Ciomadul is the youngest volcanic system in the Carpathian-Pannonian Region recording
eruptive activity from ca. 1Ma to 30ka. Based on combined zircon U-Th and (U-Th)/He
geochronology, Ciomadul volcanism is divided into two main eruptive periods: Old Ciomadul
(1Ma – 300 ka; OCEP) and Young Ciomadul Eruptive Period (160–30ka; YCEP). OCEP activity
comprises Eruptive Epochs 1–3, whereas new ages for eight lava domes and four pyroclastic
units belonging to the YCEP lead to its further subdivision into two eruptive epochs:
Eruptive Epochs 4 and 5. The extrusion of most of the lava domes occurred between
160 and 90 ka (Eruptive Epoch 4) during three eruptive episodes at ca. 155ka, 135
ka and 95 ka (Eruptive Episodes 4/1, 4/ 2 and 4/3, respectively) along a NE-SW lineament,
which is perpendicular to the regional NW-SE trend of the Călimani-Gurghiu-Harghita
volcanic chain. Eruptive Epoch 5 occurred after a ca. 40 kyr of quiescence at ca.
55–30 ka, and is mainly characterized by explosive eruptions with a minor lava dome
building activity. All of the dated pyroclastic outcrops, together with the lava dome
of Piscul Pietros, belong to the older Eruptive Episode 5/1, with an eruption age
of 55–45ka. The eruption centers of Eruptive Epoch 5 are located at the junction of
the conjugated NW-SE and NE-SW lineaments defined by the older eruptive centers. The
whole-rock geochemistry of all studied samples is fairly homogeneous (SiO2=63–69wt%,
K2O=3–4wt%). It also overlaps with the composition of the lava domes of the Old Ciomadul
Eruptive Period, implying a monotonous geochemical characteristic for the past 1 Myr.
The eruption rates for the Ciomadul volcanism were determined based on the erupted
lava dome volume calculations, supplemented with the eruption ages. The activity peaked
during the Eruptive Epoch 4 (160–90 ka), having an eruption rate of 0.1 km3 /kyr.
In comparison, these values are 0.05km3 /kyr for the YCEP (160–30ka) and 0.01 km3
/kyr for the overall Ciomadul volcanism (1Ma–30 ka). Based on the geochemical characteristics,
the quiescence periods and the lifetime of the complex, as well as the relatively
small amount of erupted material, this volcanic system can be placed in a subduction-related
post-collisional geodynamic setting, which shows strong chemical similarities to continental
arc volcanism. The commonly found long repose times between the active phases suggest
that the nature of a volcano cannot be understood solely based on the elapsed time
since the last eruption. Instead, comprehensive geochronology, coupled with the understanding
of the magma storage behavior could be a base of hazard assessment for volcanic fields,
where the last eruptions occurred several 10's of thousand years ago and therefore
they are not considered as potentially active.
