Smithsonian Institution
Global Volcanism Network Bulletin v. 19, no. 11, November 1994
Popocatepetl (Mexico) Small eruption on 21 December ends decades-
long slumber
Popocatepetl
central Mexico (19.02N, 98.62W)
All times are local (= GMT - 6 hours)
A new episode of explosive activity began at Popocatepetl volcano
on 21 December (figure 1). The eruption followed increases in
seismicity, SO2 flux, and fumarolic activity during the last 13
months (Bulletin v. 18, no. 11; v. 19, nos. 1-4, 6, 8, 10).
Although in the last year seismicity rose and fell several times,
during late-October there was a sudden, prominent (roughly 1.6- to
10-fold) increase in daily earthquakes compared to previous months.
Measurements of the volcano's total SO2 flux were consistently
large (some airborne measurements averaged >1,000 tons/day). During
October-November 1993 a cluster of steam vents in the summit crater
produced clouds that reached 6,000 m elevation, several-hundred
meters above the 5,465 m summit. These clouds sometimes stretched
for 50 km.
Eruptive activity. Near midnight on 22 December, Servando de la
Cruz sent the following report.
"The fumarolic activity that has been developing during the last
two years or so culminated on early 21 December 1994, when a series
of volcanic earthquakes, probably associated with phreatic
explosions, marked the beginning of a new stage of eruptive
activity. The seismic events, detected at 0131, 0132, 0138, 0140,
and 0148, were very impulsive, high-frequency, short-duration
signals, and were followed by a major, lower-frequency event at
0153. The events were recorded by 4 telemetric stations, within 11
km of the volcano, operated jointly by CENAPRED (National Disaster
Prevention Center) and the Institutes of Geophysics and Engineering
of the National University of Mexico (UNAM). As the day cleared an
ash plume was observed for the first time in decades emerging from
the volcano's crater. The ash emission was moderate and produced an
almost horizontal plume causing a light ashfall over the city of
Puebla, about 45 km E of the volcano's summit. A helicopter flight
at 1030 showed that most of the ash issued from near the lower NE
rim of the inclined crater. A radial fissure on the NE flank of the
cone displayed some steam-producing vents, though the cloudy
conditions made this interpretation equivocal. Old cracks in the
glacier appeared to have extended a significant amount towards the
W. A second flight at 1430 the same day revealed a substantial
increase in ash production (about 3-4 times the amount observed in
the morning). The light-gray ash appeared to be emitted
episodically, with "puffs" every few minutes.
"The seismicity consisted of mostly low-amplitude B-type
earthquakes and concurrent high-frequency A-type events. Though
this seismicity remained lower than during the night of 21
December, during the next day the seismicity again increased. At
this stage and after several consultations between the scientific
group and the Civil Protection authorities, an evacuation of the 19
most vulnerable towns and villages on the E sector of the volcano
was started around 2100 of December 21, and about 31,000 persons
were moved during the night to shelters in safer areas. Since then
the situation has remained fairly stable, though long-duration,
low-amplitude tremors appeared in the night of 21-22 December, and
continue."
Claus Siebe reported that climbers at Popocatepetl reached the
summit, which lies along the W margin of the gaping summit crater's
rim, both on the day before the eruption, and hours after the 21
December eruption started. On the day before the eruption visiting
climbers could see the crater lake and sparse fumaroles. They
reportedly heard no hissing sounds and they smelled less odor from
sulfur-bearing gases than in previous months.
Curiously, the six volcanic earthquakes that took place between
0130 and 0200 on 21 December were not felt, and the presumably
associated phreatic summit explosions were not heard by any of
about 25 mountain climbers at Tlamacas, 4 km N of the summit
(figure 1). The climbers, who said they started ascending the
mountain around 0400 on 21 December, did not notice anything
unusual until they neared the crater rim. Just prior to reaching
the rim, a few minutes before 0800, climbers were stunned by what
they thought was the sound of jet engines. At the crater rim they
saw new bombs as large as 40 cm that had been thrown out of the
250-m-deep crater and had burrowed deep impact-pits in the snow.
According to Siebe: "Most climbers who reached the summit that
morning thought that the activity was normal, because they had
never visited Popocatepetl before." At the summit, the climbers
said they could not see the crater floor even though a strong wind
was blowing. They descended the mountain without incident.
Siebe was at Tlamacas at 0900 on 21 December during clear weather.
He observed a continuous ash plume rising 100-500 m above the
crater with pulses at 1-5 minute intervals. The plume was carried
at least 60 km E. Enough silt- and sand-sized material reached
Puebla to produce a thin coating on cars. The ejecta appeared to be
non-juvenile, and contained pyrite, sulfur, and Ca-sulfate.
A report from Steve McNutt indicated that the volcano began to
quiet down on the afternoon of 25 December. During the night of
27-28 December a M 2 earthquake took place; the largest prior event
in the recent past was M 2.9. On 27 December tremor was barely
perceptible and a few small low-frequency events took place. During
the 24-hour period ending about midday on 28 December there were
~30 low-frequency events. Tremor roughly doubled between 23 and 24
December, but then during 25-28 December it dropped and became
barely detectible. No specific seismic data were available for
dates after that, though seismicity did increase again and an
explosion was heard roughly 10 km from the summit at about 1300 on
31 December. Investigators planned to install about four new
seismic stations to improve spatial and azimuthal coverage, and to
add one station close in.
By 27 December all but three of the previously evacuated towns had
been reoccupied; those towns not reoccupied were subject to lahar
hazard. A glaciologist made an initial helicopter inspection of the
glacier looking especially for signs of abnormal melting. No report
was available at the time of this publication, but plans to monitor
the glacier included both a daily inspection flight and a video
camera aimed at it from 5 km away. The last of the three previously
evacuated towns was reoccupied by 28 December.
News reports. A 21 December AP story said Popocatepetl, "spewed a
column of roiling black ash Wednesday, dusting villages and
farmland but causing no injuries" and that "television footage from
traffic helicopters showed a dense column of ash belching from the
summit."
As of 23 December, an AP report noted that the Puebla state
government said 75,000 people would be evacuated from the
countryside around the volcano. Other news reports put the number
of evacuees at ~50,000. One of the evacuated towns, Santiago
Xalitzintla, is located ~13 km NE of the summit. The town sits
along the road over "Paso de Cortez," the pass between Popocatepetl
and the adjacent Quaternary stratovolcano to the N, Iztaccihuatl
(figure 1).
A 26 December UPI report noted that "Jorge Martinez Soto, a
researcher at the University of Puebla, said the amount of smoke
and ash being emitted ... diminished by about 75 percent since last
week ...."
Plume imagery and transport modeling. Although the 21 December
eruption plume may appear on satellite imagery, to our knowledge no
investigator has yet announced having found it. There is an AVHRR
(channel 1) image of a Popocatepetl plume on 22 December at 0818
(1418 GMT). That image shows a SE-directed plume tens of kilometers
long. There are also three other AVHRR images for plumes on 26, 27,
and 28 December. All four images are available from Melissa
Seymour, LSU Earth Scan Lab. We learned of these images at press
time and although we have not had time to see them and tabulate
plume orientations, the imaged plumes reportedly trailed southward.
The Synoptic Analysis Branch (SAB) of NOAA/NESDIS first reported
Popocatepetl activity at 1530 (2130 GMT) on 26 December for an
eruption that took place at around 1300. A SIGMET (Significant
Meteorological Event) notice was posted from Mexico City announcing
that a new eruption had taken place and that the plume from this
eruption reached an altitude of about 6.7 km (22,000 feet). SAB
later described the shape of the plume associated with this
eruption based on GOES-7 and -8 data (table 1 and figure 2). A
report later on 26 December indicated that the volcano had
continued to erupt, creating a visible plume that at 1745 extended
to 50 km E. At 0745 the next day (27 December), a GOES-8 visible
satellite image of the plume suggested a gently curving,
funnel-shaped mass tracking NE (figure 2). Based on the lack of
infrared signatures and on their visible signatures, all the plumes
reported in table 1 and figure 2 were thought to be of low density.
A modeling program called "VAFTAD" was used to forecast the
transport and dispersion of the plume from the 26 December eruption
(see references and description of VAFTAD in the report for
Rinjani, Bulletin v. 19, no. 6). VAFTAD produced a series of visual
ash-cloud forecasts such as those on figure 3, which showed the
plume initially covering both quadrants in the E half of the
volcano and then traveling NE along about the same path taken by
actual plumes seen in the GOES imagery (table 1 and figure 2). The
models forecasted that after about 24 hours the plume would travel
NE over the Gulf of Mexico.
VAFTAD uses wind and pressure data updated twice daily on grids
with spacings of 91 km in the USA and 1 degree over the rest of the
globe. The model assumes the eruption delivers a mass load to the
atmosphere. The mass load is not scaled to the actual mass of the
eruption, but rather the load is assumed to be 1 gram (composed of
spherical particles with a density of 2.5 x 10^6 grams/m^-3 in a
size range of 0.3-30 fm in diameter). VAFTAD computes transport and
dispersion assuming particles are carried by advection both
horizontally and vertically, diffuse with a bivariate normal
distribution, and fall according to Stoke's law with a slip
correction. Calculated ash concentrations have been correlated with
satellite imagery for defining the visual ash cloud forecasts.
One noteworthy aspect of the Popocatepetl plumes is the relatively
large height of the summit crater (elevation ~5,215 m). Even small,
low-energy eruptions from this high-altitude vent can eject
material to 6 km (~20,000 feet) elevation.
So in essence, these ash cloud forecasts serve best for hazards
planning purposes. A key use, in fact, is to warn airline pilots of
the airspace most likely to contain volcanic ash particles. Besides
the other hazards discussed in Boudal and Robin (1989), a large
eruption from Popocatepetl could affect air travel in routes over
parts of NE Mexico and much of the Gulf of Mexico.
Eruptive history. In the Holocene Popocatepetl has produced both
effusive and pyroclastic activity. The latter has ranged from mild
steam-and-ash emissions to Plinian eruptions accompanied by
pyroclastic flows and surges. Vigorous Holocene explosive activity
took place in three periods (in years before present, ybp): a)
10,000 to 8,000, b) 5,000 to 3,800, and c) 1,200 to present (Boudal
and Robin, 1989). An effusive period from 3,800 to 1,200 ybp ended
with a vigorous explosive eruption that both enlarged the summit
crater and generated St. Vincent-type pyroclastic flows. Another
large explosive eruption, about 1,000 ybp, produced pyroclastic
flows that descended the N flank.
Historical eruptions depicted on Aztec codices date back to 1345
AD. About 30 eruptions have been reported since then, although
documentation is poor. Most historical eruptions were apparently
mild-to-moderate Vulcanian steam and ash emissions. Lava flows
restricted to the summit area may also have occurred in historical
time, but cannot be attributed to specific eruptions. Larger
explosive eruptions, possibly Plinian in character, were recorded
in 1519 and possibly 1663. The last significant activity took place
from 1920 to 1922. Then, intermittent explosive eruptions produced
6.6-km-tall columns and a small lava plug extruded onto the floor
of the summit crater. Ash clouds were also reported in 1923-24,
1933, 1942-43, and 1947.
Reference: Boudal, C., and Robin, C., 1989, Volcan Popocatepetl:
Recent eruptive history, and potential hazards and risks in future
eruptions: IAVCEI Proceedings in Volcanology 1, J.H. Latter (Ed.):
Volcanic Hazards, Springer-Verlag Berlin, Heidelberg, p. 110-128.
Information Contacts: Servando de la Cruz-Reyna, Instituto de
Geofisica, UNAM, Ciudad Universitaria, 04510 Mexico DF, Mexico
(Email: sdelacrr@tonatiuh.igeofcu.unam.mx); Claus Siebe, Instituto
de Geofisica, UNAM, Coyoacan, 04510, Mexico DF, Mexico (Email:
csiebe@tonatiuh.igeofcu.unam.mx); Steve McNutt, Alaska Volcano
Observatory, Univ. Alaska Fairbanks, Geophysical Inst., Fairbanks,
AK, 99775-0800 USA (Email: steve@deus.gi.alaska.edu); Melissa
Seymour, LSU Earth Scan Lab, Coastal Studies Institute, 412
Howe-Russell Geoscience Complex, Baton Rouge, LA 70803-7257 USA
(Email: melissa@anatres.csi.lsu.edu); Nick Heffter, National
Oceanic and Atmospheric Administration (NOAA), Air Resources
Laboratory, SSMC3, Room 3151, 1315 East West Hwy., Silver Spring,
MD 20910 USA (Email: nick@arlrisc.ssmc.noaa.gov); Jim Lynch,
Synoptic Analysis Branch, NOAA/NESDIS, Room 401, 5200 Auth Road,
Camp Springs, MD 20746 USA.
Figure 1. Base map of Popocatepetl and vicinity (elevations taken
from the 1986 Mexico City 1:250,000 topographic sheet).
Figure 2. Popocatepetl ash plume at 0745 (1345 GMT) on 27 December
(black) and 0815 (1415 GMT) on 28 December (stipple) as seen on
satellite imagery. The N edge of the longer plume just reached the
Gulf Coast near Tampico. Courtesy of Nick Heffter.
Table 1. Visible (GOES-7 and -8) satellite images reported for
Popocatepetl. The time of initial eruption for all these plumes was
around 1300 (1900 GMT) on 26 December. The third and fifth plumes
listed are shown graphically on figure 2. Courtesy of SAB.
Date Local Time Plume Length; Estimated Height in km
(GMT) Greatest Width (in 1,000's of feet)*
26 Dec 1300 (1900) 50 km; not reported 6.7 km (22,000 ft)
26 Dec 1745 (2345) 50 km E; not reported 6.7 km (22,000 ft)
27 Dec 0745 (1345) 250 km NE; ~75 km 7.6 km (25,000 ft)
27 Dec 1400 (2000) 85 km; not reported 7.0 km (23,000 ft)**
28 Dec 0815 (1415) 160 km; 40 km 6.1 km (20,000 ft)***
* Height of plumes were generally based on SIGMETs from Mexico
City.
** Based on upper air data from Mexico City at 0600 (1200 GMT).
SIGMET ALFA 2 indicated ash cloud 17,000-20,000 ft at 1500
GMT.
*** Previous SIGMETS and weather balloon (radiosonde) data from
Mexico City
Figure 3. Examples of forecasts of the Popocatepetl plume after a
large eruption. Both of these forecasts were for an initial erupted
plume height of 7.6 km (25,000 feet) and an eruption duration of 24
hours. They both portray the elevation range from 6-10 km
(20,000-35,000 feet). Forecasts were based on an eruption beginning
at 1300 (1900 GMT) on 26 December. The map on the left shows the
forecast plume 12 hours after the eruption began, the map on the
right, 24 hours after the eruption began. Courtesy of Nick Heffter.