Date:         Wed, 28 Dec 1994 16:26:57 MST
From: Rick Wunderman-Global Volcanism Network 
Subject:      Popocatepetl

                     Popocatepetl central Mexico (19.02xN, 98.62xW)
              summit elev. 5,465 m
        All times are local (= GMT - 6 hours))

1. Field Report. Claus Siebe reported that climbers at Popocatepetl
reached the summit both on the day before the eruption, and not long
after the 21 December eruption. On the day before the eruption the
climbers could see the crater lake and sparse fumaroles. They reportedly
heard no hissing sounds and smelled less odor from sulphur-bearing gases
than in previous months.
     Six volcanic earthquakes took place between 0130 and 0200 on 21
December, events probably associated with phreatic summit explosions.
These events were neither heard nor felt by any of about 25 mountain
climbers at Tlamacaz, 4 km N of the summit. The climbers, who said they
started ascending the mountain around 0400, did not notice anything
unusual until at the crater rim around 0800. Minutes before reaching the
rim 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 impacts
in the snow. Some climbers who had never visited Popocatepetl before
reached the summit unaware of any abnormal activity. At the summit, the
climbers said they could not see the crater floor even though a strong
wind was blowing. They descended back down the mountain without
incident.
     Claus Siebe was at Tlamacaz 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 (45
km ENE) to produce a thin coating on cars. The material produced so far
appeared non-juvenile, and it contained pyrite, sulphur and Ca-sulfate
recognizable in coatings and vein fillings.

2. Plume Imagery. On 26 and 27 December the Synoptic Analysis Branch
reported plumes seen in the visible but not in the infrared wavelengths.
A new eruption took place on 26 December at around 1300. Based on the
latest significant meteorological (SIGMET) advisory from Mexico City at
the time, this plume reached an altitude of about 6.7 km (22,000 ft). A
report later that day indicated that the volcano had continued to erupt
and created a plume that at 1745 reached 50 km in length, trending E.
The next day a GOES-8 visible satellite image of the plume at 0745
suggested a gently curving, funnel-shaped mass tracking NE. The plume
just touched the Gulf Coast near Tampico. Based on its lack of infrared
signature and on the visible signature, the plume was thought to be of
low density.
     Trajectory forecasts generally showed the plume engulfing the E
half of the volcano and then traveling NE. After about 24 hours, these
forecasts projected the plume traveling NE and stretching over a
significant part of the Gulf of Mexico. The forecasts indicated that it
would later reach the Mississippi delta region. About two days after the
eruption, the forecast edge of the plume would cross the SE part of the
USA and reach the E coast of Florida and the Atlantic. The forecasts
were based on initial volcanological assumptions that the plume reached
~20,000 ft altitude and that eruptions were sustained continuously for
24 hours.