Although precise information from VSI (Volcanological Survey of Indonesia)
is still needed, the November 22 eruption may signify an important change
in the location of future magma extrusion and the distribution of related
hazards at Merapi.
Over the past hundred years at least, glowing block avalanches, pyroclastic
flows and lahars associated with extrusion of the lava domes were entirely
confined to the W and SW flanks (last major eruptive phases in 1969, 1976,
1984 and 1992). Now, in 1979 (ref.1) and 1983 (ref.2) attention had been
drawn on the potential risk of new magma outbreak towards the South (from
the Gendol-Woro breach), that would threaten a more densely populated area
including the northern suburb of Yogjakarta city. This concern was based on
the following observations: increase and then persistency of very high
temperatures in the Woro and Gendol fumarolic
fields (implying a close proximity to the top of the magma column, as later
verified from magnetotelluric measurements), appearance of scattered but
new fumaroles on the Woro downslopes in 1979, and above all extreme
fumarolic alteration of this sector of the edifice (weakening its stability
or/and resistance to any magma intrusion).
The present events seem to verify it, south-eastward migration of the
eruption site being facilitated by magma blockage in the SW Batang breach
due to the presence of the huge 1992-94 lava dome.
References:
1. Allard P. & Tazieff H., Phenomenologie et cartographie thermique des
principales zones fumerolliennes du Merapi (Indonesie). C.R. Acad. Sci.
Paris, D-288, 747-750, 1979.
2. Tazieff H., Monitoring and interpretation of activity on Mt. Merapi,
Indonesia, 1977-1980: an example of practical, civil defense, volcanology.
In:
Forecasting Volcanic Events (eds H. Tazieff & J-C Sabroux), Elsevier,
Amsterdam, 485-492, 1983.
Dr Patrick ALLARD Centre des Faibles Radioactivites, CNRS-CEA
91190 Gif/Yvette, France
Tel. 33-1-69088542; fax: 33-1-69087716; e-mail:allard@asterix.saclay.cea.fr Charlie WATSON
In response to the suggestion that the top of the Merapi magma column
may be "closer" to the surface because of increased of temperature and
fumarolic activity, I would put forth another suggestion that the
hydrothermal conduits to the magma chamber may have been facilitated
in some fashion. I really don't see a major case of magma movement
developing from just fumarolic activity. There may indeed be an increased
"hazard" due to the facilitation of these conduits, hence, leading to
an eruption event, but without additional evidence (ie; seismicity,
induced geophysical investigations, etc.), I would be cautious on
reporting movements of the magma.
Among volcanologists there are often moments in which some models are
more in fashon than others.
The suggestion of P. Allard (geochemist) about the presence of a magma
column close to the surface at Merapi volcano, prior to the recent eruption, based on the high temperature of the fumarolic field, is "rejec-
ted" by C. Watson (geophysicist). The latter proposes, if we understood
him, a better thermal efficiency of the fumarolic conduits (he said
hydrothermal cobsuits ...) most likely produced the higher surface
fumarolic temperature.
We feel this is a continuation of the eternal controversy between
geophysicists and geochemists.
Neither of the two researchers point out the actual fumarolic
temperatures before the occurrence of the recent pyroclastic flow. We feel to
avoid any misinterpretation by readers, it is better the temperatures
be reported. Merapi fumaroles, Gendol fumarolic field reached more than
900 degrees C in August (measured by one of us D.T.).
The magnitude of the temperatures leads us to the following conclusions:
(1) It is inappropriate to talk in term of hydrothermal system or
conduits (cf. C. Watson) because there had been no water (liquid) in the
system for some time.
(2) The high measured temperature is evidence that the magma was already
there ... just below the surface.
The temperature was already high sometime before the recent eruption. We
suggest our Indonesian friends belonging to M.V.O. should be consulted
since they have conducted remarkable work on Merapi. They visit the area
one or two times per month, to the dome, to collect fumarolic fluids and
measure temperature.
One more thing; the very high temperature, together with the presence of
very aggressive (acidic) fluids could also be factor which helped
trigger the eruption. A partial collapse of the dome occurred in September,
and continuous destabilization of the whole dome, then sudden
depressurization is clearly possible.
Furthermore, we do not think that high temperatures alone can be used
to argue that the Merapi volcano has shown a south-eastward migration
of activity (P. Allard). We sincerely feel that this suggestion must be
verified by future activity.
We would also like to write a few words on the model proposed by Charlie
Watson. At Vulcano island (southern Italy) the fumarolic vents underwent
a steady increase of temperature from 300 to 700 degrees C, during the
period 1988-1992. During this time no (anomalous) seismicity or ground
deformation were recorded; but some scientists argued that a magma
migration-injection occurred. In this case the idea of better thermal
efficiency of fumarolic conduits (as here proposed by Watson) is highly
possible and indeed proposed by D. Tedsco (JGR, in press). But the two
cases, Merapi and Vulcano island, seem to us to be very different.
I (D.T) think that this kind of forum is very useful and interesting.
However, it seems strange to me that only very rarely do volcano
observatory researchers specifically express their own idea, or
"feelings". Usually discussion is initiated by individuals belonging
to academic institutions or national research laboratories. I am
presently the secretary of WOVO (World Organization of Volcano
Observatories) and would like to push researchers working in the observatories
to participate in this kind of debate and to stress their point of
view. If discussion is not forthcoming the gap (already existing)
between those involved with volcano modeling or volcano surveillance
can only increase, to the detriment of all researchers.
Ciao to everybody
Dario Tedesco and Greg Bignall fttcj147@jpnoucc.Bitnet
Institute for Study of the Earth's Interior
Okayama University
Yamada 827, Misasa, Tottori-ken
682-01 Japan David STEVENSON
With respect to the comments of Patrick Allard, Charles Watson, and Dario
Tedesco, regarding the interpretation of fumarole temperatures at Merapi,
there are at least two quantitative models of fumaroles: Connor et al (1993)
and Stevenson (1993). The major factors that control fumarole temperature
are magma source depth, gas ascent velocity, and the temperature field
surrounding the fumarole conduit. The last factor depends upon the presence
or absence of a hydrothermal system, and the proximity of other gas conduits.
An increase in temperature can indicate magma ascent, a higher gas flux, or
a general heating of the vent surroundings. As with most volcano
monitoring techniques, complimentary data from a different method is needed
to pin down the actual process.
David Stevenson dstevenson@email.meto.govt.uk
Atmospheric Processes Research (Room 153)
Meteorological Office
London Road
Bracknell
Berkshire RG12 2SZ
U.K.
Connor, C.B. et al (1993) Continuous monitoring of high-temperature fumaroles
on an active lava dome, Volcan Colima, Mexico: Evidence of mass flow
variation in response to atmospheric forcing.
J Geophys. Res. 98:19713-19722
Stevenson, D.S. (1993) Physical models of fumarolic flow.
J Volcanol. Geotherm. Res. 57:139-156 (Reprints available from me) Patrick ALLARD
My "short comment" about Merapi eruption has stimulated a
discussion (C. Watson, D. Tedesco and D. Stevenson mails) that can only be
theoretical at present, given our lack of detailed information upon the
actual sequence of events and changes at Merapi summit (bad weather
conditions and volcanic risks having yet prevented new field
investigations). My comment itself was speculative and just intended to
suggest that the present destruction of the southern slopes of Merapi by
pyroclastic flows and glowing surges, for the first time over decades,
might announce a S-SE shift of the summit eruption site, the potential
threat of which had been emphasized in previous works (listed references).
Before going further, we do need more information from our Indonesian
collegues of the VSI (a 28/11 VSI report indicates that the pyroclastic
flows would have resulted from partial collapse of the active lava dome.
Whether a new vent has appeared or not is not known yet).
Nevertheless, I'd like to clear some points of my comment, which
apparently led to misinterpretation:
a. First, I never spoke of magma movement, but actually of "migration of
the eruption site". This is different in the sense that magma is already
intruding and extruding at the summit of Merapi and, thus, can easily erupt
from another vent without substantial magma transfer (and seismic signals)
being required. This is a common way Merapi has been working over the past
centuries (see the abundant Dutch and Indonesian litereature). The problem
is that, because extrusion occurs at the top of the volcano, even a rather
small shift of the vent can strongly modify the flank distribution of
related hazards.
b. High temperatures in Woro and Gendol fumarole fields: I acknowledge the
comment of C. Watson that very high fumarole temperatures are not a
unique indicator of magma approach and may simply be related to increased
aperture of volcanic fractures/conduits or/and lower buffering of magmatic
gases by a hydrothermal system. The examples of Momotombo in the eighties
and Poas in 1979-1980, where fumarole temperatures reached 900 or even
1000C without any magma eruption, are good illustrations. At Merapi,
however, the persistency of very high gas temperatures in Gendol and Woro
areas is closely connected with almost continuous nearby (=BE 100m) magma
extrusion and with shallow lateral extension of the magma column beneath
this sector, as revealed by electrical measurements (M. Halbwachs, in
Forecasting Volcanic Events, Elsevier, figure p. 516, 1983). That means,
magma has been emplaced for many years at subsurface level beneath this
sector. Such conditions have favoured the development of numerous proximal
gas conduits (marked by flames at night) and extensive heating of the
conduits surroundings (important factors, as recalled by D. Stevenson).
This in turn has led to extensive fumarolic alteration of the host rocks in
this sector of the edifice (reduced to powder or crust in many parts),
weakening its mechanical resistance.
c. D. Tedesco mentionned that gas temperature in Gendol "reached more than
900C in August". Such an increase (900C had never been yet recorded at
Gendol) strengthens the idea of an increased gas pressure in the magma
column prior to the eruption (suggested by reported testimonies of a 25 mn
early phase of pressurized gas steaming, preceeding the first pyroclastic
flows; note that this observation would discard a simple gravitational
collapse of the lava dome as the cause of the events). Higher temperature
(i.e. higher gas flow) in Gendol during the pre-eruptive period is
consistent with increasing leak of magmatic gas in this area, facilitated
by blockage of the vent by the huge 1992-1994 lava dome. Such a pattern was
observed before several other eruptions of Merapi: once the lava dome has
become stabilized and voluminous, the lava extrusion rate decreases and
gas pressure progressively builds up, leading to either dome destruction
and/or opening of another vent (e.g. Hartman, 1931-1932; Van Bemmelen,
1949). Finally, note that the high fumarole temperature of 849C reported
by D.Tedesco is equivalent to the highest temperature measured in
pressurized gases sampled directly in contact with the lava dome in 1978
(Allard and Tazieff, 1979; Allard, 1980b).
I think such a discussion has nothing to do with any so-called
"eternal controversy between geophysicists and geochemists" (D. Tedesco),
whereas I entirely share the opinion (also D. Tedesco) that it could
strongly benefit from the contribution of volcano observatory researchers.
Last, I'd like to outline that our IAVCEI Commission on the Chemistry of
Volcanic Gases, whose the Fifth Workshop in Indonesia included gas sampling
of Merapi summit fumaroles (Gendol-Woro) during the last days of July, has
been very lucky: given the very tenuous warning signals of the recent
eruption (reported by a VSI team just working on top), it would have been
more than a delicate situation to be faced with urgent running down of the
quite numerous participants! In that sense, geochemists are particularly
exposed to volcanic risks.
All the best,
Patrick ALLARD Centre des Faibles Radioactivités, CNRS-CEA
91190 Gif/Yvette, France
Tel. 33-1-69088542; fax: 33-1-69087716; e-mail:allard@asterix.saclay.cea.fr Dario TEDESCO & Greg BIGNALL
The comments made by P. Allard, views we share, signal the end to this
interesting discussion on Merapi.
Patrick started, probably unintentionally, a new theme that we feel is very
important. He said that geochemists working on Merapi during the 5th
International Workshop om Volcanic Gases (August) were very lucky. This is
true!! But what should we say about the V.S.I. team, working on the
summit on the same day of the eruption? They were unbelievebly lucky, and
it was only by chance (probably a "miracle", ... Allah is great ...!!)
that they were on the other side of the dome and not close to the two
fumarolic fields at Woro and Gendol. Does this mean that despite the
sophisticated geophysical network, no premonitory signals were recorded? We
suppose so, since the team was clearly in the danger area. Did the team
receive any warning by radio? This is difficult to say, we should wait
for some answers from the Indonesian scientists.
It is true that geochemists, and all scientists, working on active
volcanoes, are exposed to risk. In 1993 (the black year for volcanologists)
three different accidents occurred on active volcanoes resulting in 15
deaths...
In two different occasions, once in the WOVO Newsletter column (no
longer published, due to a shortage of funds) and once at the International
Conference on Volcanoes (May, 1993. Accademia dei Lincei, Rome, Italy)
we have tried to introduce a new discussion, not only on the safety
measures* that scientists must follow when working on active volcanoes,
but more on the possibility of forecasting small scale eruptions
(explosions). A large eruption can usually be forecast, sometime long
enough in advance to dispatch a warning and to avoid casualties. There
is a long list of well documented eruptions which have been forecast.
However the weak point of active volcanology and particularly of
monitoring observatories is that our present knowledge doesn't allow
us to recognize the small scale eruption-explosion, large enough to
kill scientists, tourists or those curious and too close to the volcano
or crater.
Is there any team working on this subject? Is there any news from
obsertory geophysicists, geochemists or others about this kind of activity?
In our opinion, all volcanological conferences should devote time to
this subject. If it is true that small scale explosions (often phreatic
in origin) are largely responsible for most casualties on volcanoes, we
think we should try to understand and to forecast this kind of activity.
Difficult we know, but still ...
We recommend to all observatories and scientists interested in this field
to express their feelings and to make proposals. Possibly, a small scale
conference can be organized. Also IAVCEI, sensitive to the problems of
safety [*a subcommission of IAVCEI (S. Aramaki, F. Barberi and S. McNutt)
last year, produced for the first time, a memorandum for scientists
working on active volcanoes, the first version of which was published in
the WOVO Newsletter n. 4) should participate.
Just to finish, the idea of a program helping to develop observatories,
in Indonesia or elsewhere. The idea to supply observatories with e-mail
system (hardware and software) has been proposed (informally) this year
to the United Nations and to interested scientists. The eruptions at
Merapi dramatically highlights this problem. Any idea to how obtain
sponsors, funds and materials?
Ciao Dario and Greg
Dario Tedesco and Greg Bignall are at ISEI, Okayama University, Japan
e-mail fttcj147 at jpnoucc.Bitnet Graeme WHELLER
Just a small comment to the following paragraph from Dario Tedesco and Greg
Bignall:
" Just to finish, the idea of a program helping to develop observatories,
in Indonesia or elsewhere. The idea to supply observatories with e-mail
system (hardware and software) has been proposed (informally) this year
to the United Nations and to interested scientists. The eruptions at
Merapi dramatically highlights this problem. Any idea to how obtain
sponsors, funds and materials? "
Having conducted volcanological research in Indonesia and worked for two
years with the Fiji Mineral Resources Department (another developing
country), in my view the installation of e-mail facilities should very much
be secondary to the development of a country-wide, well-equipped monitoring
program with trained staff and comprehensive emergency contingency plans.
The VSI has a very large job to do, with over 120 active or fumarolic
volcanoes to deal with, many of them lying within some of the most densely
populated areas on earth. However, in general, their equipment is extremely
basic and very thinly distributed. The level of monitoring of Merapi is
probably an exception in Indonesia, to a large extent because it has
attracted international researchers.
I believe the international volcanological community should be giving
greater emphasis to government-to-government development programs in the
area of volcanic risk mitigation in developing countries. This requires
lobbying by volcanologists in both developed and developing countries to
their respective governments. This is the only way that sufficiently large
amounts of money will be made available for the development of adequate
monitoring programs.
It is therefore distressing to hear that the WOVO Newsletter has died
through lack of funds. Is this indicative of a lack of interest in volcano
monitoring? I doubt it, but if there is to be any chance of increasing
government funding for monitoring programs then publicity is the key, and
resurrecting and expanding the distribution of the WOVO Newsletter seems
like a good starting point....
Graeme Wheller (Dr) Consultant Geologist
Volcanex International Pty Ltd
Post: 521 Tinderbox Rd, Blackmans Bay, TAS, 7052, Australia
Internet: g.wheller@geol.utas.edu.au
Voice/Fax: domestic (002) 298057, international +61 02 298057