Home page > La Soufrière: field experiments > Geophysical and Geochemical Measurements in Pit Tarissan > The first samplings in the Tarissan pit: a guided tour
by GIBERT Dominique - 11 March 2006
The name of the pit comes from the name of a veterinary of the 19th century whose intrepid curiosity made him to fall in the pit. Details about this person may be found in the book La vieille Dame written by Gérard Werther who narrates the story of La Soufrière.
The Tarissan pit, viewed from the North in the picture on the right taken in May 2002, is an opening located at the center of the summit plateau of the lava dome of La Soufrière. The pit appears as a narrow conduit with a cross-section of about 15 by 8 meters and with an elliptic overture with steep slopes and an area of about 1500 square meters.
The Tarissan pit plays a key role in La Soufrière eruptive activity because it always get active during each phreatic eruption which occurred since the lava dome was created near the end of the 15th century. The last activity occurred during the 1976-1977 eruptive crisis when the pit emitted a large quantity of steam, ashes and rocks of metric size. Afterward, the activity gradually decreased, and, several years later, the Tarissan pit appeared quiet with no steam emission. This calm period allowed the French speleologists Jean-Claude Sallot and Vincent Silve to visit the pit in 1991. An account of this impressive exploration is given by Michel Ferrier in an article published in Spelunca , and Jean-Claude Sallot who descended down to a small lake located about 90 meters under the mouth of the pit estimated that the temperature of water was near 50°C.
The activity of the Tarissan pit renewed in 1999 with tenuous plumes of steam which progressively increased to become today a quasi continuous warm and dense panache.
In November 2002, we decided to explore the pit again. However, the present activity disables any attempt to visit the pit as Sallot and Silve did, and we constructed a sampling device to be dropped in the pit with a rope. The pictures below show Gilbert Hammouya and Alberto Tarchini preparing the sampling nacelle composed of a teflon bottle, a thermistor, and a contact switch which automatically closes when plunged in a liquid. The electric cable measures 150 meters and also serves to support the nacelle.
When all was ready, the whole staff went onto the volcano to proceed to the first sampling operation in the Tarissan pit. Everyone conjectured about what might be found inside. Water or not ? Hot or not ? It is time to go on and to make the last adjustments. Below, on the left, Gilbert and Alberto together with Laurent Mercier proceed to a last connexion. On the right, Gilbert, Alberto and Dominique Gibert reinforce the weak parts of the electrical circuit in order to hopefully prevent any short-circuit in the case where liquid is present in the pit.
Now the sampling nacelle is ready and Alberto guides it during the starting phase while Gilbert fights with the rope. Finally, the nacelle is placed in the middle of the conduit by Florence Nicollin who pulls the rope on the opposite side of the pit, and the descent may begin.
The descent is controlled by Gilbert who stops every 5 meters in order to measure the resistance of the thermistor from which the temperature is deduced. In the upper part of the conduit, the temperature fluctuates around 40 Celsius degrees, and it augments around 50 degrees below. Suddenly, the switch closes, indicating a contact with liquid. After some tests, the contact is confirmed and coincides with a sharp drop of the temperature near 98 Celsius degrees.
After a couple of minutes, the nacelle is pulled up and appears strongly deformed with its metallic parts quite corroded. Also, the teflon bottle is full of a black hot liquid whose pH is immediately measured by Gilbert.
The measurements are made at the laboratory of the volcanological observatory, and the pH = 0.23 mainly due to HCl acid.
This first sampling confirmed that the activity of the Tarissan pit is renewing and that the temperature of the liquid, which was around 50 degrees in 1991, is now the boiling temperature at the altitude of La Soufrière plateau. More sampling will be done during each future electrical tomography experiment , and a cable will be installed in july 2006 to measure the temperature, the pressure and the noise level in the acid lake.
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