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

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.     

Holocene-historical volcanism and active faults as natural risk factors for Armenia and adjacent countries

Examples of Holocene-historical volcanism in the territory of Armenia and adjacent areas of Eastern Anatolia and Western Iran are discussed. Holocene-historical activity is proved for the volcanoes of Tskhouk–Karckar, Porak, Vaiyots-Sar, Smbatassar and Ararat. Based on the analysis of remote sensing data, field work, and historical and archeological information, it is demonstrated that there was a considerable number of cases of volcanic activity in Armenia and adjacent regions of Turkey, Syria and Iran during the historical period. The Holocene volcanic centers are situated within pull-apart basin structures and controlled by active faults. Situated in an area prone to many types of natural hazards, Armenia and adjacent countries face high natural risk. The evidence presented shows that volcanic hazard also contributes to the natural risk for these countries.     

Structure of the Nemrut caldera (Eastern Anatolia, Turkey) and associated hydrothermal fluid circulation

Plio-Quaternary volcanism played an important role in the present physical state of Eastern Anatolia. Mount Nemrut, situated to the west of Lake Van is one of the main volcanic centers in the region, with a spectacular summit caldera 8.5 × 7 km in diameter. The most recent eruptions of the volcano were in 1441, 1597 and 1692. Nemrut Lake covers the western half of the caldera; it is a deep, half-bowl-shaped lake with a maximum depth of 176 m. Numerous eruption centers are exposed within the caldera as a consequence of magma–water interaction. Current activity of Nemrut caldera is revealed as hot springs, fumaroles and a small, hot lake.Self-potential and bathymetric surveys carried out in the caldera were used to characterize the structure of the caldera and the associated hydrothermal fluid circulation. In addition, analyses based on digital elevation models and satellite imagery were used to improve our knowledge about the structure of the caldera. According to SP results, the flanks of the volcano represent “the hydrogeologic zone”, whereas the intra-caldera region is an “active hydrothermal area” where the fluid circulation is controlled by structural discontinuities. There is also a northern fissure zone which exhibits hydrothermal signatures. Nemrut caldera collapsed piecemeal, with three main blocks. Stress controlling the collapse mechanism seems to be highly affected by the regional neotectonic regime. In addition to the historical activity, current hydrothermal and hydrogeologic conditions in the caldera, in which there is a large lake and shallow water table, increase the risk of the quiescent volcano.     

Hidden calderas evidenced by multisource geophysical data; example of Cappadocian Calderas, Central Anatolia

The Cappadocian volcanic field in central Anatolia (Turkey) is characterised by a sequence of 10 Neogene ignimbrites. The associated calderas have been partly dismantled and buried by subsequent tectonic and sedimentary processes and, therefore, cannot be readily recognized in the field. Recent progress in the understanding of the stratigraphic correlations and flow patterns has identified two main probable source areas for the ignimbrites. Detailed study of these areas, based on gravity surveys, remote sensing data (SPOT and ERS1 images) and digital elevation models (DEM), has provided evidence for two major caldera complexes and their relationship to old stratovolcanoes and Neogene tectonics. The older Nevsehir–Acigöl caldera complex, located between the towns of Acigöl, Nevsehir and Cardak, is inferred to be the source of the Kavak and Zelve ignimbrites. The Nevsehir–Acigöl caldera complex is defined mainly by a −35 mGal circular gravimetry anomaly about 15 km in diameter. The boundaries of this, now buried, caldera complex are shown by high gradients on the Bouguer gravity anomaly map. The younger Derinkuyu caldera complex, located between the Erdas stratovolcano and the Ciftlik basin, is inferred to be the source of the Sarimaden, Cemilköy, Gördeles and Kizilkaya ignimbrites. It is well-defined by a rectangular (35×23 km) gravity low (−30 mGal) with a positive high (+20 mGal) in the center. Gravity, remote sensing data and the DEM provide evidence that the Erdas stratovolcano, on the northern margin of the Derinkuyu caldera complex, represents the remnants of a large stratovolcano partly cut by one or more caldera collapses. The positive anomaly within the Derinkuyu caldera complex is centered on the 15-km-wide Sahin Kalesi volcanic massif. Field evidence and structural features inferred from the DEM and remote sensing data strongly suggest that this massif is a resurgent doming associated with the Gördeles ignimbrite eruption. High-resolution ERS1, SPOT and DEM images reveal that the transtensive regime, active at least since the Miocene, influenced the location of eruptive centers and caldera complexes in Cappadocia. The two caldera complexes are located in transtensive grabens. The subsidence of these grabens, continuing after the caldera collapse events, most likely resulted in the burying of the calderas and could explain the difficulties in identifying them in the field.     

Genesis of dacitic magmatism at batur volcano, Bali, Indonesia: Implications for the origins of stratovolcano calderas

Batur is an active stratovolcano on the island of Bali, Indonesia, with a large, well-formed caldera whose formation is correlated with the eruption about 23,700 years ago of a thick ignimbrite sheet. Our study of the volcanic stratigraphy and geochemistry of Batur shows the formation of the caldera was signalled by a change in the composition of the erupting material from basaltic and andesitic to dacitic. The dacitic rocks are glassy, possess equilibrium phenocryst assemblages, and display compositional characteristics consistent with an origin by crystal-liquid fractionation from more mafic parent magmas in a shallow chamber, possibly at 1.5 km depth and 1000–1070°C.However, although separated by a gap of 6 wt.% SiO₂, the dacitic rocks are clearly related in their minor- and trace-element geochemistry to those basalts and basaltic andesites erupted after the caldera was formed rather than to the andesites erupted immediately before the dacites first appeared. We infer from this and published experimental modelling of the possible crystallization behaviour of basaltic magma chambers that a magmatic cycle involving caldera formation began independently of the previous activity of Batur by formation of a new, closed-system magma chamber beneath the volcano. Fractional crystallization, possibly at the walls of the chamber, led to the early production of derivative siliceous magmas and, consequently, to caldera formation, while most of the magma retained its original composition. The postcaldera Batur basalts represent the largely undifferentiated core liquids of this chamber.This model contrasts with the traditional evolutionary model for stratovolcano calderas but may be applicable to the origins of calderas similar to that of Batur, particularly those in volcanic island arcs.     

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

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.     

Evolution of the Bayramiç magmatic complex, northwestern Anatolia

During the Oligocene–Middle Miocene period widespread magmatic activity developed in Western Anatolia, following the continental collision between the Sakarya continent and the Tauride–Anatolide platform. This produced both intrusive and extrusive rocks, which appear to be associated in space and time, as exemplified from the Bayramiç area. In the Bayramiç area, the magmatic activity started with the intrusion of the Evciler granite, and the coeval lower volcanic association. This was followed by the development of the upper volcanic association. These rock groups form collectively the Bayramiç magmatic complex, which was generated under an on-going north–south compressional regime. The Bayramiç magmatic complex has a subalkaline composition, displaying a calcalkaline trend. Trace elements and REE contents resemble to island-arc and collision-related magmas. According to the isotope values the Bayramiç magmatic complex was derived from the magmas of lithospheric mantle origin, which were later contaminated, while passing through the thick continental crust, in a post-collisional tectonic setting, during the Oligocene–Early Miocene period. The latest product of the magmatism is the Late Miocene–Pliocene basalt lavas. Their geochemical properties are clearly different from the Oligocene–Early Miocene magmatic rocks. The basalts were generated when the north–south compression gave way to the north–south extensional regime.     

Neogene ignimbrites of the Nevsehir plateau (Central Turkey): stratigraphy, distribution and source constraints

In Anatolia (Turkey), extensive calc-alkaline volcanism has developed along discontinuous provinces from Neogene to Quaternary times as a consequence of plate convergence and continental collision. In the Nevsehir plateau, which is located in the Central Anatolian Volcanic Province, volcanism consists of numerous monogenetic centres, several large stratovolcanoes and an extensive, mainly Neogene, rhyolitic ignimbrite field. Vent and caldera locations for the Neogene ignimbrites were not well known based on previous studies.In the Neogene ignimbrite sequence of the Nevsehir plateau, we have identified an old group of ignimbrites (Kavak ignimbrites) followed by five major ignimbrite units (Zelve, Sarimaden Tepe, Cemilköy, Gördeles, Kizilkaya) and two smaller, less extensive ones (Tahar, Sofular). Other ignimbrite units at the margin of the plateau occur as outliers of larger ignimbrites whose main distributions are beyond the plateau. Excellent exposure and physical continuity of the units over large areas have allowed establishment of the stratigraphic succession of the ignimbrites as, from bottom to top: Kavak, Zelve, Sarimaden Tepe, Cemilköy, Tahar, Gördeles, Sofular, Kizilkaya. Our stratigraphic scheme refines previous ones by the identification of the Zelve ignimbrite and the correlation of the previously defined ‘Akköy’ ignimbrite with the Sarimaden Tepe ignimbrite. Correlations of distant ignimbrite remnants have been achieved by using a combination a field criteria: (1) sedimentological characterisitics; (2) phenocryst assemblage; (3) pumice vesiculation texture; (4) presence and characteristics of associated plinian fallout deposits; and (5) lithic types. The correlations significantly enlarge the estimates of the original extent and volume of most ignimbrites: volumes range between 80 km³ and 300 km³ for the major ignimbrites, corresponding to 2500–10,000 km³ in areal extent.The major ignimbrites of the Nevsehir plateau have an inferred source area in the Derinkuyu tectonic basin which extends mainly between Nevsehir and the Melendiz Dag volcanic complex. The Kavak ignimbrites and the Zelve ignimbrite have inferred sources located between Nevsehir and Derinkuyu, coincident with a negative gravity anomaly. The younger ignimbrites (Sarimaden Tepe, Cemilköy, Gördeles, Kizilkaya) have inferred sources clustered to the south between the Erdas Dag and the Melendiz Dag volcanic complex. We found evidence of collapse structures on the northern and southern flanks of the Erdas Dag volcanic massif, and of a large updoming structure in the Sahinkalesi Tepe massif. The present-day Derinkuyu tectonic basin is mostly covered with Quaternary sediments and volcanics. The fault system which bounds the basin to the east provides evidence that the ignimbrite volcanism and inferred caldera formation took place in a locally extensional environment while the basin was already subsiding. Drilling and geophysical prospecting are necessary to decipher in detail the presently unknown internal structure of the basin and the inferred, probably coalesced or nested, calderas within it.     

On the conditions of sheet injections and eruptions in stratovolcanoes

Nearly all eruptions in stratovolcanoes (composite volcanoes, central volcanoes) are supplied with magma through fractures. Consequently, a primary physical condition for an eruption to occur in a stratovolcano is that a magma-driven fracture is able to propagate to the surface. Magma-filled fractures, frozen or fluid, are referred to as sheet intrusions. More specifically, they are named dykes when subvertical, and inclined (or cone) sheets when inclined. Field observations indicate that most sheet intrusions do not reach the surface to feed eruptions but rather become arrested at various crustal depths. For this reason periods of volcanic unrest with sheet injections are much more common than volcanic eruptions. Whether a sheet intrusion becomes arrested or, alternatively, propagates to the surface depends primarily on the stress field in the stratovolcano. A stratovolcano normally consists of layers of contrasting mechanical properties, such as soft (low Youngs modulus) pyroclastic units and stiff (high Youngs modulus) lava flows. We present numerical models indicating that volcanoes composed of such layers commonly develop stress fields encouraging sheet and dyke arrest. The models indicate that a necessary condition for a sheet intrusion to reach the surface and feed a volcanic eruption is that the stress field along the sheet pathway becomes homogenised. We propose that much of the activity in a stratovolcano during a volcanic cycle encourages stress-field homogenisation. Field studies show that the sheet intrusions in individual stratovolcanoes have various dips: some are vertical dykes, others inclined sheets, and still others horizontal sills. Analytical models indicate that the dip of a sheet reaching the surface can have great effects on the magma transport during an eruption. This effect is normally greater for a flat volcano such as a collapse caldera than for a stratovolcano that forms a topographic high. We conclude that the shallower the dip of a sheet intrusion, the less will be its volumetric magma transport to the surface of a stratovolcano.Editorial responsibility: D Dingwell     

Double Glacier Volcano,a ‘new’ Quaternary volcano in the eastern Aleutian volcanic arc

The Double Glacier Volcano (DGV) is a small dome complex of porphyritic hornblende andesite and dacite that is part of the Cook Inlet segment of Quaternary volcanoes of the eastern Aleutian arc. Its discovery reduces the previously described large volcano gap in Cook Inlet segment to a distance similar to that between other volcanoes in the area. DGV lavas are medium-K, calcalkaline andesites and dacites with concentrations of major and minor elements similar to the other Quaternary volcanoes of the Cook Inlet segment. Available K-Ar ages indicate that DGV was active 600–900 ka.     

Reactivation of basement faults beneath volcanoes: a new model of flank collapse

In order to explain the presence of voluminous volcanic debris avalanche deposits around a stratovolcano, reactivation of vertical faults beneath a volcanic cone has been tested using analogue models. Reactivation of a single vertical fault beneath a cone generates a normal fault and an upturning of the layers creating a bulge on the flank. The upturning induces a flank collapse characterized by a typical horseshoe-shaped scar called an avalanche caldera. Reactivation of two vertical faults beneath a cone also generates a normal fault and a summit bulge. This bulge may result from the movement along a reverse fault. A large collapse is generated within the angle created by the two vertical faults. The angle of the collapse can be up to 140° whereas this angle is typically 120° for a dome intrusion. Collapse is instantaneous and is favoured by the presence of ductile layers (ash-and-pumice formations in the example considered) in a stratovolcano complex. The model may be applicable to volcanoes in a state of dormancy (or extinction) in regions with active regional tectonism. We suggest this mechanism of collapse in the case of the Cantal stratovolcano (Massif Central, France) to explain the presence of voluminous volcanic debris avalanche deposits around this volcano.     

Relationships between mineralization and silicic volcanism in the Central Andes

Studies of late Tertiary silicic volcanic centres in the Western and Eastern Cordilleras of the Central Andes show that three volcanic environments are appropriate sites for mineralization: (1) ring-fracture extrusions post-dating large calderas; (2) similar extrusions within ignimbrite shields; and (3) isolated, small silicic volcanoes. Subvolcanic tin mineralization in the Eastern Cordillera is located in silicic stocks and associated breccias of Miocene age. The Cerro Rico stock, Potosi, Bolivia, contains tin and silver mineralization and has an intrusion age apparently millions of years younger than that of the associated Kari Kari caldera. Similar age relationships between mineralization and caldera formation have been described from the San Juan province, Colorado. The vein deposits of Chocaya, southern Bolivia, were emplaced in the lower part of an ignimbrite shield, a type of volcanic edifice as yet unrecognized in comparable areas of silicic volcanism. The El Salvador porphyry copper deposit, Chile, is related to silicic stocks which may have been intruded along a caldera ring fracture. Cerro Bonete, Chile, provides a modern example of the volcanic superstructure which may have overlain isolated mineralized stocks and breccia pipes such as that of Salvadora at Llallagua, Bolivia.Existing models for the genesis of porphyry copper deposits suggest that they formed in granodioritic stocks located in the infrastructure of andesitic stratovolcanoes. Sites of porphyry-type subvolcanic tin mineralization in the Eastern Cordillera of Bolivia are distinguished by the absence of such andesitic structures. The surface expression of a typical subvolcanic porphyry tin deposit was probably an extrusive dome of quartz latite porphyry, sometimes related to a larger caldera structure. Evidence from the El Salvador porphyry copper deposit in the Eocene magmatic belt in Chile suggests that it too may be more closely related to a silicic volcanic structure than to an andesitic stratovolcano.The dome of La Soufriere, Guadeloupe is proposed as a modern analog for the surface expression of subvolcanic mineralization processes, the phreatic eruptions there suggesting the formation of hydrothermal breccia bodies in depth. Occurrence of mineralized porphyries, millions of years after caldera formation, does not necessarily indicate that intrusions and mineralization are not genetically related to the sub-caldera pluton, but may be a consequence of the long thermal histories (1–10 million years) of the lowermost parts of large plutons. Caldera formation can only inhibit mineralization by dispersal of ore metals when these are of magmatic origin, and ignimbrites should not be taken as being unlikely to be associated with porphyry mineralization. Whether ore metals are of wall rock or magmatic origin, the key to understanding the relationships between silicic volcanism and mineralization lies in the fractionation of trace elements within large zoned magma chambers during their igneous history, and their subsequent hydrothermal migration. Small, highly mineralized intrusions formed late in a caldera cycle (such as the Cerro Rico) may be due to the introduction of fresh supplies of mafic magma into the lower parts of the main pluton.     

Fission track dating obsidians in Central and Northern Anatolia

Twenty obsidian samples from Central and North Anatolia were dated by the fission track method. Split samples analyzed in two different laboratories give consistent results. Data presented refer to three volcanic fields and are summarized as follows: Orta-Sakaeli (Northern Anatolia) obsidians with ages from 21–23 Ma; Göllü Dagi dome complex near Çiftlik (Central anatolia) has given ages between 1.33–0.98 Ma; Acigöl caldera (Central Anatolia) shows three age groups: (1) Eruptions of precaldera (Bogazköy) obsidians with ages 0.18–0.15 Ma; (2) Formation of the large intracaldera dome Kocadag-Taskesiktepe at about 0.08 Ma (uncorrected apparent age of 0.075 Ma); and (3) Acigöl peripheral obsidian domes with plateau ages of 0.020 and 0.019 Ma, and an additional uncorrected age of 0.018 Ma form a distinct age group. This fission track parameters discriminate between the various obsidian sources of prehistoric obsidian artifacts.     

Volcanoes of the Cape Hoskins area,New Britain,territory of Papua and New Guinea

One active and ten extinct Quaternary volcanoes are described from the Cape Hoskins area, on the north coast of New Britain. They are mostly strato volcanoes built up of lava flows, lava domes, pyroclastic flows, lahars, tephra, and derived alluvial sediments. The volcanic products range in composition from basalt to rhyolite, but basaltic andesite and andesite predominate. Much of the area is covered by tephra, several metres thick, consisting mainly of rhyolitic pumice. The active volcano, Pago, is built up of several glacier-like lava flows, the last of which was formed during an eruption in 1914–18. Pago lies within a well-preserved caldera forming the central part of a broad low-angle cone, named Witori, which consists largely of welded and unwelded pyroclastic flow deposits. C-14 dates obtained on charcoal indicate that the caldera eruption occurred about 2500 years B. P. Another caldera of similar age lies south of Witori. Of the other eight volcanoes described four are relatively well-preserved steep-sided cones formed mainly of lava flows, one is a remnant of a low-angle cone with a caldera, and three are deeply eroded cones which have none of their constructional surfaces preserved.     

Contractional tectonics and magma paths in volcanoes

This study aims to contribute a possible explanation for magma migration within volcanoes located in contractional tectonic settings, based on field data and physically-scaled experiments. The data demonstrate the occurrence of large stratovolcanoes in areas of coeval reverse faulting, in spite of the widely accepted idea that volcanism can develop only in extensional/transcurrent tectonic settings. The experiments simulate the propagation of deformation from substrate reverse faults with different attitudes and locations into volcanoes. The substrate fault splits into two main shear zones within the volcano: A shallow-dipping one, with reverse motion, propagates towards the lower volcano flank, and a steeper-dipping one, with normal motion, propagates upwards. In plan view, the reverse fault zone is arcuate and convex outwards, whereas the normal fault zone is rectilinear. Structural field surveys at volcanoes located in contractional settings show similar features: The Plio–Quaternary Trohunco and Los Cardos–Centinela volcanic complexes (Argentina) lie above Plio–Quaternary reverse faults. The Late Pleistocene–Holocene El Reventador volcano (Ecuador) is also located in a coeval contractional tectonic belt. These volcanoes show curvilinear reverse faults along one flank and rectilinear extensional fracture zones across the crater area, consistent with the experiments. These data consistently suggest that magma migrates along the substrate reverse fault and is channelled along the normal fault zone across the volcano.     

Late Quaternary deformation on the island on Pantelleria: New constraints for the recent tectonic evolution of the Sicily Channel Rift (southern Italy)

Structural observations carried out on the volcanic Island of Pantelleria show that the tectonic setting is dominated by NNE trending normal faults and by NW-striking right-lateral strike-slip faults with normal component of motion controlled by a ≈N 100°E oriented extension. This mode of deformation also controls the development of the eruptive fissures, dykes and eruptive centres along NNE–SSW belts that may thus represent the surface response to crustal cracking with associated magma intrusions. Magmatic intrusions are also responsible for the impressive vertical deformations that affect during the Late Quaternary the south-eastern segment of the island and producing a large dome within the Pantelleria caldera complex. The results of the structural analysis carried out on the Island of Pantelleria also improves the general knowledge on the Late Quaternary tectonics of the entire Sicily Channel. ESE–WNW directed extension, responsible for both the tectonic and volcano-tectonic features of the Pantelleria Island, also characterizes, at a greater scale, the entire channel as shown by available geodetic and seismological data. This mode of extension reactivates the older NW–SE trending fault segments bounding the tectonic troughs of the Channel as right-lateral strike-slip faults and produces new NNE trending pure extensional features (normal faulting and cracking) that preferentially develop at the tip of the major strike-slip fault zones. We thus relate the Late Quaternary volcanism of the Pelagian Block magmatism to dilatational strain on the NNE-striking extensional features that develop on the pre-existing stretched area and propagate throughout the entire continental crust linking the already up-welled mantle with the surface.     

Evolution of the Cañadas edifice and its implications for the origin of the Cañadas Caldera (Tenerife, Canary Islands)

The volcano-stratigraphic and geochronologic data presented in this work show that the Tenerife central zone has been occupied during the last 3 Ma by shield or central composite volcanoes which reached more than 3000 m in height. The last volcanic system, the presently active Teide-Pico Viejo Complex began to form approximately 150 ka ago. The first Cañadas Edifice (CE) volcanic activity took place between about 3.5 Ma and 2.7 Ma. The CE-I is formed mainly by basalts, trachybasalts and trachytes. The remains of this phase outcrop in the Cañadas Wall (CW) sectors of La Angostura (3.5–3.0 Ma and 3.0–2.7 Ma), Boca de Tauce (3.0 Ma), and in the bottom of some external radial ravines (3.5 Ma). The position of its main emission center was located in the central part of the CC. The volcano could have reached 3000 m in height. This edifice underwent a partial destruction by failure and flank collapse, forming debris-avalanches during the 2.6–2.3 Ma period. The debris-avalanche deposits can be seen in the most distal zones in the N flank of the CE-I (Tigaiga Breccia). A new volcanic phase, whose deposits overlie the remains of CE-I and the former debris-avalanche deposits, constituted a new volcanic edifice, the CE-II. The dyke directions analysis and the morphological reconstruction suggest that the CE-II center was situated somewhat westward of the CE-I, reaching some 3200 m in height. The CE-II formations are well exposed on the CW, especially at the El Cedro (2.3–2.00 Ma) sector. They are also frequent in the S flank of the edifice (2.25–1.89 Ma) in Tejina (2.5–1.87 Ma) as well as in the Tigaiga massif to the N (2.23 Ma). During the last periods of activity of CE-II, important explosive eruptions took place forming ignimbrites, pyroclastic flows, and fall deposits of trachytic composition. Their ages vary between 1.5 and 1.6 Ma (Adeje ignimbrites, to the W). In the CW, the Upper Ucanca phonolitic Unit (1.4 Ma) could be the last main episode of the CE-II. Afterwards, the Cañadas III phase began. It is well represented in the CW sectors of Tigaiga (1.1 Ma–0.27 Ma), Las Pilas (1.03 Ma–0.78 Ma), Diego Hernández (0.54 Ma–0.17 Ma) and Guajara (1.1 Ma–0.7 Ma). The materials of this edifice are also found in the SE flank. These materials are trachybasaltic lava-flows and abundant phonolitic lava and pyroclastic flows (0.6 Ma–0.5 Ma) associated with abundant plinian falls. The CE-III was essentially built between 0.9 and 0.2 Ma, a period when the volcanic activity was also intense in the ‘Dorsal Edifice' situated in the easterly wing of Tenerife. The so called ‘valleys' of La Orotava and Güimar, transversals to the ridge axis, also formed during this period. In the central part of Tenerife, the CE-III completed its evolution with an explosive deposit resting on the top of the CE, for which ages from 0.173 to 0.13 Ma have been obtained. The CC age must be younger due to the fact that the present caldera scarp cuts these deposits. On the controversial origin of the CC (central vertical collapse vs. repeated flank failure and lateral collapse of mature volcanic edifices), the data discussed in this paper favor the second hypothesis. Clearly several debris-avalanche type events exist in the history of the volcano but most of the deposits are now under the sea. The caldera wall should represent the proximal scarps of the large slides whose intermediate scarps are covered by the more recent Teide-Pico Viejo volcanoes.     

Tyatya Volcano, southwestern Kuril arc: Recent eruptive activity inferred from widespread tephra

Abstract Tyatya Volcano, situated in Kunashir Island at the southwestern end of Kuril Islands, is a large composite stratovolcano and one of the most active volcanoes in the Kuril arc. The volcanic edifice can be divided into the old and the young ones, which are composed of rocks of distinct magma types, low‐ and medium‐K series, respectively. The young volcano has a summit caldera with a central cone. Recent eruptions have occurred at the central cone and at the flank vents of the young volcano. We found several distal ash layers at the volcano and identified their ages and sources, that is, tephras of ad 1856, ad 1739, ad 1694 and ca 1 Ka derived from three volcanoes of Hokkaido, Japan, and caad 969 from Baitoushan Volcano of China/North Korea. These could provide good time markers to reveal the eruptive history of the central cone, which had continued intermittently with Strombolian eruptions and lava flow effusions since before 1 Ka. Relatively explosive eruptions have occurred three times at the cone during the past 1000 years. We revealed that, topographically, the youngest lava flows from the cone are covered not by the tephra of ad 1739 but by that of ad 1856. This evidence, together with a report of dense smoke rising from the summit in ad 1812, suggests that the latest major eruption with lava effusion from the central cone occurred in this year. In 1973, after a long period of dormancy, short‐lived phreatomagmatic eruptions began to occur from fissure vents at the northern flank of the young volcano. This was followed by large eruptions of Strombolian to sub‐Plinian types occurring from several craters at the southern flank. The 1973 activity is evaluated as Volcanic Explosivity Index = 4 (approximately 0.2 km³), the largest eruption during the 20th century in the southwestern Kuril arc. The rocks of the central cone are strongly porphyritic basalt and basaltic andesite, whereas the 1973 scoria is aphyric basalt, suggesting that magma feeding systems are definitely different between the summit and flank eruptions.     

Thermo-magnetic measurements as a possible tool in the prediction of volcanic activity in the volcanoes of the Rabaul Caldera, Papua New Guinea

Volcano-magnetic effects provide a possible way of monitoring temperature or stress changes within a volcano. A systematic study of several volcanoes in and close to the Rabaul caldera in east New Britain, Papua New Guinea, has been made to identify those in which thermal changes would be expected to cause changes in magnetic field. Two out of nine were chosen as suitable for a programme of magnetic monitoring which commenced in February 1974 and is continued at approximately six monthly intervals. The main criterion used for selection is that the volcano must have a magnetic structure causing a strong anomaly in magnetic field with its source located where its temperature would respond to changing conditions in the volcano. In the first three years of monitoring, no thermomagnetic effects have been observed, but the ongoing programme may be a useful tool in the prediction of volcanic eruption. Estimation of the magnitude of possible field changes depends on determining a satisfactory model of the source of the magnetic anomaly and how the remanent magnetization of the source depends on temperature. The former is a standard problem in geomagnetism and may be solved by computer modelling methods. Measurements of the temperature dependence of thermo-remanent magnetization (TRM) for rocks collected in the Rabaul caldera show the latter to be a much more difficult problem to solve in detail. This is because the temperature range in which TRM changes most rapidly with temperature varies greatly from specimen to specimen, and depends both on the magnetic minerals present and the cooling history of the rock. Rocks from within the anomaly sources were not accessible so doubt remains as to how similar their magnetic properties are to those of rocks collected from the surface. Within this limitation, it is shown that for at least one of the volcanoes studied, changes of total magnetic field of several hundred nanoteslas (1 nT = 1γ) are possible prior to a future eruption.