest in the early 16th century, Popo has erupted several times but documentation of these events by witnesses is fragmentary and varies in quality (Waitz, 1921). These historic eruptions seem to have a common characteristic: Energy release was relatively gentle with repeated formation of small domes inside the summit crater. Related vulcanian explosions produced 1-10 km-high ash plumes with accompanying ashfall. Such activity lasted for several years to a few decades and no major damage or casualties were reported. The pattern of activity during one of these smaller eruptions is insufficiently understood to connect them with cataclysmic eruptions.

Present eruption and prognosis for the next 5 years

On March 5, 1996, ash emissions resumed with renewed intensity. By March 29, a new lava dome appeared in the crater. Within one month the lava had covered the entire crater floor to a thickness of at least 50 m. On April 30, a small vulcanian explosion from the dome blasted meter-sized boulders from the crater killing five mountaineers. Gravel 3 to 4 cm in diameter fell at a distance of 6 km, clasts as large as 0.5 cm fell on the roofs of Xalitzintla at a distance of 12 km, and sand-sized ash fell in Tlaxcala. Since then, a total of 9 domes have formed in the crater. Dome growth is normally preceded and accompanied by harmonic tremor and increased fumarolic activity.

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