April 30, 1997

Several colleagues have asked me about the current state of activity at
Popocatepetl Volcano so, I want to make more extensive the information I
have at hand to the volcano community:

Popocatepetl had several explosions last week starting on Thursday (April
24), had several others of minor magnitude during the weekend and on Monday
(April 28).  One of the explosions on Thursday reached >10,000 m (4,000 m
above the summit, the same day, another explosion reaching >8,000 m
occurred during the afternoon. On Friday, April 25, more than 19 explosions
occurred but the column of ashes did not reach high elevations. The
activity during the weekend decreased significantly and on Monday the
seismic activity was at low levels. Last Tuesday (April 29) at 1 AM (local
time) several explosions occurred and were heard by the people living in
the surroundings. Ashes were reported at towns like San Buenaventura
Nealticon (about 20 km from the vent).

Some people from the surroundings recorded images of fire in the forest
using a video camera during the night which were shown by the local TV. The
media  "interpreted" those images as lava flows coming out from the
volcano. Needless to mention the impact of such versions.

Roberto Quaas, Alicia Martinez, Javier Lermo, Teodoro Hernandez, and myself
flew over the volcano on Tuesday to make observations on the state of
activity at the crater. We flew on a helicopter of the Procuraduria General
de la Republica.

Observing the volcano's inside, it was noticed that the lava dome that was
growing since January 1997, was destroyed (by last week's explosions or
yesterday's), no new lava dome was growing inside. The lavas did not show
the shape they had a week before and seemed to be covered by rubble of the
dome itself. During previous similar events, after the explosions,  there
were traces of small pyroclastic flows on the flanks., blocks of the
previous domes and juvenile material was observed around the volcano on the
flanks. This time no juveniles, no large amounts of blocks outside the
crater, no recent (fresh) small pyroclastic flows were observed. Impacts of
blocks on the glacier's surface on the northwestern side are seen though.
Juvenile material and the amount of fragments and blocks seemed not to be
involved in a significant amount during these last explosions. The lava
flows advertised by the media were fires, some of which were present before
the eruption. Some other fires however, were produced by hot blocks
reaching the grass of the uppermost part of the mountain.

SO2 fluxes have been very stable (in the last 2 months the fluxes have been
around 10,000 t/d in average). The first explosions of the last week were
preceded by an increase in SO2 flux 3 days before (up to 16,000  t/d) and a
drop 2 days later (down to 7,000 t/d), however, this has happened before
without any eruption. The following measurements including the one before
the explosions of Tuesday indicate fluxes of nearly 8,000 t/d. There were
no significant variations.

The large fluxes of gas sustained for more than a year, suggest a large
amount of magma that has not showed up yet. However, open vent degassing
has resulted to be very efficient. The last events have been gas bursts
driven by a large amount of gas accumulated beneath the volcano perhaps
trapped by a partially crystallized magma body which has undergone intense
degassification and do not allow the gas to come out from time to time.

Hugo Delgado

Dr. Hugo Delgado Granados Instituto de Geofisica, U.N.A.M. Circuito Cientifico, C.U. 04510, Mexico D.F. Phone: (525) 622-4145 Fax: (525) 550-2486 Internet: hugo@tonatiuh.igeofcu.unam.mx