There are indications that some long-dormant or seemingly inactive volcanoes may have potentially active magma storage systems. One such system is Ciomadul volcano, which is located at the south-eastern terminus of the Carpathian volcanic chain (Romania). With the last eruption occurring at ~30 ka, this is the youngest volcano in eastern-central Europe. Understanding the nature and structure of the magma plumbing system is crucial to elucidating the evolution of the volcano and to assessing its hazard potential. This includes the depth, size, and geometry of the magma storage region, the amount and composition of the melt present, and the link between mantle and crustal processes.

Ciomadul is situated in a geodynamically active region about 50 km from the Vrancea zone, where deep earthquakes are frequent. These earthquakes may represent the descent of a dense lithospheric slab beneath a continental collision zone and this may imply an asthenospheric upwelling due to return flow of mantle material. To the north-west of Ciomadul lies a chain of older volcanic complexes, the Călimani–Gurghiu-Harghita volcanic complex; about 40 km west of Ciomadul towards the Transylvanian Basin, a monogenetic basaltic volcanic region was developed at 1.2–0.5 Ma (Perşani volcanic field). Seismic tomography has revealed low-velocity columns through the lithosphere beneath both Ciomadul and Perşani. However, high-resolution images of the complex geometry of the system are lacking.

We report here on a 3-D electrical resistivity model of the region that was generated from 41 magnetotelluric measurements acquired in 2022 that form a 75 km by 75 km array. The data typically had reliable periods from 128 Hz to 4,100+ s. Choosing appropriate locations for measurement was critical, away from sources of cultural electromagnetic noise that can contaminate the signals, as was careful data processing, including applying data pre-selection schemes and manual time windows in addition to standard approaches using robust statistics.

Phase tensor analysis suggests that the data are 3-D at all scales. The 3-D electrical resistivity model reveals conductive anomalies (<10 ohm-m) in the subvolcanic crust. These are interpreted as melt-bearing magma reservoirs distributed in the mid-lower crust (depths of ~10–25 km) and a quasi-vertical conduit extending to the near surface. The crustal reservoir is oriented north-south, has its western margin beneath the surface vent of Ciomadul, and extends ~20 km eastward. These results are consistent with the quantitative petrological models placing the upper melt-bearing silicic crystal mush reservoir at a depth of 5–20 km beneath Ciomadul, and a magma-generation area in the asthenosphere (85–105 km depth). In contrast, no strong conductive anomaly is observed in the crust below Perşani, which fits the magma evolution model, i.e. small batches of mantle-derived magmas ascend rapidly through the crustal column. Our results suggest that Ciomadul, a seemingly inactive volcano, is still underlain by a melt-bearing magma body and therefore can be regarded as having potential for reactivation and further volcanic eruptions. ... Read more

Ciomadul volcano, located at the south-eastern terminus of the Carpathian volcanic arc (Romania), is the youngest volcano in eastern-central Europe, with the last eruption occurring at 32 ka. Petrological and geophysical constraints indicate a melt-bearing silicic crystal mush body beneath Ciomadul, approximately 5–20 km below surface. This suggests that long-dormant or seemingly inactive volcanoes may have potentially active magma storage systems. However, the geometry and size of the magma storage region is unknown. Understanding the nature and structure of the volcanic plumbing system is crucial to understanding the evolution of the system, as well as to assess the hazard potential.

In this presentation, we will report on 41 new magnetotelluric measurements acquired in 2022. The survey design consists of an irregular grid with dimensions of approximately 75 km by 75 km, centered around Ciomadul in the corner of the south-eastern Carpathian Mountains. This makes an array suitable for three-dimensional modelling. A 100 km long NW-SE transect across the array has a measurement spacing of less than 15 km.

The newly acquired data complement previous measurements (recorded in 2010) near Ciomadul and Băile Tuşnad within an area of approximately 5 km by 10 km. The new data extend westward to the Perşani volcanic field (Racoș, Homorod), about 40 km west of Ciomadul, towards the Transylvanian Basin, and to the south-east to the edge of the seismically active Vrancea zone, often attributed to the descent of a slab, more than 50 km south-east of Ciomadul, towards the Focșani Basin. Both regions are targets for future measurement campaigns.

Recordings were carried out at each location for approximately 1–5 days. The data typically had reliable periods of up to 1,000–4,000 s. The data are not homogenous and have characteristics of three-dimensionality. At some locations, cultural electromagnetic noise contaminated the signals and degraded the data; thus choosing appropriate locations for measurement was critical. Estimating local and inter-station transfer functions required special care, such as applying data pre-selection schemes and manual time windows, in addition to standard approaches including using robust statistics and the remote reference method. Overall, the preliminary analysis is encouraging and the results show that it is possible to obtain usable magnetotelluric data in this region. ... Read more