Conditions favouring catastrophic landslides on Tenerife (Canary Islands)

Catastrophic failures of volcano flanks represent one of the most hazardous geological phenomena. These immense mass movements originate either by increasing the destabilizing forces (driving forces) or by reducing the strength of the materials involved, or both. The study of large volcanic landslides on Tenerife suggests that the presence of weak residual soils (palaeosols) in combination with the pre-existence of deep, narrow canyons created by fluvial erosion, have played a fundamental role in the initiation of large-scale sector collapses of the volcanic edifice, regardless of the triggering mechanism considered. Residual soils strongly reduce the material strength during undrained loading, while pre-existing canyons control the lateral limits of the landslide. The existence of a wet climate in some sectors of the island favours these circumstances.     

Paleoliquefaction features on Tenerife (Canary Islands) in Holocene sand deposits

On Tenerife, one of the Canary Islands, a series of clastic dikes and tubular vents is attributed to liquefaction of sediments during a high-intensity paleoearthquake. Geotechnical, geological, tectonic, and mineralogical investigations have been carried out to identify the soil composition and structure, as well as the geological processes operating in the area. Geochronological analysis has indicated an age ranging from 10,081±933 to 3490±473 years BP for the liquefaction features. The area in which these liquefaction features are found has undergone tectonic uplift and is affected by two faults. One of these faults was responsible for displacing the Holocene materials. The paleoearthquake responsible for this liquefaction has been analysed in terms of its peak ground acceleration (pga) and magnitude by back calculation analysis based on the cyclic stress and Ishihara methods. A range of 0.22–0.35g was obtained for the pga, with the value of 0.30g being selected as most representative. From this, an earthquake-modified Mercalli intensity of I_MM=IX was estimated for the liquefaction site. The magnitude-bound method and energy-based approaches were used to determine the magnitude of the paleoearthquake, providing a moment magnitude M in the range of 6.4–7.2; M=6.8 is taken as the representative figure.     

Glass-bearing crustal xenoliths (buchites) erupted during the recent activity of Stromboli (Aeolian Islands)

Three xenoliths erupted as ejecta during recent violent explosion of Stromboli volcano (Aeolian Islands) were investigated in this paper. They consist of high-temperature mineral association (cordierite, hercynite spinels, sillimanite, ±plagioclase, ±mullite, ±corundum) and abundant glass (10–70 vol.%), and may be classified as buchites. The peraluminous composition of the xenoliths, their trace element distribution and REE patterns support their origin from granulite- and amphibolite-facies metapelites of the Calabrian continental crust, which is considered the crystalline basement beneath Stromboli. Buchites have an isotopic composition comparable to that of Stromboli extrusives and significantly different from that of the Calabrian basement.

The glass is generally colourless and has a Si–Al–alkali-rich composition, except for one sample where a Fe–Mg–Ca-rich reddish-brown glass also occurs. These two kinds of glass show complicated textures where patches of reddish-brown glass are often surrounded by plagioclase and/or cordierite or forms streaks and swirls with the colourless glass. Cordierite, plagioclase and oxides have different compositions according to their position in the xenoliths. Ca-rich plagioclase (An_72–95), Mg-poor cordierite (Mg-values 47–66) and Al-rich spinels are in the inner portions of the xenoliths and associated with colourless glass; on the contrary, close to the contact with the host lava or associated with coloured glass, cordierite shows higher Mg-values, Ti–Fe-bearing oxides occur and plagioclase is chemically similar to the basalt phenocrysts (An_66–71).

The abundant and fresh glass and the idiomorphic shape of the high-temperature minerals suggest that the xenoliths were hold in the basaltic magma, before its extrusion, for a significant time to allow their partial or nearly complete melting and subsequent nucleation and growth of new phases. During this stage, the interaction between the anatectic liquid and the basaltic magma affected the original isotopic composition of the xenoliths and, in some cases, produced glass and mineral phases (cordierite, plagioclase and oxides) with different composition.     

Assessment and Modelling of Lava Flow Hazard on Lanzarote (Canary Islands)

This paper presents an evaluation of the lava flowhazard on Lanzarote (Canary Islands) by means of aprobabilistic maximum slope model. This model assumesthat the topography plays the major role indetermining the path that a lava flow will follow. Thearea selected for containing future emission centreshas been chosen taking into account thecharacteristics of the recent eruptive activity andthe present activity of the island. The results of thesimulations constitute hazard maps whose values ateach point represent the probability of being coveredby lava. These results are qualitatively analysed toprovide some indication of the risk to the lifelines(electricity, drinking water etc.) of the island.     

Peralkaline acid tendencies in Gran Canaria (Canary Islands)

The study of a volcanic series from the island of Gran Canaria (Canary Islands) in which alkaline and peralkaline, saturated and undersaturated rocks coexist, is reported here. Materials with high volatile content (ignimbritic trachytes) were first emitted and the series ended with the eruption of phonolitic lavas. The average peralkalinity index in these rocks is typically about 1.0 and, therefore, peralkaline rocks coexist with non-peralkaline ones. However, a maximum in peralkalinity is found in the ignimbritic rocks of the lower part of the series. In spite of the evident acid peralkaline tendencies of these rocks, it does not seem appropriate to classify them as pantellerites or comendites. Nor are they consistent with the genetic processes proposed for rocks of similar composition and oceanic environment.The crystallization of the feldspars controls the variation trends among the different magmas but the fractionation alone does not sufficiently explain the genesis of successive fluids. Various factors seem to point to the important role which a gas-transfer process causing a geochemical stratification inside the magmatic chamber may have played.The occurrence of peralkaline silicics at Gran Canaria, which is located for away from the active Mid-Atlantic ridge, is not related to transitional basalts. These rocks are a deviation from the main undersaturated alkalic trend which characterizes the volcanism of the Canary Islands, their genesis being related to the realization of favourable local volcanic conditions.     

The Cambrian of Bennett Island (New Siberian Islands)

The paper presents new data on the Cambrian stratigraphy of Bennett Island, one of the least explored East Arctic islands. The section, about 500 m of total thickness, comprises four lithological units that store a record of the deposition history: (1) clastic sediments including storm sandstones; (2) shallow-marine mudstone; (3) lagoonal variegated mudstone and limestone; (4) black shale. It is suggested to classify the units as formations with their proper names. The section spans all epoches of the Cambrian stratigraphy constrained by trilobite fossils. In the Cambrian, territory of the island belonged to Siberia rather than to some exotic terrane, judging by abundant endemic Siberian trilobite species in the Bennett section. This inference is supported by synchronicity in recorded deposition events of Bennett Island and northeastern Siberia (Kharaulakh Mountains). The Cambrian sediments of the two areas were deposited in different parts of a single shallow sea which extended as far as Taimyr.     

Early Paleozoic tectonics for the New Siberian Islands terrane (Eastern Arctic)

The New Siberian Islands archipelago is one of the few research objects accessible for direct study on the eastern Arctic shelf. There are several models that have different interpretations of the Paleozoic tectonic history and the structural affinity of the New Siberian Islands terrane. Some infer a direct relationship with the passive continental margin of the Siberian paleocontinent. Others connect it with the marginal basins of Baltica and Laurentia, or the Chukotka-Alaska microplate. Our paleomagnetic investigation led us to create an apparent polar wander path for the early Paleozoic interval of geological history. Based on it we can conclude that the New Siberian Islands terrane could not have been a part of these continental plates. This study considers the possible tectonic scenarios of the Paleozoic history of the Earth, presents and discusses the corresponding global reconstructions describing the paleogeography and probable mutual kinematics of the terranes of the Eastern Arctic.     

New data on the age of the glaciation in the New Siberian Islands (Russian Eastern Arctic)

To estimate the age of the glaciation in the New Siberian Islands, fossils of small mammals from the Sana-Balagan site (Faddeevsky Island, Russian Eastern Arctic) have been studied. The evolutionary degree of this fauna, which indicates the age of the sediments underlying the glacial deposits, suggests that the glaciation of the New Siberian Islands began no earlier than 190–210 thousand years ago. The new biochronological data in combination with ²³⁰Th/²³⁴U dates show that the geological event in question was synchronous with the Moscow glaciation or the Moscow stage of the Dnieper glaciation (the Vychegda glaciation, the final stage of the Saalian, the final stage of the Riss, the end of the MIS 6, 180–140 thousand years ago).     

Rifting,recurrent landsliding and Miocene structural reorganization on NW-Tenerife (Canary Islands)

We studied mechanisms of structural destabilization of ocean island flanks by considering the linkage between volcano construction and volcano destruction, exemplified by the composite Teno shield volcano on Tenerife (Canary Islands). During growth, Tenerife episodically experienced giant landslides, genetically associated with rifting and preferentially located between two arms of a three-armed rift system. The deeply eroded late Miocene Teno massif allows insights into the rifting processes, the failure mechanisms and related structures. The semicircular geometry of palaeo-scarps and fracture systems, breccia deposits and the local dike swarm reconfigurations delineate two clear landslide scarp regions. Following an earlier collapse of the older Los Gigantes Formation to the north, the rocks around the scarp became fractured and intruded by dikes. Substantial lava infill and enduring dike emplacement increased the load on the weak scarp and forced the flank to creep again, finally resulting in the collapse of the younger Carrizales Formation. Once more, the changing stress field caused deformation of the nearby rocks, a fracture belt formed around the scarp and dikes intruded into new (concentric) directions. The outline, size and direction of the second failed flank of Teno very much resembles the first collapse. We suggest structural clues concerning mechanisms of recurrent volcano flank failure, verifying the concept that volcano flanks that have failed are prone to collapse again with similar dimensions.     

Erupted cumulate fragments in rhyolites from Lipari (Aeolian Islands)

Over the last ~267 ky, the island of Lipari has erupted magmas ranging in compositions from basaltic andesites to rhyolites, with a notable compositional gap in the dacite field. Bulk geochemical and isotopic compositions of the volcanic succession, in conjunction with major and trace elemental compositions of minerals, indicate that the rhyolites were dominantly generated via crystal fractionation processes, with subordinate assimilation. Radiogenic (Sr, Nd, and Pb) and stable (O) isotopes independently suggest ≤30 % of crustal contamination with the majority of it occurring in mafic compositions, likely relatively deep in the system. Within the rhyolites, crystal-rich, K₂O-rich enclaves are common. In contrast to previous interpretations, we suggest that these enclaves represent partial melting, remobilization and eruption of cumulate fragments left-over from rhyolite melt extraction. Cumulate melting and remobilization is supported by the presence of (1) resorbed, low-temperature minerals (biotite and sanidine), providing the potassic signature to these clasts, (2) reacted Fo-rich olivine, marking the presence of mafic recharge, (3) An_38–21 plagioclase, filling the gap in feldspar composition between the andesites and the rhyolites and (4) strong enrichment in Sr and Ba in plagioclase and sanidine, suggesting crystallization from a locally enriched melt. Based on Sr-melt partitioning, the high-Sr plagioclase would require ~2300 ppm Sr in the melt, a value far in excess of Sr contents in Lipari and Vulcano magmas (50–1532 ppm) but consistent with melting of a feldspar-rich cumulate. Due to the presence of similar crystal-rich enclaves within the rhyolites from Vulcano, we propose that the eruption of remobilized cumulates associated with high-SiO₂ rhyolites may be a common process at the Aeolian volcanoes, as already attested for a variety of volcanic systems around the world.     

Partial melting below Tubuai (Austral Islands, French Polynesia)

Process identification diagrams based on trace element data show that mafic lavas from Tubuai, including alkali basalts, basanites, analcitites and nephelinites, result from different degrees of partial melting of an isotopically homogeneous mantle source. Our fractionation-corrected data are consistent with a batch melting model or a dynamic melting model involving a threshold value for melt separation close to 1% and degrees of melting ranging from 5–8% (alkali basalts) to 1.5–3% (nephelinites). The relative source concentration pattern, calculated using an inverse numerical method, shows an enrichment in highly incompatible elements. We propose that the Tubuai lava suite was derived from a two-stage partial melting process. Melting first affected the plume material located within the transition zone between garnet and spinel domains, producing alkali basalts and basanites. Then, the melting zone migrated upwards to the base of the overlying spinel-bearing lithospheric mantle, producing highly silica-undersaturated lavas. The lower lithosphere had previously been enriched by intrusion of pyroxenite veins representing plume-derived melts which percolated away from the main magma conduits. Received: 11 June 1996 / Accepted: 8 January 1997     

Permian-Triassic trap magmatism in Bel'kov Island (New Siberian Islands)

The study was inspired by information on Paleozoic andesites, dacites, and diabases in Bel'kov Island in the 1974 geological survey reports used to reconstruct the tectonic evolution of the continental block comprising the New Siberian Islands and the bordering shelf. We did not find felsic volcanics or Middle Paleozoic intrusions in the studied area of the island. The igneous rocks are mafic subvolcanic intrusions, including dikes, randomly shaped bodies, explosion breccias, and peperites. They belong to the tholeiitic series and are similar to Siberian traps in petrography and trace-element compositions, with high LREE and LILE and prominent Nb negative anomalies. The island arc affinity is due to continental crust contamination of mantle magma and its long evolution in chambers at different depths. The 252±5 Ma K-Ar biotite age of magmatism indicates that it was coeval to the main stage of trap magmatism in the Siberian craton at the Permian-Triassic boundary. The terrane including the New Siberian Islands occurred on the periphery of the Siberian trap province where magmatism acted in a rifting environment. Magma intruded semiliquid wet sediments at shallow depths, shortly after their deposition. Therefore, the exposed Paleozoic section in Bel’kov Island may include Permian or possibly Lower Triassic sediments, of younger ages than it was believed earlier.     

Recent dolomitization of quaternary biocalcarenites from fuerteventura (Canary Islands)

Quaternary marine and eolian biocalcarenites in the supratidal breaker and spray zone along the Barlovento coast of the peninsula Jandía, Fuerteventura (Canary Islands) are dolomitized by percolating brines with a high Mg/Ca ratio resulting from evaporation of seawater on the sediment's surface. Only fragments of calcareous algae primarily consisting of high-magnesian calcite are replaced by a cryptocrystalline variety of dolomite. Dolomite also occurs in large euhedral crystals in intraparticle and interparticle pore spaces. In the marine biocalcarenite dolomite has a composition of Ca₅₆Mg₄₄(CO₃)₂. It is well ordered.     

Origins and antiquity of the island fox (Urocyon littoralis) on California's Channel Islands

The island fox (Urocyon littoralis) is one of few reportedly endemic terrestrial mammals on California's Channel Islands. Questions remain about how and when foxes first colonized the islands, with researchers speculating on a natural, human-assisted, or combined dispersal during the late Pleistocene and/or Holocene. A natural dispersal of foxes to the northern Channel Islands has been supported by reports of a few fox bones from late Pleistocene paleontological localities. Direct AMS ¹⁴C dating of these “fossil” fox bones produced dates ranging from ∼ 6400 to 200 cal yr BP, however, postdating human colonization of the islands by several millennia. Although one of these specimens is the earliest securely dated fox from the islands, these new data support the hypothesis that Native Americans introduced foxes to all the Channel Islands in the early to middle Holocene. However, a natural dispersal for the original island colonization cannot be ruled out until further paleontological, archaeological, and genetic studies (especially aDNA [ancient DNA]) are conducted.     

Palynological Evidence for Climatic Change and Human Activity during the Holocene on Minorca (Balearic Islands)

Four pollen diagrams from Minorca (Balearic Islands) have been correlated with other previously studied sequences from Majorca and Minorca to define a Holocene landscape sequence for the region from 8000 yr B.P. to the present. The lower part of the pollen diagrams reflects a climatic phase with more rain and less-marked seasonality than today. Significant quantities ofCorylus, Buxus,and mesophilous taxa are found. In the middle part, between 5000 and 4000 yr B.P., a strong change is recorded in composition and structure of the vegetational landscape, with vegetation appearing that was adapted to Mediterranean conditions. This episode coincided with the first human colonization of the island and also with a widespread climatic change in the western Mediterranean region. The change in taxa was complex and some sclerophyllous taxa suchOleaplayed an important role in the transformation of the landscape physiognomy from the mid-Holocene until the present. Although human activities have removed much of the Mediterranean vegetation on the Balearic Islands, it seems clear that the changes have been brought about, in part, by increasing dryness.     

Geologic model of the magmatichydrothermal system of vulcano (Aeolian Islands, Italy)

Summary This paper presents a model of the active magmatic-hydrothermal (high-sulfidation) system of La Fossa volcano, based on mineralogical and geochemical studies of hydrothermal alteration on the surface and in the subsoil (geothermal wells and lithic clasts from explosive eruptions).The main engine of this system is represented by the shallow magmatic feeding system of La Fossa, which produces substantial degassing of volatiles (H₂O, S, Cl). The introduction of magmatic fluids into the conduit system causes high temperature recrystallisation and metasomatism of the volcanic and sub-volcanic rocks. Lateraly to the volcanic conduits, the magmatic fluids undergo a primary neutralization, forming neutral low permeability hydrothermal zones. During their rise to the surface, the magmatic vapours may condense in groundwater, forming acid solutions that react with rocks to form superficial hydrothermal alteration. Silicic, advanced argillic and intermediate argillic alteration facies develop. This reflects the progressive neutralisation of extremely acid fluids. High contents of trace elements, like TI and Bi, supporting evidence for magmatic fluid transport, were found close to the high temperature fumaroles (up to 500°Q in the silicic alteration zone of La Fossa.

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Ein geologisches Modell des magmatisch-hydrothermalen Systems von Vulcano, Aeolische Inseln, Italien

Zusammenfassung Diese Arbeit stellt ein Modell für das aktive magmatisch-hydrothermale (highsulfidation) System des La Fossa Vulkans vor. Dieses beruht auf mineralogischen und geochemischen Studien der hydrothermalen Umwandlung an der Oberfläche und im Untergrund (geothermale Bohrungen und lithische Klasten von explosiven Eruptionen). Als Energiequelle fungiert das seichte magmatische Zufuhrsystem von La Fossa, das signifikante Entgasung volatiler Phasen (H₂O, S, HCl) mit sich bringt. Das Eindringen magmatischer Fluide in die Zufuhrkanäle verursacht Rekristallisation und Metasomatose der vulkanischen und subvulkanischen Gesteine bei hohen Temperaturen. In lateralen Bereichen der vulkanischen Zufuhrkanäle erfahren die magmatischen Fluide eine primäre Neutralisation, wobei neutrale hydrothermale Zonen niedriger Permeabilität entstehen. Während des Aufstiegs an die Oberfläche können die magmatischen Fluide im Grundwasser kondensiert werden, wobei sie saure Lösungen bilden, die wiederum mit den Gesteinen reagieren und zu Hydrothermalalteration führen. Dabei entstehen silizische, fortgeschrittene argillische und intermediäre agillische Umwandlungsfazies. Dies entspricht der zunehmenden Neutralisation extrem saurer Fluide. Hohe Gehalte an Spurenelementen, wie TI und Bi können als zusätzliche Hinweise für magmatischen Fluidtransport gesehen werden, sie treten in der Nähe der HochTemperatur-Fumarolen (bis 500'C) in der silizischen Alterationszone von La Fossa auf.

     

Upper cretaceous (Senonian) bathyal pelagic sediments with Globotruncana from the Solomon Islands

Extremely fine‐grained pelagic oozes overlie basal basaltic lavas on Malaita Island, on the Pacific flank of the Solomon Islands group. These sediments contain up to 20% planktonic (but not benthonic) Foraminifera, but have less than 5% acid‐insoluble clay and little or no terrigenous material. They are associated with radiolarian chert and with finely disseminated marine manganese. The Foraminifera include several species of Globotruncana, referred to G. arca, G. havanensis, G. lapparenti and G. tricarinata. The sediments accumulated in a low‐energy, deep‐water environment (exceeding 1,000 m.) during Late Cretaceous (Senonian) time, probably in the late Senonian. These sediments are the oldest in the Solomon group and older than any so far found in the outer part of the Melanesian re‐entrant. Their age supports the idea that the Solomon Islands began their geological history in the Cretaceous.     

Characteristics of the Shetland Islands (UK) peat slides of 19 September 2003

An extreme rainfall event over the southern Shetland Islands in northern Scotland, UK, on 19 September 2003, triggered at least 20 significant peat slides and at least 15 smaller landslides of varying types. The peat slides were examined and surveyed to characterise and explain the distinctive morphological features that were produced. The failures varied in size from 0.4 to 7.3 ha (2,300 to 59,000 m³ displaced volumes of peat) and involved blanket peat up to 3 m deep and slope gradients as low as 4°. Almost all of the failure surfaces were located at the peat–mineral interface. The morphological features included large areas (up to 0.5 ha) of intact peat that moved without breaking up, linear compression and thrust features and unusual occurrences of mineral debris. These features suggest peat of high tensile strength throughout its depth and the generation of high and sometimes artesian water pressures at the base of the peat during the event. However, the variations between peat slides highlight some of the difficulties of trying to assess the susceptibility of blanket peat to failure without full knowledge of the local peat geotechnical properties and structural features within the peat mass.     

Trace elements in phosphorites from Kotelny Island (New Siberian Islands)

Doklady Earth Sciences - The first study of the full chemical composition of Triassic phosphorites from Kotelny Island and Jurassic–Cretaceous phosphorite from the Egorjevsk deposit showed...