The pre-ignimbrite (phreato) plinian and phreatomagmatic phases of the Akdag-Zelve ignimbrite eruption in Central Anatolia, Turkey

The eruptive sequence of the 7.6-Ma-old Akdag-Zelve ignimbrite (AZI) eruption comprises five units totalling up to > 50 m on Akdag mountain. They are, in stratigraphic order: (a) the main pumice fall deposit (up to 10 m); (b) the lower surge series (up to 4 m); (c) the upper pumice beds (up to 0.8 m); (d) the upper surge series (up to 5 m); and (e) the Akdag-Zelve ignimbrite (up to 40 m). The main pumice fall deposit shows a second distal thickness maximum about 20 km from the source, whereas the upper pumice beds form a simple NNE-extending fan. The source area, about 6 km north-northeast of Kaymakli, is inferred from fallout thickness, size and amount of lithic clasts, depositional facies and flow directions of the surge deposits. The main pumice fall deposit encloses accretionary lapilli up to 6 cm in size indicating a steady phreatomagmatic influence on the eruptive style from the very beginning. The major surge sequence beneath the ignimbrite is similar to that of the 1400 yr B.C. Minoan eruption on Santorini, Greece, and resembles the Minoan B stratigraphic unit attributed to phreatomagmatic eruptions due to ingression of seawater into the conduit. Similarly, the depositional environment of marshes and marginal flats of a lacustrine basin in Cappadocia accounts for the external water supply and the generation of surges during the AZI eruption. Estimates of eruption column height of ≥ 45 km indicate that the AZI eruption ranks in the order of the 1956 Bezymianny, 1815 Tambora and the Waimihia Plinian pumice eruption of New Zealand. Volume estimates of the erupted tephra are on the order of 41.5 km3 for the main pumice and 0.35 km3 of the upper pumice.