Geoelectrical structure of the central zone of Piton de la Fournaise volcano (Réunion)

Abstract

A study of the geoelectrical structure of the central part of Piton de la Fournaise volcano (Réunion, Indian Ocean) was made using direct current electrical (DC) and transient electromagnetic soundings (TEM). Piton de la Fournaise is a highly active oceanic basaltic shield and has been active for more than half a million years. Joint interpretation of the DC and TEM data allows us to obtain reliable 1D models of the resistivity distribution. The depth of investigation is of the order of 1.5 km but varies with the resistivity pattern encountered at each sounding. Two-dimensional resistivity cross sections were constructed by interpolation between the soundings of the 1D interpreted models. Conductors with resistivities less than 100 ohm-m are present at depth beneath all of the soundings and are located high in the volcanic edifice at elevations between 2000 and 1200 m. The deepest conductor has a resistivity less than 20 ohm-m for soundings located inside the Enclos and less than 60-100 ohm-m for soundings outside the Enclos. From the resistivity distributions, two zones are distinguished: (a) the central zone of the Enclos; and (b) the outer zone beyond the Enclos. Beneath the highly active summit area, the conductor rises to within a few hundred meters of the surface. This bulge coincides with a 2000-mV self-potential anomaly. Low-resistivity zones are inferred to show the presence of a hydrothermal system where alteration by steam and hot water has lowered the resistivity of the rocks. Farther from the summit, but inside the Enclos, the depth to the conductive layers increases to approximately 1 km and is inferred to be a deepening of the hydrothermally altered zone. Outside of the Enclos, the nature of the deep, conductive layers is not established. The observed resistivities suggest the presence of hydrated minerals, which could be found in landslide breccias, in hydrothermally altered zones, or in thick pyroclastic layers. Such formations often create perched water tables. The known occurrence of large eastward-moving landslides in the evolution of Piton de la Fournaise strongly suggests that large volumes of breccias should exist in the interior of the volcano; however, extensive breccia deposits are not observed at the bottom of the deep valleys that incise the volcano to elevations lower than those determined for the top of the conductors. The presence of the center of Piton de la Fournaise beneath the Plaine des Sables area during earlier volcanic stages (ca. 0.5 to 0.150 Ma) may have resulted in broad hydrothermal alteration of this zone. However, this interpretation cannot account for the low resistivities in peripheral zones. It is not presently possible to discriminate between these general interpretations. In addition, the nature of the deep conductors may be different in each zone. Whatever the geologic nature of these conductive layers, their presence indicates a major change of lithology at depth, unexpected for a shield volcano such as Piton de la Fournaise.

Electric potential anomaly induced by humid air convection within Piton de La Fournaise volcano, La Réunion Island

“…nearly 400 m under the actual bottom of Dolomieu collapse. Such a thickness is compatible with the high-resistivity (8-100 k ) layers found by Lénat et al (2000), which may correspond to unsaturated porous basalts. Considering a 200 mW m −2 heat flux for PdF volcano, corresponding to a temperature contrast of 70 • C across the unsaturated layer, an equivalent Rayleigh number of 300 is derived for air convection.…”

“…The central part of the cavity is filled with a material with a resistivity greater than 10 5 m whereas the flanks have a resistivity as low as 1.2 × 10 4 m. This supports the idea that the flanks act as barriers for the fluid flow. The resistivity of a water saturated basaltic soil is about 10 3 m (Lénat et al, 2000). Thus, given the high resistivities of the soil of Formica Leo, only a partial filling of the porosity with water is conceivable.…”

Hydrogeological settings of a volcanic island (San Cristóbal, Galapagos) from joint interpretation of airborne electromagnetics and geomorphological observations

“…electrical, electromagnetic, and audiomagnetotelluric methods (Aizawa et al, 2009;Descloitres et al, 2000;Gómez-Ortiz et al, 2007;Lénat et al, 2000;Lienert, 1991;MacNeil et al, 2007;Revil et al, 2004). Yet, the development of landbased surveys is often long and costly over large, inaccessible areas.…”

Lead isotopes behavior in the fumarolic environment of the Piton de la Fournaise volcano (Réunion Island)

“…The lack of high-temperature springs on the flank of the volcano suggests little interaction between the hydrologic and the magmatic systems, perhaps because of the existence of an impervious shell around the shallow magma chamber (Violette et al, 1997). Hydrothermal activity seems to be confined to the summit crater, under which a low-resistivity zone was identified (Lénat et al, 2000). Lower hydrothermal activity has also been evidenced with self-potential technique along the North-South rift zone crossing the summit area (Barde-Cabusson et al, 2012).…”