Extinct or asleep volcanoes: hints from Ciomadul (Romania)

Mickael Laumonier
Laboratoire Magmas et Volcans, University of Clermont Ferrand (France)

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Volcanic eruptions are unpredictable. A volcano is considered as “extinct” after a repose-time longer than 10 thousand years. However, we know that some volcanoes have extremely long unrest periods (e.g. Yellowstone in USA, Cerro Utunruncu in Bolivia), posing difficult risk assessment because how they endure and stir back to “life” is poorly constrained.

Ciomadul (Romania) belongs to this class of supposedly extinct volcanoes with no eruption in the last 32 thousand years. Its last activity led to the building of a dacitic lava dome complex alternated with explosive phases (Fig. A). However, several lines of evidence suggest a potentially active magma storage beneath the volcano. For instance, significant CO₂ emanations with mantle origin, or geophysical anomalies revealed by magnetotelluric surveys. Therefore, volcano status should be based on the characterization of the plumbing system rather than the absence of activity for an arbitrary duration.

To constrain Ciomadul’s status and better understand the class of seemingly inactive volcanoes, I gathered a team of researchers with complementary skills to tackle the problem with a multidisciplinary approach (Fig. B). First, we studied the petrology of Ciomadul’s erupted products through thermobarometry and hygrometry to provide an estimation of the storage conditions (Temperature, Pressure, H₂O content). We found several amphibole populations allowing us to recognize both deep and shallow storage zones and pre-eruptive conditions: we attribute temperatures >900°C to deep storage conditions, oftentimes encountered in amphiboles’ core while a temperature of 715°C (+/- 20) reflects shallow and colder storage conditions (Fig. B-1). Almost all amphiboles have a rim characterized by a temperature of ~ 815°C (+/- 20) interpreted as pre-eruptive conditions. These constraints were used in numerical simulations to model the thermal evolution of a potential magma reservoir beneath Ciomadul. The results are surprising: despite the modest size of the potential reservoir (based on the geophysical anomaly), temperatures are still relatively warm making the existence of melt (liquid) realistic, around 15% in average and up to 40% locally (Fig. B-2). Then, we tested such predictions by measuring in situ (at magmatic conditions) the electrical conductivity of the molten dacite erupted at Ciomadul with various water content. The modeling of the experimental results was used to explain the value of the electrical anomaly observed during geophysical survey: we need between 20 and 58% of hydrous melt with a dacitic to rhyolitic composition in the reservoir, validating numerical predictions (Fig. B 3-4).

Significant amounts of melt beneath Ciomadul do not mean that an eruption will necessarily happen. All we can say today is that melt-bearing reservoirs likely exist for a protracted time under active but also supposedly extinct volcanoes. The active vs. inactive status of a volcano must be defined from the persistence of a magmatic reservoir, and more attention should be directed towards seemingly inactive volcanoes as well. All details can be found in the article of Mickael Laumonier: Evidence for a persistent magma reservoir with large melt content beneath an apparently extinct volcano, which recently came out in Earth Planetary Science Letters.