Luigina Vezzoli

Continental arc volcanism and tectonic setting in Central Anatolia, Turkey

Abstract The Neogene and Quaternary volcanism of Central Anatolia represents the central sector of the Anatolian Volcanic Arc. related to continental collision between the Afro-Arabian and Eurasian plates. It is closely associated with a complex system of tectonic depressions related to brittle deformations of transtentional type and which commenced in the Late Miocene. The volcanism here considered can be divided into three main periods of activity, separated by important deformative and erosive events. The first period is represented by a mostly andesitic effusive activity. The second period is characterized by the emplacement of a thick ignimbritic sequence and shows an areal distribution up to 11,000 km 2 . Seven ignimbrite units have been recognized. The three main units were found at a distance of more than 100 km from the presumed source area. Geological and sedimentological data lead us to recognize the Melendiz Dag volcanic complex and the Ciftlik caldera as the probable ignimbrites source vent. During the third period great andesitic-basaltic stratovolcanoes and a number of prevalently acid monogenic centres developed. The relationship between the volcanic activity is clearly conditioned by the main transcurrent fault systems present in Central Anatolia. The Neogene-Quaternary volcanic activity prevalently developed along the ENE-WSW Karaman-Sivas lineament. Most of the great central volcanoes developed at the intersection between the ENE-WSW trends and the Ecemis and Tuz Golu transcurrent faults. The structural interpretation of the Quaternary monogenic centres is more difficult. Probably they are related to the very recent N-S fault swarms which cross the Anatolides and the Taurus Range.

Discovery of a Plinian basaltic eruption of Roman age at Etna volcano, Italy

Basaltic Plinian eruptions are rare and poorly known volcanic phenomena. Etna is an active basaltic volcano, the activity of which is dominated by effusive eruptions that represent a continuous threat to a large populated area. We report on a Plinian eruption of basaltic composition that occurred in 122 b.c. Lapilli fallout caused extensive damage to the southern flank of the volcano. The discovery of this large explosive eruption raises important issues for previous hazard assessment at Etna and other basaltic volcanoes: An effusive basaltic volcano, generally nonhazardous for humans, can become very dangerous.

Quaternary stratigraphy of distal tephra layers in the Mediterranean—an overview

This paper reviews the results of fifty years of tephra studies in the Mediterranean basin. The tephrostratigraphy of the Eastern Mediterranean is defined well and has been developed in conjunction with detailed biostratigraphy. Currently about thirty marker tephra have been identified and characterised, some being widespread over large sectors of the basin. Sedimentation in both Tyrrhenian and Aegean Seas is influenced by the proximity of the source volcanoes and hence primary tephras frequently exhibit evidence for reworking and post-depositional phenomena. In contrast to the long marine tephrostratigraphy there are still only a few studies reporting continental tephra deposits; these mainly concern lacustrine sequences and archaeological excavations. Recent investigations have employed electron probe microanalysis of discrete glass shards. These have illustrated the considerable geochemical variability of the principal tephra markers including those related to large-volume explosive events. In some instances this variability can be shown to mirror the heterogeneous composition of related pyroclastic formations in the proximal areas. Integration of a wide range of these findings, has enabled the construction and update of distribution maps for the main markers. This will permit revised estimations of the volume of the erupted material enabling consideration of the ecological and climatic impacts of the related explosive events. However, in spite of the increased chronostratigraphical information available for many of the source areas, some significant tephra layers remain uncorrelated. This uncertainty should stimulate further investigation to enable evaluation of the volcanic hazard in these densely populated regions. q 1999 Elsevier Science B.V. All rights reserved.

Plinian pumice fall deposit of the Campanian Ignimbrite eruption (Phlegraean Fields, Italy)

Abstract A plinian pumice fall deposit associated with the Campanian Ignimbrite eruption (36 ka, Phlegraean Fields caldera, Italy) occurs at the base of the distal grey ignimbrite in 15 localities spread over an area exceeding 1500 km2 between Benevento and the Sorrentina peninsula. In the thickest stratigraphic section at Voscone (130 cm), 45 km east of the Phlegraean caldera centre (Pozzuoli), the deposit consists of two units: the lower fall unit (LFU) is well sorted, exhibits reverse size grading and is composed of equidimensional light-grey pumice clasts with very subordinate accidental lithics; the upper fall unit (UFU) is from well to poorly sorted, crudely stratified, richer in lithics and composed of both equidimensional and prolate pumice clasts. The two fall units show slightly different dispersal axis: N90° for the LFU and N95° for the UFU. Volumes calculated with the method of Pyle (1989) are about 8 km3 for the LFU and 7 km3 for the UFU. The maximum height of the eruptive columns are estimated, using the model of the maximum lithic clasts dispersal, at 44 km for the LFU and 40 km for the UFU, classifying both fall units as ultraplinian in character. Reverse size grading within the LFU suggests an increase of the height of the column and magma discharge rate with time. Moderate sorting and crude stratification of the UFU are consistent with short-period oscillation of the column, possibly associated with repeated partial column collapses. Sharp increases in lithic content at the transition to UFU and within the UFU suggest that changes in the eruptive behaviour was produced by a dramatic increase in conduit/vent erosion. The phase of column instability preceded the emplacement of widely dispersed pyroclastic flow. The ultraplinian nature of the fall fits well with the wide dispersal of the Campanian Ignimbrite with an estimated aspect ratio of 3–4×10−4 (LARI).

Tephra layers in Bannock Basin (Eastern Mediterranean)

Several volcanic ash layers were identified in cores collected in the Bannock Basin (Eastern Mediterranean) during cruises BAN-84, BAN-86 and BAN-88 of the R.V. Bannock. Lithological, microscopic, mineralogical and chemical analyses together with stratigraphic position help in identifying them as tephra layers Y-1, Y-5, X-2 and W-1 of Keller et al. (GSA Bull. 89, 1978). Tephra layer Y-1 is stiff and usually deformed, and is black to very dark brown in colour. It is mainly composed of highly vesicular micropumice with dark brown and colourless limpid glass and it has a benmoritic chemical composition, which is typical of Mt. Etna material. Tephra layer Y-5 is composed of soft, limpid vitric shards and subordinate micropumice ranging in composition from alkaliphonolitic trachyte to latite and may be correlated with the Campanian Ignimbrite eruption which occurred about 35,000 years ago in the Phlegrean Fields (the Campanian volcanic area in Italy). Tephra layer X-2 is olive-grey in colour, and is composed of micropumice and glass shards with a chemical composition ranging from alkali-phonolitic trachyte to latite; its source is probably the Campanian volcanic area. Tephra layer W-1 is dark grey and made up of micropumice and glass shards with a chemical composition ranging from tephrite-phonolite to alkali-phonolitic trachyte; its source is probably the Roman volcanic area in Italy. The volcanic layers have been identified in cores of the basin sill, in the central bulge of the basin and on the basin flanks leading up to the “cobblestone topography” of the Mediterranean Ridge; they could not, however, be identified in any core raised from beneath the anoxic hypersaline brines. Important volcanological results are: (a) an extension of the areal distribution of Y-1, X-2 and W-1, (b) correlation of Y-1 with the Biancavilla Ignimbrite of Mt. Etna dated in previous works at about 14,000 yrs B.P., and (c) determination of the bimodal chemical composition of Y-5 showing that the latter has a composition in accordance with that of the Campanian Ignimbrite in the Phlegrean Fields.

Miocene magmatism and tectonics of the easternmost sector of the Calama–Olacapato–El Toro fault system in Central Andes at ~24°S: Insights into the evolution of the Eastern Cordillera

The Miocene Las Burras–Almagro–El Toro magmatic complex lies ~300 km to the east of the Central Andes volcanic arc, in the easternmost sector of the transverse Calama–Olacapato–El Toro fault zone. The magmatic rocks of the Las Burras–Almagro–El Toro complex comprise a monzogabbro to monzogranite laccolith like intrusion and basaltic andesite to dacite volcanic rocks that include seven lithostratigraphic members. New Rb-Sr dates indicate that the intrusive rocks are ca. 14 Ma, and K-Ar dates suggest emplacement ages of ca. 12.8–6.4 Ma for the volcanic rocks. The emplacement of the intrusion was controlled by N-S–striking strike-slip faults in a context of oblique convergence; the volcanism, which occurred along WNW-ESE– and N-S–striking extensional faults, relates to the Calama–Olacapato–El Toro fault zone. Two magmatic phases were recognized. Intrusive and volcanic rocks of the older magmatic phase (ca. 14–13 Ma) are characterized by Ba/Nb (7–14), La/Ta (11–18), and isotopic ratios ( 87 Sr/ 86 Sr: 0.704339–0.705281, 143 Nd/ 144 Nd: 0.512713–0.512598), which are intraplate characteristics. The source of the older magmas was isotopically depleted lithospheric mantle rich in K, Rb, and Th. Energy constrained–assimilation and fractional crystallization (EC-AFC) modeling indicates that fractional crystallization and crustal assimilation moderately modified magma composition during its residence in the crust. The products of the younger magmatic phase (ca. 11–6 Ma) have higher Ba/Nb (24–42) and La/Ta (24–30) and 87 Sr/ 86 Sr (0.706738–0.708729) and lower 143 Nd/ 144 Nd (0.512433–0.512360). The results of EC-AFC modeling exclude a significant role for the upper crust in the generation of the most primitive magmas of this phase. Their compositions can be explained by (1) contamination of the primary magmas having originated in a depleted mantle with a mafic crust, or (2) the contribution of isotopically enriched mantle zones. Shallow differentiation and moderate contamination by continental crust can explain the composition of the intermediate and evolved products of the younger phase. The variation of the magma source characteristics at 11 Ma is discussed in the frame of the complex geo-dynamical setting in this region.

Easter Island, SE Pacific: An end-member type of hotspot volcanism

Easter Island (Rapa Nui, Chile) is an intraoceanic volcanic island on the Easter hotspot, ~350 km E of the Eastern Pacific Rise. We match new field data with previously published age and petrochemical data to reconstruct the general evolution of the Island. This consists of three main volcanoes (Poike, Rano Kau, and the larger Terevaka), which experienced an overall similar and nearly coeval evolution, characterized by two periods: (1) buildup of a basaltic shield, culminating in the development of a summit caldera and the emission of more evolved highly porphyritic lavas (ca. 0.78–0.3 Ma); and (2) rifting along the shield flanks, by means of fissure eruptions (0.24–0.11 Ma). The trend of most eruptive fissures, NNE-SSW to NE–SW, appears to be controlled by the ~NE-SW elongated, emerged, and submerged morphology of the island. However, while the fissure-forming period at Rano Kau and Poike appears to be associated with reduced magma supply to the reservoir, at Terevaka it is characterized by the arrival of new basic magma, rejuvenating the system. The comparison to other intraoceanic volcanic islands suggests that, because of its tectonomagmatic features (low eruptive rate, scattered rift zones, and scarce lateral collapses), Easter Island represents an end-member type of hotspot volcano that is contrary to Hawaii, which represents the opposite end member.

The Cerro Mencenares volcanic center, Baja California Sur: Source and tectonic control on postsubduction magmatism within the Gulf Rift

At the end of active subduction along the North American plate (12.5 Ma), in the Baja California peninsula arc magmatism was replaced mainly by alkali-rich high-Mg basaltic andesites and andesites with peculiar and unusually high K/Rb, Sr/Rb, and LREE/HREE ratios defined “bajaite” by Rogers et al. (1985) and “Baja alkalic suite” by Sawlan (1991). Two interesting features of this magmatic change are represented by (i) emplacement of transitional, high-K calc-alkaline andesites that preceded, in places, the Baja alkalic suite magmatism and (ii) subduction unrelated medium-K calc-alkaline magmatism in Baja California Sur, active in areas within the Gulf of California Rift, for example, Tres Virgenes–Reforma caldera, Isla Coronado. A previously unknown example of this calc-alkaline activity was studied in the area of Cerro Mencenares located north of Loreto, Baja California Sur. The Mencenares volcanic complex is located along the Gulf coast, east of the main Gulf escarpment. Activity of the Mencenares volcanic complex occurred in three stages: (i) an initial phase of rhyolitic dome emplacement; (ii) andesitic to dacitic volcanism that produced the Mencenares stratovolcano; (iii) late-stage emplacement of silicic domes and flows, mostly dacite and rhyolite, along north-south–trending normal faults. Tephra and lava flows interfinger with marine sediments of the Pliocene Loreto Basin and the youngest volcanic products were probably emplaced during Quaternary times. This volcanism is coeval with the Baja alkalic suite magmatism west of the Gulf escarpment. In the latter areas, Baja alkalic suite magmas ascended rapidly with only limited differentiation. In the Mencenares volcanic complex, a complex tectonic framework favored the formation of crustal magma chambers where more complex phenomena such as extensive differentiation and mixing could take place. A change from medium-K calc-alkaline toward Baja alkalic suite–like chemical characteristics is present in the younger products of the Mencenares volcanic complex and mixing possibly occurred between medium-K calc-alkaline and Baja alkalic suite magmas. Magma evolution in the Loreto area suggests that (i) Baja alkalic suite and medium-K calc-alkaline magmas can coexist in time and interact in the same volcanic field; (ii) if a relic, subduction-related component is the main cause of medium-K or high-K calc-alkaline volcanism then this component appears to decrease from east to west; and (iii) although prior calc-alkaline magmatism may not be unquestionably considered a necessary condition for the generation of Baja alkalic suite magmas, episodes of medium-K or high-K calc-alkaline melt extraction from mantle sources precede or accompany Baja alkalic suite magmatism in many instances.

Time dimension in the geochemical approach and hazard estimates of a volcanic area: The isle of Ischia case (Italy)

Abstract The volcano-tectonics and magmatic evolution of the Isle of Ischia, one of the active volcanoes of the Naples area, has been reconstructed in order to estimate the volcanic hazards in the region. Taking advantage of previous chronological and petrochemical studies, five successive volcanic phases and the magmatic processes characterizing each of them have been identified. Moreover, the high-resolution K/Ar time-scale and the trace-element, Sr-isotope and mineral chemistry distributions enable an evaluation of both the duration and the trends of these processes. Each phenomenon is related to a specific volcano-tectonic setting, including negative or positive “calderic” movements. The most recent phase is clearly related to the Mt. Epomeo horst uplift ( > 780 m in the last 33,000 years). This last phase corresponds to scattered eruptions distributed at the periphery of the horst. The geochemical features of historical eruptions suggest an evolution of the system toward a more basic bulk composition. It does not give rise to a high eruptive hazard. On the contrary, the morphological disequilibrium due to the rapid uplift of M. Epomeo horst generates the hazard of screes and mudflows. This hazard is increased by seismic crises like in the Casamicciola disaster in 1883.

Last 100 Ka Tephrostratigraphic Record of Mount Etna

The tephro- and chrono-stratigraphic synthesis of Etna pyroclastic deposits of the last 100 ka is presented here. Deposits correlation over the whole volcanic edifice allows the reconstruction of a continuous pyroclastic succession from about 100 ka to the Present. Tephra layers are composed of scoria or pumice lapilli and ash, representing pyroclastic fall or flow deposits interbedded with continental volcanogenic sedimentary deposits. They are grouped into five stratigraphic units corresponding to main periods of explosive activity. On the basis of the tephro- and chrono-stratigraphic data they are: (A) about 100 ka basaltic strombolian activity; (B) 80-100 ka benmoreitic plinian eruption that ends the activity of the Trifoglietto volcano; (C) 16-80 ka strombolian to subplinian eruptions, basaltic to mugearitic in composition, of the post-Trifoglietto (Giannicola, Salifizio and Cuvigghiuni and Ellittico) volcanoes; (D) 15-15.5 ka benmoreitic-trachitic caldera-forming plinian eruptions that end the activity of the Ellittico volcano; and (E) the last 12 ka basaltic subplinian eruptions and the 122 BC plinian eruption of the Mongibello volcano. The tephrostratigraphic reconstruction indicates that the explosive activity presented different features during the history of Etna. Eruptive styles cover a range from strombolian to plinian, producing some marker beds that have a great importance in the new geological reconstruction of Etna. High volatile content of Etna magmas seems to be the key factor for the origin of this strong explosive activity. The characterization of Etna explosive activity has given this kind of activity a new relevance and important implications for the volcanic hazard assessment at Etna, a volcano commonly considered, before these studies, just effusive.