by GIBERT Dominique - 18 March 2006
This article is adapted from Electrical Tomography of La Soufrière of Guadeloupe Volcano: Field Experiments, 1D Inversion and Qualitative Interpretation by Florence Nicollin, Dominique Gibert, François Beauducel, Georges Boudon, and Jean-Christophe Komorowski, Earth and Planetary Sciences Letters, Vol. 244, 709-724, 2006.)
Location map of the Soufrière lava dome with morphostructures, manifestations of the hydrothermal system and common sites (see inset legend). Hydrothermally altered and fumarollic areas: (1) Route de la Citerne, (2) Morne Mitan, (3) Forage-Col de l’échelle, (4) Col de l’échelle (Soufleur, Chaudières), (5) Fumerolles du Carbet I, (6) Fumerolles du Carbet II, (7) Fumerolles Collardeau, (8) Napoléon, (9) Cratère Sud and Lacroix-Napoléon fumerolles supérieur, (10) Fumerolles Lacroix inférieur, (11) Fente du Nord, (12) Faujas-Chemin des Dames, (13) Bains Chauds du Galion, (14) Fumerolle de la Matylis. Primary latitude and longitude graduations in 500 meters, WGS84 geodetic system, UTM20N projection). This map must not be reproduced without permission from his author J.-C. Komorowski .
Picture taken from the Volcanological Observatory and showing the lava dome of La Soufrière located within more ancient volcanic structures. From left (i.e. west) to righ (i.e. east): the vertical scarp turned toward the dome is the Nez Cassé (broken nose), the smooth summit located at the basis of the dome is the rim of the Amic crater, the summit located on the eastern side of the dome is the Echelle, and the horizontal plateau visible on the eastern side of the Echelle is La Citerne crater. The summit located in front of the dome is the Tarade block where you can observe a light brown spot which marks the landslides caused by the 2004, 21st november earthquake (see the IPGP webside for more informations)
La Grande Découverte - la Soufrière composite volcano located on the Basse-Terre Island of Guadeloupe is one of the active volcanoes of the recent inner arc in the Lesser Antilles. The IPGP (Observatoire Volcanologique et Sismologique de Guadeloupe) maintains an extensive integrated monitoring network on this volcano located within 5 to 9 km North of the towns of Saint-Claude and Basse-Terre (population 25,000). The Soufrière lava dome (1467 m, highest point of the Lesser Antilles) was formed during the last magmatic eruption of this volcano dated around 1440 AD Vincent 1979,Boudon 1988,Boudon 1989. The lava dome is cut by several radial fractures that were opened during the successive six historical phreatic explosive eruptions of 1690, 1797-98, 1812, 1836-37, 1956, 1976-77 AD. The most substantial eruptions occurred in 1798-98, 1956, and 1976-77. Different fractures or portions of fractures have been newly opened or reactivated during these eruptions, sometimes repeatedly (Figure structures and names dome). Only the central phreatic crater (the Tarissan crater) was active in all eruptions. During the last 1976-1977 crisis the so-called July 8th and August 30th fractures opened in the eastern and south-eastern part of the lava dome, partly reactivating a fracture formed to east during the 1956 eruption.
Rock falls caused by steam blasts during phreatic eruptions:
|General view of the 1976 30th august fault which was opened by a steam blast during a phreatic eruption.||The Fente du Nord, seen from the North. Observe the strong morphological resemblance with the 30th august fault. Althought no historical report exists, it is likely that the Fente du Nord was also created by a steam blast.|
|The Faujas rock fall seen from the west.||The Breislack pit seen from the east.|
The intense hydrothermal activity associated with acidic fumaroles and hot springs that has developed in the last 10,000 years at the periphery and base of the lava dome, below the lava dome, and within the fractures on the lava dome has led to extensive argilization of geological formations enhanced by the about 10 meters of rainfall per year. Historical observations show that the nature, distribution, and intensity of these geothermal manifestations has fluctuated considerably over time Boudon 1988,Barat 1984,Zlotnicki 1992,Komorowski 2005.
Hydrothermal activity around the dome:
|The ancient fumarolic area of the Col de l’Echelle seen from the north-west (noted 4 on the general map above).||The small fumarolic zone located along La Citerne road, at the bottom of the dome (noted 2 on the general map above). Presently, these fumarolles present a very tenious inacty.|
|Left: general view of a scarp located on La Matylis gully, in the Galion canyon (noted 14 on the general map above). Tenious gaz emanations are visible in the lower part of the scarp.|
|The Bains Jaunes hot hydrothermal spring. This area is located south-west and about 200 meters below the base of the dome.||Gilbert Hammouya and Didier Mallarino are sampling water at the hot Galion spring. This area is located south and about 100 meters below the base of the dome (noted GA on the general map above).|
|Gilbert Hammouya and Didier Mallarino are sampling water at the hot Carbet spring. This area is located east and several tens of meters below the base of the dome (noted CE on the general map above).|
Phases of fumarolic reactivation were reported in 1737-1766, 1809-1812, 1879, 1890, 1896, 1899 and 1902-1903. Between the end of the 1976-77 eruption and 1984 there was a phase of progressive decline in fumarolic activity in all areas on the summit (Tarissan, Cratère Sud, Fente du Nord, Cratère 1956), on the flanks (disappearance of the Lacroix fumaroles in 1984) and at the base of the lava dome (disappearance of the Carbet fumaroles in 1979, of the Collardeau fumaroles in 1982, and of the Col de l’échelle fumaroles in 1984). A phase of minimum fumarolic activity occurred between 1984 and 1992, with no fumaroles at the summit and only minor degassing along the SW regional La Ty fracture that intersects the base of the lava dome (fumaroles of the Route de la Citerne and of the Morne Mitan) Zlotnicki 1992,OVSG reports.
A phase of systematic progressive increase in fumarolic degassing with reactivation of summit fumaroles began in 1992 at Cratère Sud Zlotnicki 1992,OVSG reports, continued in 1996-97 at Napoléon Fracture/Crater, and finally involved Tarissan crater in 1997 with an increase since 1999 Komorowski 2005,OVSG reports,Komorowski 2001. Since 1992 the volcano observatory has recorded a systematic and progressive increase in shallow low-energy seismicity, significant development of three acid-sulfate thermal springs at the SW base of the volcano with a slow rise of temperatures Villemant 2005, and most noticeably, a significant increase in summit fumarolic activity associated with -rich and acid gas emanations Komorowski 2005,OVSG reports. Currently there is no significant fumarolic activity at the base of the dome except weak non-pressurized emanations from the stable areas of Morne Mitan and Route de la Citerne.
Activity at the summit of the dome:
|In some rare instances of quiet meteorological conditions with no wind and no clouds, three panaches may be observed at the summit of the dome: the leftmost one corresponds to the Tarissan pit, and the other two come from the north and south parts of the Cratère Sud. These two panaches merge quickly and only their bases are disjoint. Picture taken from the Volcanological Observatory.|
|Two short movies showing the fumarolic activity at the Cratère Sud located on the summit plateau of the lava dome (noted 9 on the general map above). This is the presently most active fumarolic area of the volcano. The movie above shows the south panache of the Cratère sud seen from the South. The movie below shows the panache of the northern part of the Cratère Sud seen from north.|
|Sulfur deposits at the Cratère Sud. Left: view of the main eruptive vent of the southern part of the Cratère Sud. Above: small (about 30 cm) vent located on the eastern side of the Cratère Sud. The sulfur deposits are more important since 2003.|
Numerous geological works were carried on the volcano Boudon 1988,Komorowski 2005,OVSG reports,Komorowski 2001 and a good knowledge of its past history is now obtained. During its construction, la Grande Découverte - la Soufrière volcano has experienced a series of flank-collapse events Boudon 1988,Boudon 1989,Komorowski 2005,Boudon 1984,Komorowski 2002, particularly during its recent stage, making this volcano one of the most unstable of the world. The last one occurred probably at the beginning of the 1440 AD eruption and the lava dome is built inside a small horseshoe-shaped crater opened to the South as the precedent ones. Evidence from the geological record indicates that in the last 15,000 years the frequency of partial edifice collapse has increased although the volume of the collapses has decreased. Several factors suggest that the Soufrière lava dome is locally mechanically weak and thus pre-disposed to flank instability:
1) numerous fractures formed and reactivated during phreatic eruptions,
3) morphological constraints (steep slope),
4) the pervasive extensive hydrothermal alteration of parts of the Soufrière dome,
5) the reactivation of the hydrothermal system involving acid fluids,
6) a ring of thermal springs at the base of the lava dome with a discharge rate that can reach several kilogrammes per second. These springs promote the development of head-ward erosion in concave embayments.
|Left: general view the Tarissan pit seen from La Découverte summit. This pit plays a particular role in the eruptive history of La Soufrière in the sense that it became active at every eruptive episode. After a period of quiescence following the 1976-1977 crisis, the Tarissan pit displays an increasing activity (see Geophysical and Geochemical Measurements in Pit Tarissan). The Napoléon crater, located on the eastern side of the Tarissan pit also display a progressive increase of its fumarollic activity as can be seen on the general view and on the detailled view above.|
Detailed geophysical imaging of the upper part of volcano edifices is of primary importance for the interpretation of data provided by the permanent sensors networks influenced by the heterogeneous character of the medium \citeKagiyama 1999. Self-potential measurements are among the most widely used geophysical observables to monitor the activity of volcanoes Pham 1990,Hashimoto 1995,Zlotnicki 2003,Zlotnicki 2003B,Aizawa 2004, and a correct interpretation of these data needs the knowledge of the electrical conductivity distribution Gibert 2001,Aizawa 2005.
Mechanical and dynamic modelling of the stability of the lava dome and its potential partial collapse requires a better knowledge of its past activity but also of its internal structure. Moreover, obtaining volumetric estimates of the mechanically weak areas susceptible to collapse within the lava dome as well as of the hydrothermal active cells susceptible to participate to a phreatic explosion remain key objectives for any improved hazard and risk analysis concerning the future evolution of the lava dome.
Relevant scientific papers:
P.M. Vincent, N. Vatin-Perignon, M. Semet, J.L. Cheminée, Le dôme de la Soufrière (Guadeloupe), son âge et son mode de mise en place, C. R. Acad. Sci. Paris II 288 (1979) 51-54.
G. Boudon, J. Dagain, M. Semet, D. Westercamp, Notice explicative de la carte géologique à 1/20000 du massif volcanique de La Soufrière (Département de la Guadeloupe, Petites Antilles), BRGM, Paris, 1988, 43 pp.
G. Boudon, M. Semet, P.M. Vincent, The evolution of La Grande Découverte (La Soufrière) volcano, Guadeloupe (F.W.I.), Volc. Hazards, Springer-Verlag, Berlin-Heidelberg, 1989, pp. 86-109.
A. Barat, Etude du rôle des eaux souterraines dans le mécanisme des éruptions phréatiques. Application à la Montagne Pelée de Martinique et à La Soufrière de Guadeloupe, Thèse de spécialité, Univ. Bordeaux 3, 1984, 232 pp.
J. Zlotnicki, G. Boudon, J.L. Le Mouël, The volcanic activity of La Soufrière of Guadeloupe (Lesser Antilles): structural and tectonic implications, J. Volcanol. Geotherm. Res. 49 (1992) 91-104.
J.-C. Komorowski, G. Boudon, M. Semet, F. Beauducel, C. Anténor-Habazac, S. Bazin, G. Hammouya. Guadeloupe. In: J.M. Lindsay, S. Ali, R.E.A. Robertson, J.B. Shepherd & L. John (Eds), Volcanic Atlas of the Lesser Antilles, University of the West Indies, Seismic Research Unit, Trinidad, and IAVCEI (2005) 65-102.
Observatoire Volcanologique et Sismologique de la Guadeloupe (OVSG), IPGP, Bilan Mensuel de l’Activité Volcanique et de la Sismicité régionale de l’Observatoire Volcanologique de la Soufrière, (see OVSG link from the IPGP web site at http://www.ipgp.jussieu.fr) (1999-2005).
J.-C. Komorowski, G. Boudon, C. Antenor-Habazac, G. Hammouya, M. Semet, J. David, J., F. Beauducel, J.-L. Cheminée, M. Feuillard, L’activité éruptive et non-éruptive de la Soufrière de Guadeloupe: problèmes et implications de la phénoménologie et des signaux actuellement enregistrés, INSU Lesser Antilles Volcanic Hazard Workshop, Paris, 18-19 janvier 2001, abstract volume, (2001) 18-21.
B. Villemant, G. Hammouya, A. Michel, M. Semet, J-C. Komorowski, G. Boudon, J.-L. Cheminée, The memory of volcanic waters: shallow magma degassing revealed by halogen monitoring in thermal springs of La Soufrière volcano (Guadeloupe, Lesser Antilles), Earth and Planet. Sci. Lett. (accepted) (2005).
G. Boudon, M. Semet, P.M. Vincent, Flank failure-directed blast eruption at Soufrière, Guadeloupe, French West Indies: A 3,000-yr-old Mt. St. Helens?, Geology 12 (1984) 350-353.
J.-C. Komorowski, G. Boudon, M. Semet, B. Villemant, G. Hammouya, Recurrent flank-collapses at Soufrière of Guadeloupe volcano: implications of acid hydrothermal fluids on edifice stability Mount Pelée 1902-2002; Explosive volcanism in subduction zones, IPGP-INSU-IAVCEI International Congress, Martinique, 12-16 mai 2002, abstract volume p. 69.
T. Kagiyama, H. Utada, T. Yamamoto, Magma ascent beneath Unzen Volcano, SW Japan, deduced from the electrical resistivity structure, J. Volcanol. Geotherm. Res. 89 (1999) 35-42.
V.N. Pham, D. Boyer, G. Boudon, S. Gruszow, J. Zlotnicki, Anomalies de polarisation spontanée sur la Soufrière de Guadeloupe. Relations avec la structure interne du volcan, C. R. Acad. Sci. Paris II 310 (1990) 815-821.
T. Hashimoto, Y. Tanaka, A large self-potential anomaly on Unzen volcano, Shimabara peninsula, Kyusyu island, Japan, Geophys. Res. Lett. 22 (1995) 191-194.
J. Zlotnicki, Y. Nishida, Review on morphological insights of self-potential anomalies on volcanoes, Surveys in Geophys. 24 (2003) 291-338.
J. Zlotnicki, Y. Sasai, P. Yvetot, Y. Nishida, M. Uyeshima, F. Fauquet, H. Utada, H. Takahashi, G. Donnadieu, Resistivity and self-potential changes associated with volcanic activity: The july 8, 2000 Miyake-jima eruption (Japan), Earth and Planet. Sci. Lett. 205 (2003) 139-154.
K. Aizawa, A large self-potential anomaly and its changes on the quiet Mt. Fuji, Japan, Geophys. Res. Lett. 31(5) (2004) L05612, doi:10.1029/2004GL019462.
D. Gibert, M. Pessel, Identification of sources of potential fields with the continuous wavelet transform: Application to self-potential profiles, Geophys. Res. Lett. 28 (2001) 1863-1866.
K. Aizawa, R. Yoshimura, N. Oshiman, K. Yamazaki, T. Uto, Y. Ogawa, S.B. Tank, W. Kanda, S. Sakanaka, Y. Furukawa, T. Hashimoto, M. Uyeshima, T. Ogawa, I. Shiozaki, A.W. Hurst, Hydrothermal system beneath Mt. Fuji volcano inferred from magnetotellurics and electric self-potential, Earth and Planet. Sci. Lett. in the press (2005).