Erkan Aydar

The Hasan Dagi stratovolcano (Central Anatolia, Turkey) : evolution from calc-alkaline to alkaline magmatism in a collision zone

Abstract The Hasan Dagi volcano is one of the two large Plio-Quaternary volcanoes in Cappadocia (Central Anatolia, Turkey). Three stages of edifice construction have been identified for this volcano: Paleovolcano, Mesovolcano and Neovolcano. Most samples from Hasan Dagi volcano are calc-alkaline and define an almost complete trend from basaltic andesite to rhyolite. However, the more recent (Neovolcano) mafic samples are alkaline basalts. The mineralogical and geochemical characteristics of the oldest lavas (Kecikalesi (13 Ma) and Paleo-Hasan Dagi (7 Ma)) are significantly different from those of the younger lavas (Meso- and Neo-Hasan Dagi ( 87 Sr / 86 Sr than the Paleo- and Mesovolcano basalts, whereas the Mesovolcano basalts display more radiogenic Pb than Paleovolcano samples. Magma mixing processes between initially heterogeneous and/or variably contaminated magmas may account for the genesis of the less differentiated and intermediate lavas (48–57% SiO2). Meso- and Neovolcano differentiated lavas (60–68% SiO2) are either derived from the analyzed basalts or from more primitive and more depleted magmas by fractional crystallization±some crustal contamination (AFC). Furthermore, the highly differentiated samples (72–75% SiO2) are not strongly contaminated. The strong calc-alkaline character of Hasan Dagi lavas, in the absence of contemporaneous subduction, must reflect the heritage of the early subduction of the Afro–Arabian plate under the Eurasian plate. The evolution towards alkaline compositions through time is clearly related to the development of extensional tectonics in Central Anatolia in the Late Miocene.

Morphological analysis of active Mount Nemrut stratovolcano, eastern Turkey: evidences and possible impact areas of future eruption

Abstract Mount Nemrut, an active stratovolcano in eastern Turkey, is a great danger for its vicinity. The volcano possesses a summit caldera which cuts the volcano into two stages, i.e. pre- and post-caldera. Wisps of smoke and hot springs are to be found within the caldera. Although the last recorded volcanic activity is known to have been in 1441, we consider here that the last eruption of Nemrut occurred more recently, probably just before 1597. The present active tectonic regime, historical eruptions, occurrence of mantle-derived magmatic gases and the fumarole and hot spring activities on the caldera floor make Nemrut Volcano a real danger for its vicinity. According to the volcanological past of Nemrut, the styles of expected eruptions are well-focused on two types: (1) occurrence of water within the caldera leads to phreatomagmatic (highly energetic) eruptions, subsequently followed by lava extrusions, and (2) effusions–extrusions (non-explosive or weakly energetic eruptions) on the flanks from fissures. To predict the impact area of future eruptions, a series of morphological analyses based on field observations, Digital Elevation Model and satellite images were realized. Twenty-two valleys (main transport pathways) were classified according to their importance, and the physical parameters related to the valleys were determined. The slope values in each point of the flanks and the Heim parameters H/L were calculated. In the light of morphological analysis the possible impact areas around the volcano and danger zones were proposed. The possible transport pathways of the products of expected volcanic events are unified in three main directions: Bitlis, Guroymak, Tatvan and Ahlat cities, the about 135 000 inhabitants of which could be threatened by future eruptions of this poorly known and unsurveyed volcano.

The geology of Mount Hasan stratovolcano, central Anatolia, Turkey

Mount Hasan is a double-peaked stratovolcano, located in Central Anatolia, Turkey. The magmas erupted from this multi-caldera complex range from basalt to rhyolite, but are dominated by andesite and dacite. Two terminal cones (Big Mt. Hasan and Small Mt. Hasan) culminate at 3253 m and 3069 m respectively. There are four evolutionary stages in the history of the volcanic complex (stage 1: Kecikalesi volcano, 13 Ma, stage 2: Palaeovolcano, 7 Ma, stage 3: Mesovolcano and stage 4: Neovolcano). The eruptive products consist of lava flows, lava domes, and pyroclastic rocks. The later include ignimbrites, phreatomagmatic intrusive breccias and nuees ardentes, sometimes reworked as lahars. The total volume is estimated to be 354 km 3 , the area extent 760 km 2 . Textural and mineralogical data suggest that both magma mixing and fractional crystallization were involved in the generation of the andesites and dacites. The magmas erupted from the central volcanoes show a transition with time from tholeite to calc-alkaline. Three generations of basaltic strombolian cones and lava flows were emplaced contemporaneously with the central volcanoes. The corresponding lavas are alkaline with a sodic tendency.

Geochemical approach to magmatic evolution of Mt. Erciyes stratovolcano Central Anatolia, Turkey

Erciyes stratovolcano, culminating at 3917 m, is located in the Cappadocian region of central Anatolia. During its evolution, this Quaternary volcano produced pyroclastic deposits and lava flows. The great majority of these products are calc-alkaline in character and they constitute Kocdag and Erciyes sequences by repeated activities. Alkaline activity is mainly observed in the first stages of Kocdag and approximately first-middle stages of Erciyes sequences. Generally, Kocdag and Erciyes stages terminate by pyroclastic activities. The composition of lavas ranges from basalt to rhyolite .48.4-70.5 wt.% SiO . Calc-alkaline rocks are represented mostly by andesites and dacites. Some compositional 2 differences between alkaline basaltic, basaltic and andesitic rocks were found; while the composition of dacites remain unchanged. All these volcanics are generally enriched in LIL and HFS elements relative to the orogenic values except Rb, Ba, Nb depleted alkaline basalt. 87 Srr 86 Sr and 143 Ndr 144 Nd isotopic composition of the volcanics range between 0.703344-0.703964, 0.512920-0.512780 for alkaline basalts and change between 0.704322-0.705088, 0.512731-0.512630 for alkaline basaltic rocks whereas calc-alkaline rocks have relatively high Sr and Nd isotopic ratios 0.703434-0.705468, . 0.512942-0.512600 . Low Rb, Ba, Nb content with high ZrrNb, low BarNb, LarYb ratio and low Sr isotopic composition suggest an depleted source component, while high Ba, Rb, Nb content with high LarYb, BarNb, low ZrrNb and low 87 Srr 86 Sr ratios indicate an OIB-like mantle source for the generation of Erciyes alkaline magma. These elemental and ratio variations also indicate that the different mantle sources have undergone different degree of partial melting episodes. The depletion in Ba, Rb, Nb content may be explained by the removal of these elements from the source by slab-derived fluids which were released from pre-collisional subduction, modified the asthenospheric mantle. The chemically different mantle sources interacted with crustal materials to produce calc-alkaline magma. The BarNb increase of calc-alkaline samples indicates the increasing input of crustal components to Erciyes volcanics. Sr and Nd isotopic compositions and elevated LIL and HFS element content imply that calc-alkaline magma may be derived from mixing of an OIB-like mantle melts with a subduction-modified asthenospheric mantle and involvement of crustal materials in intraplate environments. q 1998 Elsevier Science B.V. All rights reserved.

Volcanological evolution of Mount Erciyes stratovolcano and origin of the Valibaba Tepe ignimbrite (Central Anatolia, Turkey)

Abstract Mount Erciyes (3917 m) is the largest stratovolcano of Central Anatolia (Turkey). The volcanological evolution of Mount Erciyes from Pliocene–Quaternary to historical times exhibits two distinct stages: (1) Koc Dag and (2) Erciyes. During the Koc Dag stage, basaltic and andesitic lava flows were emitted from the cinder cones of Kizil Tepe and Topakkaya Tepe. Then, Koc Dag pyroclastics were emplaced, leading to a caldera collapse of 14×18 km in diameter with a volume estimated at 110 km3. Two eruption phases separated by scoria fall and mud flow deposits are recognised associated with the caldera forming event. During Phase 1, plinian fall and pumice flows were emplaced on the eastern part of the volcano. Plinian fall deposits occur more than 50 km away from the source area. The volume of tephra is estimated at 63.3 km3 (16 km3 dense rock equivalent (DRE)), including 62 km3 of plinian fall and 1.3 km3 pumice flow deposits. Phase 2 mainly consists of pyroclastic flow deposits (two pumiceous flow units and Valibaba Tepe Ignimbrite (VTI)). The pumiceous flow deposits (4.2 km3 DRE) extend 30 km from the inferred source area towards the north and northeast. The final pyroclastic flows of Phase 2 occurred 2.8 Ma ago and produced the VTI. The VTI is a low aspect ratio welded ignimbrite and its volume is estimated at 40 km3 DRE. Plinian fall deposits (0.8 km3 DRE) preceded the VTI and are only observed on the eastern part of the volcano, covering an area of 1500 km2. During the Erciyes stage, two eruptive cycles are defined. The first cycle (2.6–0.17 Ma) is characterised by effusive, extrusive and weakly explosive activity with emplacement of andesitic lava flows, dacitic dome flows, basaltic andesite lava flows, and andesitic cones. The second cycle involved dacitic extrusive and explosive activity, located at the summit area, and produced block-and-ash flows derived from rhyodacitic domes. Plinian fall, surge and pumice flow deposits were emplaced prior to rhyodacitic dome extrusions. The last event was the emplacement of debris avalanche deposits related to sector collapse caldera.

Early Miocene to Quaternary evolution of volcanism and the basin formation in western Anatolia: a review

Abstract Western Anatolia, largely affected by extensional tectonics, witnessed widespread volcanic activity since the Early Miocene. The volcanic vents of the region are represented by epicontinental calderas, stratovolcanoes and monogenetic vents which are associated with small-scale intrusions as sills and dykes. The volcanic activity began with an explosive character producing a large ignimbritic plateau all over the region, indicating the initiation of the crustal extension event. These rhyolitic magmas are nearly contemporaneous with granitic intrusions in western Anatolia. The ignimbrites, emplaced approximately contemporaneous with alluvial fan and braided river deposits, flowed over the basement rocks prior to extensional basin formation. The lacustrine deposits overlie the ignimbrites. The potassic and ultrapotassic lavas with lamprophyric affinities were emplaced during the Late Miocene–Pliocene. The volcanic activities have continued with alkali basalts during the Quaternary.

Texture discrimination of volcanic ashes from different fragmentation mechanisms: A case study, Mount Nemrut stratovolcano, eastern Turkey

Abstract Multicondition-driven mechanisms may produce pyroclastic deposits varying in fundamental properties such as dispersal, grain size, vesicularity and morphology of juvenile clasts, and the abundance of lithic or “wall rock” ejecta (xenoliths). Volcanic ash particles from different fragmentation mechanisms have different surface textures and morphologies. The analysis of the volcanic clast shape remained largely qualitative. A new method for ash particle characterization based on quadtree decomposition and surface gradient analysis is introduced. The approach is applied for assessing fragmentation mechanisms operating during eruptions. The surface descriptor variables like the number of quadtree blocks (nQT), the mean block size (mQT), the standard deviation of block sizes (sQT) and the surface descriptors derived from gradient analysis seem to be suitable for quantifying the structural changes of the ash surface due to variable explosion conditions. These parameters are presented in volcanology as distinctive key parameters for different eruption types. This may enrich our capabilities for effective prediction for the basis of planning to overcome the impending danger of eruptions.

Identifying the volcanic eruption depicted in a neolithic painting at Catalhoyuk, Central Anatolia, Turkey.

A mural excavated at the Neolithic Çatalhöyük site (Central Anatolia, Turkey) has been interpreted as the oldest known map. Dating to ∼6600 BCE, it putatively depicts an explosive summit eruption of the Hasan Dağı twin-peaks volcano located ∼130 km northeast of Çatalhöyük, and a birds-eye view of a town plan in the foreground. This interpretation, however, has remained controversial not least because independent evidence for a contemporaneous explosive volcanic eruption of Hasan Dağı has been lacking. Here, we document the presence of andesitic pumice veneer on the summit of Hasan Dağı, which we dated using (U-Th)/He zircon geochronology. The (U-Th)/He zircon eruption age of 8.97±0.64 ka (or 6960±640 BCE; uncertainties 2σ) overlaps closely with 14C ages for cultural strata at Çatalhöyük, including level VII containing the “map” mural. A second pumice sample from a surficial deposit near the base of Hasan Dağı records an older explosive eruption at 28.9±1.5 ka. U-Th zircon crystallization ages in both samples range from near-eruption to secular equilibrium (>380 ka). Collectively, our results reveal protracted intrusive activity at Hasan Dağı punctuated by explosive venting, and provide the first radiometric ages for a Holocene explosive eruption which was most likely witnessed by humans in the area. Geologic and geochronologic lines of evidence thus support previous interpretations that residents of Çatalhöyük artistically represented an explosive eruption of Hasan Dağı volcano. The magmatic longevity recorded by quasi-continuous zircon crystallization coupled with new evidence for late-Pleistocene and Holocene explosive eruptions implicates Hasan Dağı as a potential volcanic hazard.

Quantitative scanning-electron microscope analysis of volcanic ash surfaces: Application to the 1982–1983 Galunggung eruption (Indonesia)

Qualitative analyses of volcanic ash are time-consuming and subjective, whereas quantitative analyses are methodical and automated. Not only volcanic ash particles, but also many natural particles have been widely described and quantified by their outlines. The qualitative data of volcanic ash surfaces need to be expressed quantitatively, supported by supplementary methods such as statistical analysis and artificial intelligence. Well-defined surface descriptors can be applied to volcanic ash particles. In this study, roughness and texture descriptors of pyroclastic material from the 1982–1983 eruption of Galunggung (Java, Indonesia) were used to describe the vesicle surfaces of the particles, alteration intensity, and/or fine particle abundance. These parameters are important for distinguishing the products of magmatic eruptions from those of phre-atomagmatic eruptions. Further application of this method may allow these descriptors to be easily converted to alteration grade, vesicularity index, intensity of the fragmentation mechanism, and relative proportions of the pyroclast types. Hence, discrimination between products of different fragmentation mechanisms may permit forecasting of volcanic hazards.

Clustering of volcanic ash arising from different fragmentation mechanisms using Kohonen self-organizing maps

In this study, we present the visualization and clustering capabilities of self-organizing maps (SOM) for analyzing high-dimensional data. We used SOM because they implement an orderly mapping of a high-dimensional distribution onto a regular low-dimensional grid. We used surface texture parameters of volcanic ash that arose from different fragmentation mechanisms as input data. We found that SOM cluster 13-dimensional data more accurately than conventional statistical classifiers. The component planes constructed by SOM are more successful than statistical tests in determining the distinctive parameters.