(SVG-Bulletin, april 2001)
Pierre-Yves Burgi, Marc Caillet, et Steven Haefeli / Photos: Olivier Grunewald
The Erta' Ale is a shield volcano located in the Afar triangle in Ethiopia. Its Afar name, which means "the smoking mountain", testifies to its quasi-permanent activity. Erta' Ale is an enormous volcanic edifice stretching over 30 km according to an axis parallel to the great rift, and culminates to approximately 700m above the Danakil depression. Because of the extreme climatic conditions of the surrounding desert of Danakil, Erta' Ale has only been visited by few travellers and scientists. The Afars themselves do not approach its summit, believing that spirits of deceased herdsmen encircle it with flying horses.
The first scientific observations date from the end of the years 60s, early 70s, and are due to a Franco-Italian team led by Haroun Tazieff and Giorgio Marinelli. In 1968, there were two lava lakes at the summit, one measuring 100 m in diameter nested in a 160 m deep crater located in the north of the caldera, and a second, smaller (65 m) and less active, situated at the same depth than its neighbor, but at the bottom of a pit located more centrally in the northern lobe of the caldera. Since these first observations, the level of the two lakes has been fluctuating. In 1971 the surface of the lakes was between 10 and 20 m below the floor of the caldera. One year later, the two lakes reached the edge and even overflowed the caldera. The diameter of the central lake was then 80 m. This situation was reported until 1974, and at this time the two lakes had same dimensions. To the exception of a visit in 1976, the activity of the summit of Erta' Ale was not observed any more until November 1992. At this time, the northern lake had disappeared, hidden under a mass of rocks due to the collapse of the edges of the crater. As for the central pit, the level of its lava lake receded again to 100 m below the caldera floor, and its dimensions were 40 x 70 m. Another visit of the site in December 1995 did not reveal any new change. The situation during the visit in February 2001 by the SVG is reported to be similar, with an elliptic lake (80x100 m), 80 m below the rim of the pit, studded with lava fountains rising between 5 to 10 m.
The last thermal measurements collected at the Erta' Ale were those carried out in the seventies by Tazieff's team, while the most recent temperature reported in the literature are those obtained by infra-red measurements using satellites (work mainly carried out by Oppenheimer, Francis and Rothery). The validity of remote sensing remains debatable for several reasons, one of which being the low-spatial resolution of the satellite images, since a point of the image corresponds in the best case to a circle approximately 40 m in diameter. Consequently, measurements on the ground carried out at a distance of a few meters of the lake by using a pyrometer and a thermocouple are difficult to substitute.
The thermal measurements collected by our team between February 13 and 15 2001 are summarized in this document. More details on this field work are available in the article "Field Temperature Measurements at Erta' Ale Lava Lake" (authors P.-Y Burgi, M. Caillet, S. Haefeli) submitted for publication in a journal of volcanology.
© Géo-Découverte
(photo O. Grunewald)
Temperature measurements with the optical pyrometer from the edge of the pit-crater, carried out during the night to avoid pollution by solar radiation. Approximately 25 °C should be added to the values obtained to compensate for the distance. Data recorded on PC (continuous measurements).
© Géo-Découverte
(photo O. Grunewald)
Descent in the pit ...
© Géo-Découverte
(photos O. Grunewald)
Measurements performed from the lake's shores. The two pictures illustrate punctual (at top) and continuous (at bottom) pyrometric acquisitions.
© Géo-Découverte
(photo O. Grunewald)
The steel plate deposited on the lake's crust with a "fishing rod"
Our team used a pyrometer for the temperature measurements. A pyrometer works by remotely measuring the infrared radiations emitted by the lava. Although there is a direct relation (formula of Max Planck) between the quantity of this radiation and the temperature of a black body, which represents an ideal case, there is a factor of proportionality to determine, the emissivity factor, to interpret the temperatures of the objects. For example, for a metal this factor is very small (approximately 0.1 for aluminium), whereas for basalt it ranges from 0.6 to 0.99 (this last number being close to a black body). The first experiment thus consisted in determining this factor. That is possible if one correlates the temperature measured with a thermocouple with that measured with the pyrometer. Obviously, this experiment required an approach of the lava lake. For the temperature measurement of the crust (only accessible part of the lake), the following protocol was followed:
Using a steel wire, a steel sheet of 18 cm by 18 cm (8 mm thickness), containing a hole in which the thermocouple was inserted, was deposited on the crust of the lake. Because of the significant distance separating the terrace from the lake (distance estimated at 15 m), this manipulation required the intervention of three people (Marc Caillet, Steven Haefeli, and Pierre-Yves Burgi). In particular, Marc Caillet, who was standing in a zone where the ambient temperature reached 300°C (and thus was equipped with a reflective cloth), used a large steel pole (8 m length) in order to move away the thermocouple from the wall. Once the steel sheet was in contact with the lake's crust, a temperature measurement was carried out every 30 seconds during the first 10 minutes, then each minute during the 20 following minutes, until the temperature stabilized. The temperature recorded at this time was 350°C. A pyrometric measurement performed in the same area than the location of the thermocouple indicated a temperature of 342°C (with an index of emissivity set to 0.9 on the pyrometer).
By combining the temperature obtained with the thermocouple with that obtained with the pyrometer, and knowing the wavelength used by the pyrometer, it is possible to calculate the factor of emissivity according to a procedure described, for example, in the article of P.-Y Burgi, bulletin SVG January 1998. According to this procedure, we obtained a factor of 0.74. By collecting a sample of basalt, it was possible to confirm this value by the use of a furnace (for a description of the procedure, see the article of S. Haefeli, bulletin SVG October 1997).
© Géo-Découverte
(photos O. Grunewald)
The manipulation proceeded in 3 stages: thermal isolation of the electric wire, deployment of the setting and fishing ...
The specific acquisition of the temperatures at various places of the lake was carried out by pyrometry. Moreover, continuous pyrometric measurements were taken over periods of several tens of minutes (with a measurement each second) and were collected on a portable PC. The crust, the faults (many), as well as the fountains of lava were the three types of areas considered. These measurements were made from the edge of the pit, and from the lower terrace. The measurements made near the lake were of primary importance since the absorption of radiations by magmatic gases between the source and the observer, as well as the enlarging of the field of view of the pyrometer with the distance are two factors which tend to distort pyrometric measurements. A difference of about 25 degrees was actually observed between the maxima measured from the edge and the bottom of the pit. In addition, the temperature measurements were taken at night in order to avoid any pollution due to the solar radiation (which can distort up to 90% the pyrometric values).
The surface of the lake is renewed in ten minutes approximately. Observations made of the lake's activity, particularly the frequency and the duration of the fountains of lava, were carried out by Yves Bessard and Alain De Chambrier, cumulating 14 hours of observation. From these observations, the statistically equivalent number of fountains of lava, which would constantly be activated, has been estimated to approximately 0.8, that is to say the equivalent of a fraction of a fountain.
The principal results of these thermal measurements can be summarized as follows: The highest recorded temperature was found in a lava fountain and amounted to 1217°C. Such a high temperature is characteristic of a basaltic magma, which is consistent with the low gas content observed on the lake. It should be noted that such magma constitutes the majority of the oceanic crust. The temperature of the crust of the lake was very variable, 290°C near the cliffs, 520°C in the centre of the lake, with an average of 474°C. On the basis of all these temperature, the thermal flow due to the radiation was estimated to (approximately) 100 MW.
An active lava lake such as found at the Erta' Ale consists of a volume of magma in convective circulation renewed permanently by a major source of magma. Without such a renewal, the lake would cool quickly and would be solidified within a few months. A lava lake constantly active can be regarded as a window opened on a column of magma, from which geophysical models of the underlying volcanic mechanisms can be derived. Of particular interest is the determination of the thermal flow, which in turn allows to deriving the mass flow necessary to compensate for the heat losses. Estimates of such a quantity of moving magma constrain the models of the volcanic edifice, and eventually help distinguishing the model of intrusions of magma in the edifice from mechanisms involving the recycling of magma in magmatic chambers. Thermal measurements such as those performed by our team are thus of obvious interest for studying this kind of volcano. However, only iterative measurements distributed over many years will make possible to draw a more accurate image of the mechanisms underlying the activity of the Erta'Ale volcano.
© Géo-Découverte
(dessin J. Metzger)
Vu plan (en haut) et coupe (en bas) du pit-crater sud:diamètre env.170m
© Géo-Découverte
(photos O. Grunewald)
Puits d’effondrement (pit-crater), Erta Ale,
février 2001
février 2001