The breath of Erta Ale, Ethiopian volcano

Paris, April 3, 2003

From March 18 to 31, 2003, a team of fourteen scientists(*) set up geophysical instruments around the Ethiopian volcano Erta Ale, located in the Danakil rift, where a chain of volcanoes has formed over millions of years. The scientists took samples of gases and lava. This is the first time since 1974 that a French scientific mission financed by the CNRS has been able to return to study this region of the Afar triangle.

Erta Ale is an exceptional volcano in more ways than one. It is one of three volcanoes in the world that has had a continuously active lava lake, probably since at least 1873. In addition, Erta Ale is part of a chain of active volcanoes located in the center of one of the six Afar rifts where an open ocean has formed in connection with a hot spot (inner Earth volcanism). The mission was organized in the framework of the "The Earth's Interior" Program and the "National program for major natural risks" of the CNRS National Institute for Sciences of the Universe. It was organized by laboratories belonging to the "Institut de physique du globe" in Paris (IPGP), in association with the CNRS, with the logistic support of Géo-découverte.

The initial objectives of the mission included understanding how a lava lake works: the structure of the volcano's feeder conduits under the lake and the processes involved in eruption. Judging by the lake's size, 80 meters in diameter in 2001, its depth is estimated to be at least 100 meters. Given its age, scientists think the lake must have a continuous link to a shallow magma chamber, which is itself fed on a regular basis by magma uplifts more directly connected to the formation of the rift. The activity of the lava lake is characterized by several points of gas discharge that release bubbles and by what appears to be rather stable convection. The spots where the lava uplifts correspond to the location of gas release. The study of convection in the lava lake (the number of conduits or rising feeding currents, the depth at which bubbles appear, etc.) is a key parameter in the dynamics of lava lakes. During the mission, the scientists observed that the level of the lake had gone down 10 meters since compared to the level recorded in 2001.

On March 18, the team of scientists set up its equipment along the edges of the volcano and made a first helicopter flight to photograph the small rift that was forming along the axis along which Erta Ale formed. A second flight enabled team members to photograph the relief from a different angle. These images will be used to create a digital elevation model, i.e., a highly accurate three-dimensional relief map.

During the mission, the scientists took continuous recordings of geophysical measurements for eight days. A network of temporary seismological stations was set up around the crater to obtain an image of the deep structure of the volcano under the lake, and to locate the position, shape and size of what is probably the magma chamber. It recorded the ground vibrations caused by volcanic activity. This technique is based on the same principle as ultrasound, except that the emission of seismic waves recorded by seismologists is natural. The network was composed of four so-called large-band stations (which register seismic waves in a wide range of frequencies) located in a radius near the lava lake and of five short-term stations (which record only high frequencies) placed in a radius of two kilometers. This system should make it possible to obtain an image of the structure of the volcano under the lake, reaching a depth of around two kilometers.

In order to hear the "breathing" of the volcano, a set of microphones and micro-barometers were arranged around the edge of the crater to study the way the gas bubbles are released, and thus determine the depth and shape of the conduit that likely connects the lake with the magma chamber. Lastly, during the entire mission two cameras (one infrared and one digital) took pictures in order to study the means of convection inside the lake. The scientists observed magma surfaces at relatively stable locations, which indicate that rising currents are associated with the gas bubbles. The lava then flows to the surface and descends in what are also rather stable locations. GPS readings were also taken around the crater to monitor the deformations over time was also performed.

In addition to setting up instruments for taking geophysical measurements, samples of gas and lava were also obtained. Ongoing fumarolian activity occurs in the volcano. Scientists took samples of gases in the lava lake and at a fumarole site. Some of the analyses were carried out while on-site (for example, SO2 content) using a spectrometer, while other analyses will be made in the laboratory. The gas samples were sealed in glass vials. Samples of lava projections due to the formation of "pit craters" and/or caldeira were also taken to determine their distribution and composition.

The surface map developed using aerial photographs will enable the scientists to create a digital elevation model and conduct an in-depth study of the small rift in formation. Seismological data of the deep structure of the eruptive conduits and the underlying longitudinal magma chamber should make it possible to analyze the interactions between the tectonic activity and the magma activity near the volcano.

(*) In particular: