Report on Popocatepetl (Mexico) — April 1994
Bulletin of the Global Volcanism Network, vol. 19, no. 4 (April 1994)
Managing Editor: Richard Wunderman.
Popocatepetl (Mexico) Seismicity, SO2 flux measurements, and crater observations reported
Please cite this report as:
Global Volcanism Program, 1994. Report on Popocatepetl (Mexico) (Wunderman, R., ed.). Bulletin of the Global Volcanism Network, 19:4. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199404-341090.
19.023°N, 98.622°W; summit elev. 5393 m
All times are local (unless otherwise noted)
Seismicity during March and April consisted primarily of B-type events. During March, 99 B-type events were recorded, an increase from the 62 recorded in both January and February (19:02). On two days (6 and 22 March) as many as 7 B-type events were recorded, and during a 3-day period (26-28 March) 16 events were registered. A- and AB-type events (11 and 6, respectively, during the month) were also recorded during periods of increased B-type activity. The number of B-type events continued to increase in April, reaching a total of 164 during the month. From 16 to 19 April, 33 events were recorded. The highest daily total was on 29 April, when 13 events were registered. Except for two A-type events on 2 April, no A- or AB-type events were detected. The seismic station, part of the Mexican National Seismic Network, is located at 3,900 m elevation on the N flank.
A new series of ultraviolet absorption correlation spectrometry (COSPEC) measurements were made by Univ de Colima scientists on 4-5 May from a Mexican Navy airplane. The measurements were requested by the Secretaria de Gobernacion through the Centro Nacional para la Prevencion de Desastres (CENAPRED). Between 1151 and 1359 on 4 May, the plume was traversed 20 times at an altitude of 3,900-4,000 m in partially cloudy to overcast conditions. Another 11 traverses were made at 3,950 m altitude between 0936 and 1148 the next day. The weather was again partially cloudy to overcast, and rainfall was detected upon returning. The aircraft's global positioning system (GPS) computed the wind speed independently for each traverse. These measurements were each used to make individual SO2 flux calculations, removing the need to calculate an SO2 estimate based on an average wind speed. This procedure is advantageous when the wind speed varies significantly. A statistical analysis of the time series was also performed.
The SO2 flux on 4 May ranged from 485 to 1,462 metric tons/day (t/d), with a standard deviation of 232 t/d and an average value of 900 t/d. This result is close to the value of 1,200 ± 400 t/d measured on 1 February (19:01). SO2 values dropped the next day to a range of 386-684, with a standard deviation of 89 and an average of 502 t/d. The 5 May results are not thought to be representative of the actual emissions because weather conditions were much more humid, resulting in a more effective gas-to-particle conversion that produced more H2SO4 aerosol droplets. An adequate baseline reference value for SO2 output has not yet been established, so any interpretations are preliminary and should be made with caution. A regular monthly schedule for additional COSPEC measurements is currently planned.
On 30 January 1994, Delgado, Siebe, and Tobias Fischer (Arizona State Univ) installed sampling boxes near the crater rim to monitor gas emissions. These boxes hold an open container of 500 ml 25% KOH solution to absorb acidic gases and allow measurement of variations in S/Cl/CO2 ratios (Noguchi and Kamiya, 1963). Since then, visits to the crater have taken place every three weeks. The following is an account of observations made during those visits through mid-May.
Intense fumarolic activity has been observed in the crater, in and around the inner dome, and from the crater walls. Sulfur deposits around the fumarolic vent glowed red during the day. Intense reddish glow at night was more common, although dense emissions often hampered observations. Hissing noises from several vents were due to powerful gas emissions that rose up to 1,000 m before being blown downwind. Most of the fumarolic column was bent by the wind to the E, NE, and SE during January-April. A tall, long plume from the volcano could be observed at distances of 40-60 km. The most dense fumarolic emissions came from the crater floor. Several small fumarolic vents associated with fractures in the crater walls and near the rim produced gas emissions, these were smelled by mountain climbers visiting the summit, and they contaminated the surrounding ice cap with sulfur. Most of the fumaroles were on the inner part of the crater rim, but some diffuse vents were observed on the outer SE and E flanks as low as ~5,000 m elevation.
During the 19 February crater visit, the small milky-green lake nested in the central dome had a temperature of 65°C and a pH of 1.5 ± 0.5. This crater lake description contrasts with observations from 1986 in color (very clear with a greenish tint), temperature (29°C), and pH (6.5) (11:01). Temperatures at fumarolic vents, measured using a thermocouple, ranged from 250 to 380°C. Fumaroles surrounded by the intensely glowing red sulfur deposits were inaccessible. The measured fumaroles were the same as those with temperatures of 97-99°C in 1986. Since late March-early April, additional water introduced to the system by precipitation has increased the vapor phase, causing the plumes to look more dense and whitish when emitted, but more diffuse when dispersed out of the crater. If no wind was present at 5,000-6,000 m altitude to disperse the plume, there was the appearance of a dense, dirty cloud near the volcano.
Increased fumarolic activity has caused alarm in surrounding towns and villages. Concern about future eruptions has prompted many people to climb the summit for observation of the activity. This has led to a proliferation of reports by untrained people describing "molten lava" (overheated sulfur) and "phreatic eruptions" inside the crater (reported as unconfirmed in 18:11). On 29 April a commercial airline pilot reported an "ash cloud" at ~5,800 m altitude 35 km SE of México City. No ash cloud was seen the following day during a routine visit to refill the KOH solution in the boxes. Careful observations inside the crater and on the ice around the crater did not reveal any recent ash emissions. The pilot very likely observed a fumarolic cloud, but the report of ash caused alarm among air-traffic authorities in México City.
Popocatépetl rises to 5,420 m above the Mexico and Puebla valleys, basins with >20 million inhabitants. The last significant eruptive period at this stratovolcano was in 1920-22, with fumarolic activity through 1927; minor ash explosions were reported in 1933, 1943, and 1947.
Following destruction of an ancestral volcano by a Bezymianny-type eruption during which a debris avalanche formed a 6.5 x 11 km caldera, the modern cone was constructed in two stages. The El Fraile volcano, formed prior to 10,000 years BP, was partly destroyed by later explosive activity. The current summit of Popocatépetl was formed to the south of El Fraile by repeated lava effusions until about 1,200 years BP. About 25-30 eruptions have occurred in the last 600 years, most of them apparently consisting of weak summit explosions.
Reference. Noguchi, K., and Kamiya, H., 1963, Prediction of volcanic eruption by measuring the chemical composition and amounts of gases: Bulletin of Volcanology, v. 26, p. 367-378.
Geologic Background. 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: Guillermo González-Pomposo and Carlos Valdés-González, Instituto de Geofísica, UNAM; Ignacio Galindo, Arturo Gonzalez, Juan Carlos Gavilanes and Carlos Navarro, CUICT Univ de Colima; Hugo Delgado and Claus Siebe, Instituto de Geofísica, UNAM, Circuito Exterior.