Introduction

Whether a volcano is considered ‘extinct’ or is merely experiencing a prolonged period of inactivity is the key question volcanologists often contended with. In the context of risk assessment, this question is reframed to assess the renewal of volcanic activity or if such potential occurrence is extremely low (IAEA 2016). This comprehension relies on understanding the history of the tectonic and magmatic processes, especially for volcanic systems that lack historical or Holocene activity, as the eruptive record in such cases is often limited, resulting in high uncertainty in hazard estimates. Absence of Holocene activity, however, does not strongly support the argument for a negligible potential of future volcanic activity (IAEA 2016; Aspinall et al. 2023). For example, intervals of inactivity lasting tens of thousands of years or even longer have been documented for the Tequila volcano in Mexico (Lewis-Kennedi et al. 2005), and the Ciomadul Volcano in Carpathians (Harangi et al. 2015), subsequently succeeded by active periods (IAEA 2016).

Here, we focus on the Hasandağ volcano, one of the most active and dangerous volcanoes in the Central Anatolian Volcanic Province (CAVP; Fig. 1a, b) since the middle Pleistocene (e.g., Friedrichs et al. 2020a). The volcano is currently experiencing an increase in local seismicity with an ongoing fumarolic activity over the last years (Ulusoy et al. 2021), thus posing a threat for future eruptive activity. This unrest holds particular significance when considering the mid-to-upper crustal low-seismic velocity zones, which could suggest the presence of potentially eruptible magma beneath the volcano (e.g., Abgarmi et al. 2017; Friedrichs et al. 2021; Tank and Karaş, 2020).

Fig. 1
figure 1

a Geographical and structural map of the Anatolian microplate and adjacent regions, showing the main structural features, continental blocks (i.e., Pontides, Kirshehir, and Tauride) and location of the Central Anatolian Volcanic Province (CAVP; red frame in the overview map). Large black arrows indicate the relative motion of the tectonic plates (from Reilinger et al. 2006). Small black arrows indicate the direction of major strike-slip fault systems (NAFZ: North Anatolian Fault Zone; EAFZ: East Anatolian Fault Zone). b Map of the CAVP showing the major volcanic centres (red triangles) and tectonic lineaments (black lines), including volcanoes within the Niğde Volcanic Complex (NVC), such as Hasandağ, Keçiboyduran, and Melendiz (west to east). c Digital elevation model (DEM) of the study area showing the location of the Belbashani Pumice exposures (white stars; see Table 1 for details). GH, Greater Hasandağ; LH, Lesser Hasandağ (red triangles). Dark blue solid line between Keçiboyduran and Melendiz volcanoes denotes the Altunhisar-Çiftlik road. Black solid line delineates the Tuz Gölü Fault Zone. Black circles indicate the towns. White circle corresponds to the location of the Pumice Fallout dated by Schmitt et al. (2014)

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The Greater (3255 m) and Lesser (3069 m) Hasandağ double-peak stratovolcano (red triangles in Fig. 1c) produced multiple large explosive eruptions during the last million years, including several widespread caldera-forming ignimbrites (Aydar and Gourgaud 1998), pumice and ash falls (Kuzucuoğlu et al. 2020), lavas, and block and ash flows (Friedrichs et al. 2020a). Although there is no historical record of volcanic eruptions, there are circumstantial lines of evidence that Hasandağ has erupted explosively during the Holocene. Schmitt et al. (2014) dated a pumice fall deposit sampled close to the Greater Hasandağ crater (Fig. 1c) at 8.97 ± 0.6 ka by (U–Th)/He double zircon dating. These authors speculate that the explosive eruption associated with this deposit may be the same event depicted in a mural found in the Neolithic settlement of Çatalhöyük (Mellaart 1967), with a 14C age of 6430 to 6790 BCE (or 8430–8790 ka; Schmitt et al. 2014).

A much larger pumice fallout was deposited on the surrounding areas of Hasandağ and widespread through the CAVP during the middle Pleistocene. This thick pyroclastic deposit was first reported by Aydar and Gourgaud (1998) and later by Kuzucuoğlu et al. (2020) along the Belbaşhanı path, a topographic high that connects Çömlekçi and Kitreli basins (Fig. 1c). This natural corridor runs ~ north–south between the Quaternary stratovolcanoes of Keçiboyduran and Hasandağ (Fig. 1c). Aydar and Gourgaud (1998) and Kuzucuoğlu et al. (2020) described a clast-supported deposit with abundant large pumice blocks (up to 30 cm) rich in biotite and classified it as the proximal facies of a pumice fallout. Aydın et al. (2014) dated this fallout deposit at ~ 339 ± 50 ka using the U/Pb method. However, no detailed volcanological studies have been conducted to reconstruct the eruption, henceforth referred to as the Belbaşhanı Pumice.

By integrating different volcanological and petrological methodologies (i.e., field observations, tephrostratigraphic correlations, U–(Th)–Pb and 40Ar/39Ar geochronology, glass geochemistry, pumice morphology, and isopach and isopleth maps), we characterize the Belbaşhanı Pumice fallout deposit and quantify the associated eruption source parameters, such as the erupted volume and column height along with the associated uncertainties. These results contribute to improve our knowledge on the volcanic history of Hasandağ volcano, also supporting future studies on volcanic hazard assessment in Central Anatolia and surrounding regions.

Geological setting of Anatolia and Hasandağ volcano

The Anatolian block, which is part of the Cenozoic Alpine-Himalayan orogenic belt, emerged by the collision and accretion of different continental fragments derived from the northern part of Gondwana (e.g., the Sakarya and Kırşehir microplates; Şengör and Yılmaz 1981; Abgarmi et al. 2017). This tectonic process, which terminated to define a threefold subdivision of orogens grouped from north to south into the Pontides, Anatolides, and Taurides (e.g., Innocenti et al. 1975; Pasquarè et al. 1988; Göncüoğlu and Toprak 1992; Fig. 1a), resulted from prolonged convergence of the current Eurasian and Afro-Arabian plates (e.g., Dewey et al. 1986; Şengör et al. 2008). The plate convergence began in the mid-Cretaceous at around 100 Ma (e.g., Yazgan and Chessex 1991) and gradually provoked the Neo-Tethys closure, being also accompanied by the consumption of its ocean plate by subduction (e.g., Piper et al. 2013). Since the Late Miocene, this process was associated with the formation of the Central Anatolian Volcanic Province (CAVP) (Pasquare et al. 1988).

The CAVP is a ~ 300-km-long and ~ 40 to 100-km-wide northeast-southwest-oriented, post-orogenic volcanic arc (e.g., Aydın et al. 2014) related to the current subduction of the oceanic African Plate beneath the Anatolian Block (e.g., Pasquare et al. 1988). Currently, the CAVP is being shifted westwards at rates of 20 mm/year (geodetic data of Jiménez-Munt et al. 2003) by the collision of the Arabian and Eurasian plates in the eastern part of Anatolia. This crustal deformation has led to the development of numerous large-scale faults and uplift of the Anatolian plateau (Toprak 1998), forming the Tuz Gölü–Ecemiş fault system (Dirik 2001), located between the conjugate Tuz Gölü (west) and the Ecemiş (east) faults (Fig. 1b; Toprak and Göncüoglu 1993). The complex tectonic setting within the CAVP has influenced the volcanic activity and vent distribution since the Miocene (Toprak 1998), leading to the formation of numerous monogenetic volcanic fields, stratovolcanoes, and calderas, including the Cappadocian Ignimbrites (~ 13.5 Ma, e.g., Innocenti et al. 1975) and the Hasandağ volcano (Aydar and Gourgaud 1998).

Aydar and Gourgaud (1998) divided the Hasandağ eruptive succession into three stages: (1) Paleohasandağ, (2) Mesohasandağ, and (3) Neohasandağ. Each stage was characterized by several explosive and effusive eruptions that produced widespread caldera-forming ignimbrites, pumice and ash falls, domes, lavas, and block and ash flows (Aydar and Gourgaud 1998; Kuzucuoğlu et al. 2020; Friedrichs et al. 2020a). The northern region of the volcano (i.e., Helvadere area; Fig. 1c) is covered by a debris-avalanche deposit forming a hummocky terrain between the Hasandağ volcano and Ihlara river (Fig. 1c; Kuzucuoğlu et al. 2020; Ulusoy et al. 2021). Parts of this avalanche deposit are distributed within the Kitreli basin, which lies towards the northeast of Hasandağ and spans the territory between the towns of Yenipınar, Ihlara, and Kitreli (Fig. 1c). The avalanche deposit is not present within the Çömlekçi basin that is located towards the southeast of Hasandağ. These two basins (Kitreli and Çömlekçi) are connected by the Belbaşhanı path, which is a topographic high between the Quaternary Hasandağ and Plio-Pleistocene Keçiboyduran volcanoes (Fig. 1c). Due to the prevalent elevated topography (Fig. 1c), this geographic domain poses challenges in conserving loose materials, particularly pyroclastic fall deposits. The rugged terrain and heightened altitude in the area are factors contributing to ongoing erosion processes, thereby leading to the gradual remobilization of such unconsolidated materials.

Materials and methods

Fieldwork

The fieldwork was guided by published geological maps, stratigraphic information, and deposit descriptions reported in Aydar and Gourgaud (1998) and Kuzucuoğlu et al. (2020). A preliminary remote sensing analysis of the area was conducted using satellite images and a Digital Elevation Model (DEM) with a resolution of 30 m (Copernicus Global DEM; Coord.Sys. UTM, datum WGS84; European Space Agency- Sinergise 2021) to identify available outcrops (Table 1).

Table 1 Details of the Belbashanı Pumice in each studied outcrop, including samples, log number, geographic coordinates, thickness, juvenile, and lithic maximum clast sizes (WGS84 coordinate system). For pumice and lithic materials, three dimensions were measured for the five largest clasts in each outcrop and their mean was calculated
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The fieldwork involved three different campaigns (November 2021, June 2022, and April–May 2023) aimed at characterizing the Belbaşhanı Pumice fallout deposit. Field observations were carried out in exposures throughout the plateau and valleys between Hasandağ and Keçiboyduran volcanoes (from north to south, Kitreli basin, Belbaşhanı path, and Çömlekçi basin), along the road Altunhisar-Çiftlik that runs between Keçiboyduran and Melendiz volcanoes, and around the towns of Ihlara and Güzelyurt (Fig. 1c). Fresh, lapilli-sized pumices were sampled for glass chemical and geochronological investigations. Grading, colour, and sorting of the Belbaşhanı Pumice were described and integrated into a total of 18 detailed and representative stratigraphic columns (Fig. 2) according to the nomenclature reported in Branney and Kokelaar (2002). Data collected in the field were used to reconstruct isopach and isopleth maps.

Fig. 2
figure 2

Stratigraphic columns logged in the studied outcrops. Orange area indicates the Belbaşhanı Pumice deposit. The inset map shows the location of the stratigraphic logs. BNE base not exposed

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Glass chemistry

Pumice clasts were cleaned, crushed, washed, dried, and sieved at the HP-HT Laboratory of Experimental Volcanology and Geophysics (HP-HT Lab) of the Istituto Nazionale di Geofisica e Vulcanologia (INGV) in Rome (Italy). Glass shards larger than 63 μm were mounted in epoxy resin and polished for major and trace element microanalyses.

Major element analyses were carried out on 30 different glass shards per sample using an electron probe micro-analyser (EPMA) JEOL JXA-8200 equipped with five wavelength dispersive spectrometers (WDS) and installed at the HP-HT Lab. Analyses were performed under high vacuum conditions, using an accelerating voltage of 15 kV and an electron beam current of 7.5 nA, with a defocused beam diameter of 10 μm. Elemental counting times were 10 s on the peak and 5 s on each of the two background positions. Corrections for inter-elemental effects were made using a ZAF (Z: atomic number; A: absorption; F: fluorescence) procedure. Calibration used a range of standards from Micro-Analysis Consultants (MAC; https://www.macstanda rds.co.uk): albite (Si-PET, Al-TAP, Na-TAP), forsterite (Mg-TAP), augite (Fe-LIF), apatite (Ca-PET and P-TAP), orthoclase (K-PET), rutile (Ti-PET), and rhodonite (Mn-LIF). Sodium and potassium were analysed first to prevent alkali migration effects. VG-568 (rhyolite) from Smithsonian microbeam standards (Jarosewich 2002) was used as quality monitor and for the calculation of accuracy and precision. Based on counting statistics, accuracy was better than 1–5%, except for elements with abundances below 1 wt%, for which accuracy was better than 5–10%. Precision was typically better than 1–5% for all analysed elements.

Trace element analyses were conducted at the Institute of Geochemistry and Petrology of the ETH Zürich (Switzerland) with a 193 nm excimer laser coupled with a second generation two-volume constant geometry ablation cell (Resonetics S-155LR) and a high-sensitivity, sector-field laser ablation-inductively coupled plasma mass spectrometer (SF-LA-ICP-MS; Thermo-Element XR). Points with a spot size of 20 μm were set on large glass shards previously analysed by EPMA and ablated for 30 s with a pulse rate of 5 Hz and an energy density of ca. 3.5 J/cm3. The synthetic glass NIST SRM610 (Jochum et al. 2011) was used as the primary reference material for trace elements quantification and instrumental drift correction; the analytical reproducibility was checked by repeated measurements of the GSD-1G (Guillong et al. 2005) and BCR-2G (Raczek et al. 2001) glass reference materials. Raw data were reduced using Iolite 4 (Woodhead et al. 2007; Paton et al. 2011) with 29Si used as internal standard (Supplementary Material 1). Precision of individual analyses varied depending upon a number of factors such as the element and isotope analysed as well as the homogeneity of the ablated material. The 1σ errors calculated from variations in replicate analyses were invariably several times larger than the fully integrated 1σ errors determined from counting statistics alone.

Pumice microtextural descriptions

Pumice clasts of sizes between 250 and 500 µm were cleaned under acetone to remove organic and adhering material and using an ultrasonic bath for no longer than 3–5 min to preserve the original morphological properties of the glass (e.g., Heiken and Wohletz 1985; Ersoy et al. 2006; Liu et al. 2015). The samples were dried overnight in an oven at ~ 60 °C and mounted in stainless-steel stubs using double-sided stick tape. After gold-coating, samples images were acquired using a scanning electron microscope (SEM) at Hacettepe University (Ankara, Turkey), operating at secondary electron mode (SE) with 7-keV accelerating voltage.

Geochronological analyses

Lithic-free pumices were crushed to concentrate juvenile minerals for radiogenic dating: zircon crystals for U–(Th)–Pb, and plagioclase and hornblende crystals for 40Ar/39Ar. Powdered pumices were sieved and separated into different fraction sizes. Grains ranging from 250 to 500 μm and 125 to 250 μm were used to concentrate both minerals of plagioclase and hornblende, whilst grains ranging from 63 to 125 μm and 38 to 63 μm were used to concentrate zircon crystals.

U–(Th)–Pb dating

The smallest grain size fractions (i.e., 63–125 and 38–63 μm) were washed and batted using a gold pan to separate the heavy crystals (e.g., zircon) from lighter components. Zircon crystals were separated from other denser minerals using a Frantz Isodynamic Separator at Hacettepe University (Ankara, Turkey). Subsequently, the adhesive matrix glass was dissolved using a cold 40% HF solution. Representative zircons were handpicked under a binocular microscope. Following BSE imaging, zircon crystals were dated with U–(Th)–Pb systematics at the ETH Zürich using a 193 nm excimer laser coupled with a second generation two-volume constant geometry ablation cell (Resonetics S-155LR) and a high-sensitivity SF-LA-ICP-MS (Thermo-Element XR; Guillong et al. 2014). Laser parameters included a spot size of 30 μm, a repetition rate of 5 Hz, and an energy density of ~ 2 J cm−2. The ablation aerosol was mixed in the fast washout S-155 ablation cell (Laurin Technic) with carrier gas consisting of He (ca. 0.25 L min−1) and make-up gas consisting of Ar (ca. 1 L min−1) and N2 (2 mL min−1). Detailed analytical parameters can be found in Supplementary Material 2 following the community-derived guidelines (Horstwood et al. 2016). The zircon GJ-1 was used as calibration reference material (CRM). Validating reference materials (VRMs) were zircon 91500, AUSZ7-1, AUSZ7-5, Penglai, and AUSZ8-10 (Wiedenbeck et al. 1995; Li et al. 2010; Kennedy et al. 2014; von Quadt et al. 2016; Lukács et al. 2021). The VRM results show the achievable precision and accuracy of the method, which is in the range of 1.5%. The raw isotopic data was reduced using the software Iolite 4 (Paton et al. 2010, 2011) combined with VizualAge (Petrus and Kamber 2012). All ages were corrected for Th disequilibrium using IsoplotR (Vermeesch 2018), by assuming a Th/U-fractionation factor between the sample and the magma of 0.2. We also corrected for common Pb content when appropriate based on an initial 207Pb/206Pb ratio of 0.85 ± 0.01.

40Ar/39Ar dating

The largest grain size fractions (250–500 and 125–250 μm) were washed to remove glass. Following the cleaning procedure, crystals were concentrated using a Frantz magnetic separator; first, at high intensity (1.5 Ampers) to remove the magnetic Fe–Ti oxide minerals (e.g., ilmenite and magnetite), and then at lower intensities (from 1 to 0.6 Ampers) to split hornblendes and plagioclase. These mineral concentrates were sent to the OSU Argon Geochronology lab for purification and picking of final separates for 40Ar/39Ar dating. Samples were acid leached to remove any alteration material by a combination of HCl (1Normal and 6Normal solutions) and HNO3 (1Normal and 3Normal solutions), modified from Koppers et al. (2000). The mineral concentrates were then treated with a 5% HF solution for 10 min to dissolve any adhering glass on the crystals. Final separates of hornblende and plagioclase were then handpicked using a binocular microscope to a purity of <99.9%. Plagioclase separates were further scrutinized using transmitted light in order to remove any melt and/or mineral inclusions.

40Ar/39Ar ages (Supplementary Material 3 and 4) were obtained by incremental heating methods using the ThermoFisher Scientific ARGUS-VI mass spectrometer and data collection using internal lab software ArArExperiments version 4.4.0. The samples were irradiated for 1 h at the TRIGA nuclear reactor of the Oregon State University (Irradiation 23-OSU-02 in the CLICIT position). The 40Ar/39Ar reference mineral is the Fish Canyon Tuff sanidine (FCT-2-NM sanidine), with an age of 28.201 ± 0.023 Ma (Kuiper et al. 2008). Individual J-values for each sample were calculated by polynomial extrapolation of the measured flux gradient against irradiation height and typically give 0.06–0.12% uncertainties (1σ). The 40Ar/39Ar incremental heating age determinations were performed on a multi-collector ARGUS-VI mass spectrometer at Oregon State University that has 5 Faraday collectors fitted with one (all fitted with two 1012 Ohm resistors: for masses 41Ar and 40Ar and three 1013 Ohm resistors: for argon masses 39Ar, 38Ar, and 37Ar and 1 ion-counting CuBe electron multiplier) located in a position next to the lowest mass faraday collector. This configuration allows measurement of all argon isotopes simultaneously, with mass 36 on the multiplier and masses 37 through 40 on the four adjacent faradays. This configuration provides the advantages of running in a full multi-collector mode whilst measuring the lowest peak (on mass 36) on the highly sensitive electron multiplier (which has an extremely low dark-noise and a very high peak/noise ratio). Irradiated samples were loaded into Cu-planchettes in an ultra-high vacuum sample chamber and incrementally heated by scanning a Synrad Firestar defocused 30 Watt CO2 laser beam in pre-set patterns across the sample, in order to release the argon evenly. Each heating step was 50 s. After heating, reactive gases were cleaned up using four SAES Zr-Al AP10 getters for 3 min; two operated at 450 °C and two operated at room temperature (21 °C). All ages were calculated using the corrected Steiger and Jäger (1977) decay constant of 5.530 ± 0.097 × 10−10 1/yr (2σ) as reported by Min et al. (2000). For all other constants used in the age calculations, we refer to Table 2 in Koppers et al. (2003). Incremental heating plateau ages and isochron ages were calculated as weighted means with 1/σ2 as weighting factor (Taylor 1997) and as YORK2 least-square fits with correlated errors (York 1969) using the ArArCALC v2.6.2 software from Koppers (2002) available from the http://earthref.org/ArArCALC/ website.

Table 2 Major and trace element composition of glasses from the Belbaşhanı Pumice eruption
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Argon isotopic results are corrected for system blanks, radioactive decay, mass discrimination, reactor-induced interference reactions, and atmospheric argon contamination. Decay constants reported by Min et al. (2000) are utilized for age calculation. Isotope interference corrections as determined using the ARGUS VI are: (36Ar/37Ar)Ca = 0.0002703 ± 0.0000005; (39Ar/37Ar)Ca = 0.0006425 ± 0.0000059; (40Ar/39Ar)K = 0.000607 ± 0.000059; (38Ar/39Ar)K = 0.012077 ± 0.000011. Ages were calculated assuming an atmospheric 40Ar/36Ar ratio of 298.56 ± 0.113 (Lee et al. 2006). Data reduction and age calculation were processed using Ar–Ar Calc 2.7.0 (Koppers 2002). Plateau ages are defined as including > 50% of the total 39Ar released with at least three consecutive steps, where the 40Ar/39Ar ratio for each step compares well with the mean at the 95% confidence level.

Isopach and isopleth maps and estimation of the eruption source parameters

All data collected during the fieldwork, such as deposit thickness, largest juvenile, and lithic sizes, were processed and georeferenced using the open-software Quantum Gis 3.26 Buenos Aires (www.qgis.org/it/site/). Once these data were displayed on the 30-m DEM, isopach maps were hand-drawn using the thickness of the Belbaşhanı Pumice in each outcrop. For isopleth maps, we measured in each outcrop the three axes (i.e., x, y, and z) of the five largest pumice and lithics clasts observed within a single square meter of the deposit. The mean of the three axes was calculated for each clast, and then these five values were averaged together. The average values for each outcrop were used to hand-draw two different isopleth maps: one for the pumices and one for the lithics. However, only isopleth maps obtained for the lithics were considered as reliable because pumice clasts break down easily when they fall from the eruptive cloud, and therefore, size measurements in the field may not be fully representative. The isopleths based on pumices are reported as Supplementary Material 5 and used to assess the uncertainty related to the estimation of column height and wind intensity.

The Weibull integration method proposed by Bonadonna and Costa (2012; 2013) was employed to estimate the volume of the Belbaşhanı Pumice from the isopach map and the column height from isopleth maps. The enclosed areas of each isopach curve were calculated, and their square roots and deposit thickness were used to estimate the eruptive volume. To estimate the column height, we calculated the square root of the area for each isopleth contour related with a specified lithic size. To quantify the uncertainties of the volume calculation method, we applied the approach described in Bonadonna and Costa (2013) to estimate the decay length scale of the deposit thinning from isopleth distribution. The mass eruption rate (MER) was calculated applying the method of Mastin et al. (2009). To estimate the wind speed at the time of the eruption, the approach described in Carey and Sparks (1986) was employed by using input parameters such as the maximum downwind range, crosswind range, and clast size.

Results

Distribution and stratigraphy of the Belbaşhanı Pumice

Although the Belbaşhanı Pumice is a late Pleistocene, non-welded, clast-supported, and easily erodible lapilli fallout deposit, it is well-preserved in the study area, especially between the Hasandağ and Keçiboyduran volcanoes, including the Çömlekçi basin, Belbaşhanı path, and Kitreli basin (Fig. 1c). Moreover, some remnants can be found in the surrounding areas of Ihlara and Güzelyurt villages (north of Kitreli basin), and along the Altunhisar-Çiftlik road, which runs SW-NE between Melendiz and Keçiboyduran volcanoes (Fig. 1c).

The best-preserved exposure with the largest clast size (both pumice and lithics) and thickest deposits of Belbaşhanı Pumice were observed in a new quarry located along the Belbaşhanı path (e.g., stratigraphic logs 2 and 11 in Fig. 2). The pumice deposit lays above a paleosol developed on top of older volcanic products. The bottom part of the Belbaşhanı Pumice consists of a 13-m-thick layer of massive, white, coarse lapilli fallout deposit, which contains abundant block-sized pumices (up to ~ 0.3 m in diameter; Fig. 3). The upper part of the Belbaşhanı Pumice comprises at least 3.5 m of bedded, coarse to medium-fine lapilli fallout deposits.

Fig. 3
figure 3

Exposures of the Belbaşhanı Pumice fallout deposit in two quarries within the Belbaşhanı path (b,c,d, from outcrop 11 and e,f, from outcrop 2 in Fig. 1c). a Composite stratigraphic column of the proximal deposits. Th, thickness. llspL, parallel-stratified pumice lapilli. mpL, massive pumice lapilli. b Panoramic view of the large Belbaşhanı quarry. c Belbaşhanı Pumice consisting of massive medium to coarse pumice lapilli (13 m thick). d Bottom contact (paleosol) of Belbaşhanı Pumice. e Contact between the bedded uppermost part of the Belbaşhanı Pumice above the massive lapilli unit observed at old quarry (stratigraphic log 2; Fig. 1c). f Close-up of a > 20 cm pumice block within the massive bottom part of the Belbaşhanı Pumice

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In proximal to medial facies within the Kitreli basin, typically only the massive bottom part of the Belbaşhanı Pumice is preserved (up to 2.5-m thickness with pumice clasts of ~ 5 cm in diameter), whilst the bedded upper part is often partially or completely eroded. However, due to the gentle slope of this basin, the Belbaşhanı Pumice is still well-preserved within several measurable exposures (e.g., stratigraphic logs 3, 6, 7, 14, 15, and 16 in Figs. 1c and 2). Within the Çömlekçi basin, the Belbaşhanı Pumice has been largely eroded because of the steep slopes. Here, the fallout deposit has only been observed in the Çömlekçi outcrop and in the Shepard trench site (stratigraphic logs 1 and 5 in Figs. 1c and 2), where ~ 1 and ~ 2 m of fallout deposits (respectively) are preserved.

In medial to medial-distal facies, the Belbaşhanı Pumice has a thickness of up to 2 m, showing internal beds of well-sorted, clast-supported, fine to medium lapilli pumice (stratigraphic logs 8, 9, and 17 in Fig. 2). Medium lapilli-sized pumices typically range from 0.5 to 3 cm in diameter. Some of the most distal deposits of Belbaşhanı Pumice that were found in this study are exposed close to Altunhisar town (stratigraphic log 4 in Fig. 2) and along the Altunhisar-Çiftlik road, which runs southwest-northeast between the Melendiz and Keçiboyduran volcanoes (stratigraphic logs 13 and 18 in Fig. 2). In these three outcrops, the Belbashani Pumice is deposited above two other tephra layers erupted from Keçiboyduran volcano, which are obsidian lithics-rich and compositionally different from Hasandağ products (Figs. 4 and 5).

Fig. 4
figure 4

a Overall view of the bedded Belbaşhanı Pumice fallout in a quarry close to Altunhisar town (stratigraphic log 4 in Figs. 1c and 2), and overlying two tephras erupted by Keciboyduran volcano. b Stratigraphic log of the fallout deposit. Th thickness, MpL massive pumice lapilli, llsp parallel-stratified pumice lapilli, V.clast volcanoclastic material. The vertical scale of the lahar (uppermost part unit) is exaggerated

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Fig. 5
figure 5

Major and trace element glass compositions from the Belbaşhanı Pumice. a TAS (total alkali vs. silica; Le Bas et al. 1986) diagram. b SiO2–CaO bivariate plot. c La–Nb bivariate plot. New glass data from Hasandağ (red area) and Keçiboyduran (blue area) are also plotted in the figure to show the compositional field of each volcano. Error bars for SiO2, CaO, Na2O, and K2O indicate standard deviations of approximately ± 0.4, ± 0.1, ± 0.2, and ± 0.2, wt% oxide respectively. For La and Nd, standard deviations are approximately ± 2 and ± 1 ppm, respectively

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Glass compositional variation

Major and trace element glass compositions of Belbaşhanı Pumice (Table 2) are compared with the glass compositional field of Hasandağ and Keçiboyduran volcanoes (Fig. 5) in order to chemically correlate the Belbaşhanı Pumice eruption to its source. These compositional fields have been constructed by microanalyses of glasses from proximal pyroclastic deposits (see Supplementary Material 1). According to the TAS (total alkali vs. silica; Le Bas et al. 1986) diagram, all glasses from the Belbaşhanı Pumice are rhyolitic in composition (Fig. 5a). SiO2–CaO and Nb–La bivariate plots outline that these glasses span a compositional range comparable to that observed for the Hasandağ pyroclastic products, but distinct from the Keçiboyduran glasses, especially for trace element contents (Fig. 5b, c).

Pumice morphology

Pumices from the Belbaşhanı fallout unit are highly vesicular, with tubular, coalesced bubbles separated by thin walls (Fig. 6). Vesicle shapes vary from subspherical to elongated, with irregular forms of the bubble wall. The glass shards show broken sharp edges with acute angles. These features are indicative of fragmentation processes driven by the exsolution of magmatic gases (Dellino and La Volpe 1995).

Fig. 6
figure 6

Scanning electron microscope (SEM) photomicrographs of highly vesiculated pumices from the Belbaşhanı eruption. EHT = 7.00 kV, Signal A = SE, WD = a 8 mm, b 8.5 mm

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Geochronological constraints

40Ar/39Ar analyses of plagioclase (sample PSK-64; Table 1) from the Belbaşhanı Pumice yielded a plateau age of 417.2 ± 20.5 ka (2σ error; Fig. 7a and Supplementary Material 3). Similarly, 40Ar/39Ar analyses of hornblende from the same sample yielded a plateau age of 436.4 ± 27.5 ka (2σ error; Fig. 7b and Supplementary Material 4), but with larger errors and the first heating steps showing contamination by atmospheric argon. For this reason, we consider the 40Ar/39Ar data derived from plagioclase (417.2 ± 20.5 ka) to be more precise and use this age for the Belbaşhanı Pumice eruption.

Fig. 7
figure 7

40Ar/39Ar age plateau of the analysed a plagioclase and b hornblende phenocrysts from the Belbaşhanı Pumice, yielding high-resolution incremental heating steps. The black bar refers to heating steps used for the calculation of the weighted average age (± 2σ) of 417.2 ± 20.5 ka and 436.4 ± 27.5 ka for plagioclase and hornblende, respectively

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Only two zircons returned an approximate eruption age, whereas all other zircons returned crystallization ages of the inherited crystals (i.e., xenocrysts; see Supplementary Material 2). The Th-disequilibrium corrected 206Pb/238U zircon ages are 410 ± 22 ka and 352 ± 16 ka (Supplementary material 2), being comparable to the 206Pb/238U age of 339 ± 50 ka reported by Aydın et al. (2014) for the same pumice deposit, and in agreement with the new plagioclase 40Ar/39Ar age determined in this study. Secular equilibrium condition between 238U and 232Th for zircons indicates that the investigated mineral grains are older than 350 ka, corresponding to the upper detection limit of the radiogenic method (Bernal et al. 2014). Comparing these geochronological results, we suggest the Belbaşhanı Pumice eruption is older than 350 ka but younger than ~ 463 ka.

Eruption source parameter estimations and related uncertainties

Based on isopach and isopleth maps (Fig. 8), the vent of the Belbaşhanı Pumice eruption was located near the intersection of the Tuz Gölü fault and the Ulukışla caldera (Fig. 1c; Ulusoy et al. 2021; Diker et al. 2024). Isomaps indicate that volcanic material was dispersed towards the northeast. According to Carey and Sparks (1986), by considering the maximum downwind range of 18 km, crosswind range of 12 km, and the mean diameter of lithics measured within the fallout deposit (i.e., ca ~ 5 cm; Table 1), most of the winds transporting the Belbaşhanı Pumice had a range of speeds of ~ 10–20 m/s.

Fig. 8
figure 8

a Isopach map (in m) of the Belbaşhanı Pumice fallout deposit. b Isopleth map (in cm) based on largest five lithics measured in the deposit. The innermost isoline on the isopleth map represents the largest diameter of 9 cm

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Following the method of Bonadonna and Costa (2012, 2013), the volume of the Belbaşhanı Pumice is estimated to range between 0.5 and 8 km3 (bulk pyroclastic material of 1250 kg/m3 as reference density value for fallout deposits from Bonadonna and Costa 2013) with a mean volume of ~ 2 km3 (Supplementary Material 5). The altitude of the eruptive column ranged between 18 and 29 km, with a mean height of ~ 25 km (Supplementary Material 5). According to Mastin et al. (2009), and considering these uncertainties on the column height, the Mass Eruption Rate (MER) of the Belbaşhanı Pumice eruption was calculated to be between 2.19 \(\times\) 107 and 1.58 \(\times\) 108 kg/s, with a mean value of 8.7 \(\times\) 107 kg/s (Supplementary Material 5). Taking into account the ranges of the column height and MER values, the Belbaşhanı Pumice eruption was Sub-Plinian to Plinian (yellow rectangle in Fig. 9; Bonadonna and Costa 2013); however, it could be classified as a Plinian eruption based on the mean values of these parameters (yellow star in Fig. 9).

Fig. 9
figure 9

Flowchart of this study and classification of tephra deposits based on the Mass Eruption Rate (MER) vs. Plume Height diagram (Bonadonna and Costa 2013). Yellow empty rectangle represents the eruption classification range of the Belbaşhanı Pumice eruption considering uncertainties. Yellow star shows the best estimations for this eruption

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Discussions

Reconstruction of the Belbaşhanı Pumice eruption dynamics

Based on new geochronological and volcanological constraints, the Belbaşhanı Pumice is interpreted to have formed by a sub-Plinian to Plinian eruption column that deposited a widespread and thick pumice fallout layer at ~ 417 ka (Fig. 3). Since there are no paleosols or unconformities within the deposit, the eruption was likely continuous and without any long hiatus or period of quiescence within it. The massive and well-sorted coarse pumice deposit at the bottom of the Belbaşhanı Pumice indicates a steady column at the beginning of the eruption, which gradually changed to a waxing-waning column as suggested by the granulometry and internal bedding of the layered uppermost part of the fallout deposit (Fig. 3). The waxing-waning eruptive behaviour was probably due to the opening and widening of the eruptive vent (e.g., Sulpizio et al. 2005). In addition, the highly vesiculated pumices with tubular coalescent bubbles and angular glass shards morphologies suggest a dominantly magmatic fragmentation during the eruption (e.g., Heiken and Wohletz 1985; Dellino and La Volpe 1995).

Isopach and isopleth maps indicate that the dispersal axis of the Belbaşhanı Pumice plume was towards the northeast, leading to the accumulation of a pumice layer at least 17 m thick in proximal areas along the Belbaşhanı path and up to 2 m thick at a distance of ~ 18 km northeast from the vent (medial-distal areas within the Kitreli basin; Figs. 2 and 8). Isomaps point at an eruptive vent of the Belbaşhanı Pumice near the intersection of the Tuz Gölü fault and Ulukışla caldera, which is located on the eastern flank of Hasandağ volcano (Fig. 1c). In accordance, glass composition of the Belbaşhanı Pumice suggests that this fallout deposit was likely produced by the Hasandağ magmatic system (Fig. 5). Volume in DRE (considering 2400 kg/m3 of magma density) would be 0.26–4.16 km3, with a mean value of ~ 1 km3.

Magnitude and intensity of the Belbaşhanı Pumice eruption, uncertainties, and implications

The magnitude of the Belbaşhanı Pumice eruption has been estimated using only the preserved proximal deposits due to this kind unconsolidated, old fallout deposit (including most of the distal deposits) is susceptible to remobilization by rains and other atmospheric events. However, taking into account (1) the eroded deposits, (2) the decay length scale of the deposit thinning (Bonadonna and Costa 2013), and (3) other uncertainties associated with the used methods (e.g., Bonadonna et al. 2015; Primerano et al. 2021; Marzocchi et al. 2021; Constantinescu et al. 2022), the pyroclastic deposit volume is estimated to range from 0.5 to 8 km3 (with a mean value of ~ 2 km3). This corresponds to a Dense Rock Equivalent (DRE; or magma volume) of 0.26–4.16 km3 (mean value of ~ 1 km3) considering a density of silica-rich magmas of 2400 kg/m3 (Bonadonna et al. 2015). On this basis, the magnitude of the Belbaşhanı Pumice eruption was at least 4 and potentially 6 (Bonadonna and Costa 2013; Pyle 2015), similar to the Volcanic Explosivity Index (VEI) that would be in the range 4–6 (Newhall and Self 1982). Considering the estimated range of column heights (18–29 km) and MER (2.19 \(\times\) 107–1.58 \(\times\) 108 kg/s), the intensity of this eruption was ~ 11 (Bonadonna and Costa 2013; Pyle 2015). Within this order of magnitude and intensity, the Belbaşhanı Pumice eruption undoubtedly had far-reaching consequences, most likely impacting significantly on the early human populations of Central Anatolia, as reported for other large CAPV eruptions of similar age (e.g., the Upper Acigöl Tuff—UAT—tephra found in Kaletepe Deresi 3 archaeological site; Slimak et al. 2008; Tyron et al. 2009). The Belbashani Pumice eruption could also have affected the regional environment and global climate, similarly to the UAT that is recognized as a marker horizon within MIS 6 (e.g., Wegwerth et al. 2019). However, additional studies on halogens and sulphur released by the Belbashani Pumice eruption into the atmosphere are necessary to confirm these hypotheses (e.g., compositional data from melt inclusions in olivine and apatite; Marks et al. 2014).

In addition, the new data provided in this study will allow future tephrochronological investigations to identify the fine ash from the Belbaşhanı Pumice eruption in distal sedimentary records (e.g. chemical fingerprint and isochron transfer), which in turn can help refine the magnitude of the eruption by reducing related uncertainties. A similar approach has already been applied to other Late Pleistocene–Holocene eruptions from Central Anatolian volcanoes. For example, the distal tephra of the Dikkartın eruption (i.e., S1 tephra from Erciyes volcano; Fig. 1b), which erupted 1–2 km3 of pyroclastic material up to altitudes of 20 ± 5 km (VEI ~ 5 according to Friedrichs et al. 2020b), has been found in different regional sedimentary records up to ~ 1300 km southwards from the volcanic vent (Friedrichs et al. 2020b), including the Levantine Sea (Hamann et al. 2010), Dead Sea (Neugebauer et al. 2017), Tayma paleolake (Neugebauer et al. 2017), Yammouneh Basin in Lebanon (Develle et al. 2009), and Sodmein caves in Egypt (Barton et al. 2015). Maar-type explosive eruptions from the Acıgöl Caldera (i.e., Korudağ and Güneydağ) are also found in distal records. Despite their smaller eruption magnitudes compared to the Belbaşhanı Pumice, these deposits were chemically correlated with the M72/5–25-GC1 core by Cullen et al. (2014), which was drilled within the Black Sea basin (~ 500 km north of Hasandağ volcano). Thus, identifying the Belbaşhanı Pumice in distal records could improve the volcanic hazard assessments in the area and also make this deposit a new useful stratigraphic marker horizon for interdisciplinary archaeological, paleo-environmental, -climatic, and -oceanic studies.

Hazard and risk implications

The possibility of a future eruption at Hasandağ cannot be discarded, as this volcano was the source of multiple eruptions over the past 60,000 years (at least five explosive events producing pyroclastic flows and fallouts, and more than nine effusive lava flows; Friedrichs et al. 2020a), with the youngest Holocene “Çatalhöyük eruption” dated at ~ 8.97 ± 0.64 ka (Schmitt et al. 2014). Further aspects supporting this potential are the recent increase in seismicity in the region, the likely presence of eruptible magma located below the volcano, and the ongoing fumarolic activity (Ulusoy et al. 2021; Friedrichs et al. 2021). Moreover, we have analysed other pumice fallout deposits within the study area, which were already observed by Kuzucuoğlu et al. (2020), with similar Hasandağ glass chemical signatures, specifically in the surroundings of Ulukışla caldera. Available data on the past eruptions from Hasandağ volcano do not allow us to estimate the probability of occurrence of the different potential eruptive scenarios, but our study can be very useful for assessing the impact of a similar eruption in the near future. Further studies are required for a more quantitative volcanic hazard assessment.

A present-day explosive eruption at Hasandağ volcano would pose a substantial threat to the surrounding populations. Several villages, including Helvadere, Kitreli, Ihlara, Yenipınar, and Çiftlik, are within the area affected by the proximal to medial Belbaşhanı Pumice, with 2-m-thick deposits around Güzelyurt at ~ 20 km NE from the vent (Fig. 1c). Additionally, the largest cities of Central Anatolia, such as Aksaray, Niğde, Nevşehir, and Kayseri, with a total population of 5 million inhabitants, are in proximity of the Hasandağ volcano (Fig. 1b) and their infrastructures would be at risk of being damaged by a potential Plinian eruption. Other important economic sectors would also be affected, like the neighbouring tourist region of Cappadocia (Fig. 1b), which sees 3 to 4 million visitors per year. Furthermore, air traffic would be disrupted in case of a similar eruption to the Belbaşhanı Pumice (Folch and Sulpizio 2010).

Conclusions

Volcanological data from the Belbaşhanı Pumice provides new insights into the middle-to late-Pleistocene explosive eruption history of Hasandağ volcano, with implications for a better hazard assessment in the Central Anatolia region. The most important outcomes can be summarized as follows:

  • The Belbaşhanı Pumice is a white, biotite-rich, fallout deposit of 0.5–8 km3 in volume (mean value of ~ 2 km3) produced by a Sub-Plinian to Plinian eruption with a column reaching 18 to 29 km in height (mean value of ~ 25 km).

  • The best-preserved proximal deposits are located within the Belbaşhanı path, a topographic high between Hasandağ and Keciboyduran volcanoes where the deposit is up to ~ 17 m thick and comprises pumice blocks of up to 30 cm in diameter.

  • The Belbaşhanı Pumice eruption, which has been dated at 417.2 ± 20.5 ka by 40Ar/39Ar, began with a steady eruption column that was gradually losing energy moving to a waxing-waning regime towards the end of the eruption. The highly vesiculated pumices, the tubular coalescent bubbles, and the angular glass shards morphology imply that the eruption was primarily magmatic.

  • Dispersal axes of isopach and isopleth maps indicate that the eruptive plume was directed towards the northeast of Hasandağ volcano (over the Kitreli valley).

  • Isomaps also show that the volcanic vent of the Belbaşhanı Pumice was near the intersection of the Tuz Gölü fault and Ulukışla caldera, thereby supporting the idea that the pyroclastic material was erupted by the Hasandağ Volcanic Complex.

  • Major and trace element glass compositions outline that Belbaşhanı Pumice is a rhyolitic deposit, compositionally similar to the other Hasandağ pyroclastic products, but slightly different from Keciboyduran volcano products.

  • Uncertainties in the calculation of eruption parameters have been considered in this work. The volume estimate range (0.5–8 km3) gives a minimum eruption magnitude for the Belbaşhanı Pumice between 4 and 6, equivalent to a VEI of 4 to 6. The estimated range of column height (18–29 km) provides a MER between 2.19 \(\times\) 107 and 1.58 \(\times\) 108 kg/s, with a mean value of 8.7 \(\times\) 107 kg/s, suggesting an eruption intensity of ~ 11.

  • Based on the increase in seismicity in the region, the ongoing fumarolic activity, the potentially eruptible magma identified below the volcano, and a record of multiple eruptions from Hasandağ in the last thousands of years, a future Plinian eruption similar to that which produced the Belbaşhanı Pumice cannot be discarded, and its potential impact on the nearby populations and infrastructure must be taken into consideration.

Although with some limitations, the reconstruction of the Belbaşhanı Pumice proposed in this work is an important source of new information to improve our knowledge on the Hasandağ eruptive history and, consequently, represents a significant step forward in the volcanic hazard assessments in the Central Anatolian area.