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Looking inside the La Garrotxa volcanoes

A study led by the CSIC has applied the electrical resistivity tomography to see the internal structure of volcanoes in the natural park of La Garrotxa, in the northeast of Spain. The method can offer information about the eruptive dynamics of volcanoes in the past, and help to predict future activity in the same volcanic area. 

The analyzed volcanoes are the Pujalós and the Monsacopa, both in Olot, and the Puig d’Adrí, in Canet d’Adrí. The three of them are monogenetic volcanoes- This type of volcanoes bears from a unique eruption, which can last from a few days to several weeks. Once the eruption finishes, the volcanoes are not active any more.

“These volcanoes have received little attention because their short life and smaller size, contrary to the big polygenetic volcanoes, such as the Teide and the Stromboli, which are seen as more emblematic and scaring”, says volcanologist Stéphanie Barde-Cabusson, who has led the work at the CSIC’s Instituto de Ciencias de la Tierra “Jaume Almera”.

Firsts minutes of an eruption giving birth to a 'parasitic' monogenetic volcano in the summit crater of Piton de la Fournaise (Reunion Island, France) - August 2006. Right: Eruption of a 'parasitic' monogenetic volcano on the Piton de la Fournaise flank (Reunion Island, France) in July 2006. Credit: Stéphanie Barde-Cabusson.

Nevertheless, the short life of monogenetic volcanoes doesn’t mean that they don’t have a complex dynamics and structure, nor that they are not dangerous.  The work, which has been published in Geophysical Research Letters, is one of the first to explore the inside of volcanoes and it is the first one to do it using electrical resistivity tomography as a method.

This technique is based on sending an electrical current throughout the subsoil and to pick it up when it comes back to the surface. “The current intensity is affected when it passes through different rocks, which have different resistivity to the electricity. This allows us to infer which materials form the volcanoes and the structure”, explains Stephanie Barde-Cabusson.

The work has brought about an important technical challenge. A net of 48 electrodes working in pairs (two sending the signal and two collecting it) has been applied to the volcanoes’ surfaces.  The electric signal, after being processed with the specific software, has allowed the scientists to obtain the internal image of every volcano. The experimental data has been complemented with previous studies and compared to the geological evidences from the surface, as the rocks can indicate what type of eruption formed the volcano.

Preparing the electric tomography reels in the field. Right: One of the 48 stainless steel electrodes buried into the soil and connected to the electric tomography cable. Credit: Xavier Bolós

The volcanoes have been explored as far as 100 meters deep, which practically covers their whole structure. Two of the analyzed volcanoes, Puig d’Adrí and Monsacopa, have shown a complex structure with strombolian deposits (related to less dangerous eruptions) and hydromagmatic deposits (related to explosive eruptions), which shows that both volcanoes were formed with these types of eruptions.
On the other hand, scientists have found a different resistivity column in the Puig d'Adri volcano. They think that it is the old eruptive conduit. “As far as we know it is the first time something like this is detected in a geophysical study. It is interesting because it can help to place the original  point of the eruption”, says  Stephanie Barde-Cabusson.

The methodology used can allow scientists to understand and predict the future activity in the same volcanic zone.

The methodology used can offer information about the eruptive dynamic of the volcanoes in the past, and alñlow scientists to understand and predict the future activity in the same volcanic zone. “Monogenetic volcanoes often appear in groups, in volcanic fields several kilometers wide. They can be formed along cracks on the ground: in different points several volcanoes can appear in a short time, with days or years of difference”.
In regard to the volcanic field in Olot, where this work has been done, Barde-Cabusson explains that “these volcanoes are young. They are only 10.000 years old. It is said that a volcanic zone aged less than 10.000 years it is not extinct, it is only dormant”.

The work has counted on the contribution of scientists from the Instituto de Ciencias de la Tierra “Jaume Almera” (CSIC), the Universitat de Barcelona  and from the Universitat Politécnica de Catalunya.


Barde-Cabusson, S., X. Bolós, D. Pedrazzi, R. Lovera, G. Serra, J. Martí, and A. Casas (2013), Electrical resistivity tomography revealing the internal structure of monogenetic volcanoes, Geophys. Res. Lett., 40, doi:10.1002/grl.50538.