During 1986-87, a seasonal, nearly circular lake occasionally occupied the summit crater. The lake's pH was 2-2.7 and the temperature was 30°C. Continuous fumarolic activity began in August 1988. A March 1989 summit visit by Alejandro Rivera Domínguez revealed large sulfur deposits in the main and inner craters. New fumaroles (not observed in 1987-88) on the main crater wall emitted high-pressure sulfurous gas and steam to 300 m. No significant microseismicity or tilt was detected.

The Grupo de Montañismo y Exploración de la UNAM, led by Prof. José Manuel Casanova Becerra, climbed the volcano on 9 April. More than 20 new fumaroles were observed on the outer S flank about 200 m below the crater rim. These vents (up to 1 m in diameter) were not observed when the group visited the area 2 years ago. Steam columns reached 20 m height and there was a mild sulfur odor. The steam's temperature was probably near the boiling point (at about 5,100 m altitude). The average altitude of the crater rim was 5,300 m with the crater bottom 340 m below. Increased steaming (common during the season) was observed inside the crater.

One seismograph is sited near the volcano . . . . Researchers hope to build an observatory 12 km from the volcano with telemetric data capture. Current monitoring is from the Meteorological Observatory, Geophysics Dept, Univ Autónoma de Puebla, and from Yancuitlalpan Village, S of the volcano.

Geological Summary. Volcán Popocatépetl, whose name is the Aztec word for smoking mountain, rises 70 km SE of Mexico City to form North America's 2nd-highest volcano. The glacier-clad stratovolcano contains a steep-walled, 400 x 600 m wide crater. The generally symmetrical volcano is modified by the sharp-peaked Ventorrillo on the NW, a remnant of an earlier volcano. At least three previous major cones were destroyed by gravitational failure during the Pleistocene, producing massive debris-avalanche deposits covering broad areas to the south. The modern volcano was constructed south of the late-Pleistocene to Holocene El Fraile cone. Three major Plinian eruptions, the most recent of which took place about 800 CE, have occurred since the mid-Holocene, accompanied by pyroclastic flows and voluminous lahars that swept basins below the volcano. Frequent historical eruptions, first recorded in Aztec codices, have occurred since Pre-Columbian time.

Information Contacts: S. De la Cruz-Reyna, UNAM; Alejandro Rivera Domínguez, Univ Autónoma de Puebla.