Geocronología preliminar K/Ar del Campo Volcánico del Basalto Cráter, Patagonia Septentrional

El campo volcánico del Basalto Cráter (CVBC) constituye uno de los campos basálticos cuaternarios de intraplaca de la Patagonia septentrional. El estudio sistemático de la geología, volcanología y geocronología del CVBC muestra una historia eruptiva "multiepisódica" de volcanes basálticos. Las dataciones K-Ar realizadas sobre roca total son coherentes con el control estratigráfico. Las edades obtenidas para el Basalto Cráter permiten distinguir tres episodios diferentes, pero individualmente coherentes, de actividad volcánica, ocurridos hace ~1,0 Ma; 0,6 Ma y 0,3 Ma. Las diferencias de edad parecen ser significativas, aún cuando el contenido de argón radiogénico determinado en los análisis de roca total resultó menor al 10 %.The Crater Basalt volcanic field is one of the Quaternary intraplate basaltic fields in northern Patagonia. A systematic geological, volcanological and geochronological study of CBVF indicates a "multistage history" of eruptions of basaltic volcanoes. K/Ar dating, using whole rock samples shows that the measured analytical ages are fully consistent with the available stratigraphic control. The radiometric ages fall into three distinct, internally consistent age groups, which give evidence that there were at least three major episodes of volcanic activity, at about 1.0 Ma, 0.6 Ma and 0.3 Ma ago. The age differences appear to be just significant, even although less than 10 % radiogenic argon was found in the isotope analysis of whole rock samples.Fil: Pécskay, Zoltán. Hungarian Academy of Sciences. Institute of Nuclear Research; HungríaFil: Haller, Miguel Jorge F.. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Nacional Patagónico; ArgentinaFil: Németh, Karoly. Massey University; Nueva Zelanda. Geological Institute of Hungary; Hungrí

Volcanic Lakes in Africa: The VOLADA_Africa 2.0 Database, and Implications for Volcanic Hazard

Volcanic lakes pose specific hazards inherent to the presence of water: phreatic and phreatomagmatic eruptions, lahars, limnic gas bursts and dispersion of brines in the hydrological network. Here we introduce the updated, interactive and open-access database for African volcanic lakes, country by country. The previous database VOLADA (VOlcanic LAke DAta Base, Rouwet et al., Journal of Volcanology and Geothermal Research, 2014, 272, 78–97) reported 96 volcanic lakes for Africa. This number is now revised and established at 220, converting VOLADA_Africa 2.0 in the most comprehensive resource for African volcanic lakes: 81 in Uganda, 37 in Kenya, 33 in Cameroon, 28 in Madagascar, 19 in Ethiopia, 6 in Tanzania, 2 in Rwanda, 2 in Sudan, 2 in D.R. Congo, 1 in Libya, and 9 on the minor islands around Africa. We present the current state-of-the-art of arguably all the African volcanic lakes that the global experts and regional research teams are aware of, and provide hints for future research directions, with a special focus on the volcanic hazard assessment. All lakes in the updated database are classified for their genetic origin and their physical and chemical characteristics, and level of study. The predominant rift-related volcanism in Africa favors basaltic eruptive products, leading to volcanoes with highly permeable edifices, and hence less-developed hydrothermal systems. Basal aquifers accumulate under large volcanoes and in rift depressions providing a potential scenario for phreatomagmatic volcanism. This hypothesis, based on a morphometric analysis and volcanological research from literature, conveys the predominance of maar lakes in large monogenetic fields in Africa (e.g. Uganda, Cameroon, Ethiopia), and the absence of peak-activity crater lakes, generally found at polygenetic arc-volcanoes. Considering the large number of maar lakes in Africa (172), within similar geotectonic settings and meteoric conditions as in Cameroon, it is somewhat surprising that “only” from Lake Monoun and Lake Nyos fatal CO2 bursts have been recorded. Explaining why other maars did not experience limnic gas bursts is a question that can only be answered by enhancing insights into physical limnology and fluid geochemistry of the so far poorly studied lakes. From a hazard perspective, there is an urgent need to tackle this task as a community

How Polygenetic are Monogenetic Volcanoes: Case Studies of Some Complex Maar‐Diatreme Volcanoes

The increasing number of field investigations and various controlled benchtop and large‐scale experiments have permitted the evaluation of a large number of processes involved in the formation of maar‐diatreme volcanoes, the second most common type of small‐volume subaerial volcanoes on Earth. A maar‐diatreme volcano is recognized by a volcanic crater that is cut into country rocks and surrounded by a low‐height ejecta rim composed of pyroclastic deposits of few meters to up to 200 m thick above the syn‐eruptive surface level. The craters vary from 0.1 km to up to 5 km wide and vary in depth from a few dozen meters to up to 300 m deep. Their irregular morphology reflects the simple or complex volcanic and cratering processes involved in their formation. The simplicity or complexity of the crater or the entire maar itself is usually observed in the stratigraphy of the surrounding ejecta rings. The latter are composed of sequences of successive alternating and contrastingly bedded phreatomagmatic‐derived dilute pyroclastic density currents (PDC) and fallout depositions, with occasional interbedded Strombolian‐derived spatter materials or scoria fall units, exemplifying the changes in the eruptive styles during the formation of the volcano. The entire stratigraphic sequence might be preserved as a single eruptive package (small or very thick) in which there is no stratigraphic gap or significant discordance indicative of a potential break during the eruption. A maar with a single eruptive deposit is quantified as monogenetic maar, meaning that it was formed by a single eruptive vent from which only a small and ephemeral magma erupted over a short period of time. The stratigraphy may also display several packages of deposits separated either by contrasting discordance surfaces or paleosoils, which reflect multiple phases or episodes of eruptions within the same maar. Such maars are characterized as complex polycyclic maars if the length of time between the eruptive events is relatively short (days to years). For greater length of time (thousands to millions of years), the complex maar will be quantified as polygenetic. These common depositional breaks interpreted as signs of temporal interruption of the eruptions for various timescales also indicate deep magma system processes; hence magmas of different types might erupt during the formation of both simple and complex maars. The feeding dikes can interact with groundwater and form closely distributed small craters. The latter can coalesce to form a final crater with various shapes depending on the distance between them. This observation indicates the significant role of the magmatic plumbing system on the formation and growth of complex and polygenetic maar‐diatreme volcanoes

Modern analogues for Miocene to Pleistocene alkali basaltic phreatomagmatic fields in the Pannonian Basin: "Soft-substrate" to "combined" aquifer controlled phreatomagmatism in intraplate volcanic fields

The Pannonian Basin (Central Europe) hosts numerous alkali basaltic volcanic fields in an area similar to 200 000 km2. These volcanic fields were formed in an approximate time span of 8 million years producing smallvolume volcanoes typically considered to be monogenetic. Polycyclic monogenetic volcanic complexes are also common in each field however. The original morphology of volcanic landforms, especially phreatomagmatic volcanoes, is commonly modified. by erosion, commonly aided by tectonic uplift. The phreatomagmatic volcanoes eroded to the level of their sub-surface architecture expose crater to conduit filling as well as diatreme facies of pyroclastic rock assemblages. Uncertainties due to the strong erosion influenced by tectonic uplifts, fast and broad climatic changes, vegetation cover variations, and rapidly changing fluvio-lacustrine events in the past 8 million years in the Pannonian Basin have created a need to reconstruct and visualise the paleoenvironment into which the monogenetic volcanoes erupted. Here phreatomagmatic volcanic fields of the Miocene to Pleistocene western Hungarian alkali basaltic province have been selected and compared with modern phreatomagmatic fields. It has been concluded that the Auckland Volcanic Field (AVF) in New Zealand could be viewed as a prime modern analogue for the western Hungarian phreatomagmatic fields by sharing similarities in their pyroclastic successions textures such as pyroclast morphology, type, juvenile particle ratio to accidental lithics. Beside the AVF two other, morphologically more modified volcanic fields (Pali Aike, Argentina and Jeju, Korea) show similar features to the western Hungarian examples, highlighting issues such as preservation potential of pyroclastic successions of phreatomagmatic volcanoes.Fil: Németh, Karoly. Massey University; Nueva ZelandaFil: Cronin, Shane. Massey University; Nueva ZelandaFil: Haller, Miguel Jorge F.. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Centro Nacional Patagónico; Argentina. Universidad Nacional de la Patagonia "San Juan Bosco"; ArgentinaFil: Brenna, Marco. Massey University; Nueva ZelandaFil: Csillag, Gábor. Geological Institute of Hungary; Hungrí

Aufbau und pyroklastische Abfolge eines kleinen quartären (?) Maars im Pali Aike Vulkanfeld, Santa Cruz, Argentinien

An approximately 200 m long, 3 m thick continuous outcrop of a near vent crater rim tephra sequence from a young phreatomagmatic volcano of the Pali Aike Volcanic Field (Santa Cruz Province, Argentina) is one of the few outcrops where the eruptive sequence of the phreatomagmatic volcanoes of this field can be studied. Regardless of the large number of morphological features such as lake filled depressions, suggestive of the existence of maar volcanoes in the Pali Aike Volcanic Field, exposed tephra ring sequences are rare. The newly identified tephra ring section documents an eruption history of a maar that erupted through a “soft rock” environment and formed a shallow maar basin, currently filled with saline lake water. The tephra ring is rich in pre-volcanic sedimentary material. A fine grained tephra succession with abundant glassy pyroclasts indicates effective magma/water interaction and the presence of high energy bedforms suggests subsequent transportation and deposition from high energy base surges. Irregularly interbedded accidental lithic fragment rich beds suggest temporal collapse of the volcanic conduit of the active vent. The common appearance of cauliflower and breadcrust bombs together in the coarse grained tephra beds suggest that during magma/water interaction certain part of the uprising melt could have reached the surface without significant contact with the ground water.Ein etwa 200 m langer und 3 m mächtiger Aufschluss in einem schlotnahen Kraterwall eines jungen Vulkans des Pali Aike Vulkanfeldes (Provinz Santa Cruz, Argentinien) ermöglicht – als einer der wenigen Aufschlüsse – die Untersuchung der eruptiven Sequenz der phreatomagmatischen Vulkane dieses Feldes. Ungeachtet der großen Zahl von morphologischen Merkmalen wie seegefüllte Senken, die auf Maarvulkanismus hindeuten, sind aufgeschlossene Tephraringsequenzen selten. Der neu gefundene Kraterrandanschnitt dokumentiert die Eruptionsgeschichte eines Maares, das durch ein unverfestigtes Nebengestein eruptierte. Diese Eruption formte ein flaches Maarbecken, das heute mit einem Salzsee gefüllt ist. Die Tephra ist reich an prävulkanischem sedimentärem Material. Eine feinkörnige Tephraabfolge mit vielen Glaspyroklasten spricht für einen effektiven Magma-Wasser-Kontakt, und das Vorhandensein von Hochenergie-Ablagerungsstrukturen legt einen abschließenden Transport und Ablagerung aus hochenergetischen „base surges“ nahe. Unregelmäßig eingeschaltete Schichten, die reich an Gesteinsfragmenten sind, zeigen den zeitweiligen Kollaps des Förderschlotes. Das häufige Vorkommen von Blumenkohl- und Brotkrustenbomben in den grobkörnigen Tephraschichten belegt, dass während der MagmaWasser-Interaktion bestimmte Teile der aufsteigenden Schmelze die Oberfläche ohne deutlichen Kontakt mit dem Grundwasser erreicht haben könnten.Fil: Haller, Miguel Jorge F.. Universidad Nacional de la Patagonia "San Juan Bosco"; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Nacional Patagónico; ArgentinaFil: Németh, Karoly. Eötvös University. Department of Regional Geology; Hungría. Massey University; Nueva Zeland

From mantle roots to surface eruptions: Cenozoic and Mesozoic continental basaltic magmatism

Basaltic volcanism is an important process in shaping large areas of the Earth’s surface, not only in continental extensional environments and at the ocean floor. This special issue contains a collection of fifteen papers that are dedicated to recent researches on various aspects of continental basaltic magmatism from its mantle roots via the ascent paths of the melt to the surface where different styles of volcanism take place erupting lavas or through explosive volcanism depositing various types of pyroclasts. Two of the fifteen were published earlier (Downes et al. 2015; Herrero-Hernandez et al. 2015). Continental basaltic volcanism also contributes to the total terrestrial sedimentary budget not only by its primary pyroclastic deposits but also their reworked varieties. Most of the papers result from presentations at the BASALT 2013 conference, which took place from April 18–24, 2013, in Görlitz, Germany (Büchner et al. 2013). The conference was organized by the Senckenberg Museum of Natural History Görlitz and co-organized by the International Association of Volcanology and Chemistry of the Earth´s Interior (IAVCEI)—particularly its Commission on Monogenetic Volcanism—and the “Sächsische Landesstiftung Natur und Umwelt” LaNU Academy). Since the conference was held in the heart of Europe in Germany with accompanied field excursions to Poland and the Czech Republic, many contributions are related to the Cenozoic Central European Volcanic Province (Fig. 1). However, there was also a variety of contributions about Mesozoic and Cenozoic basaltic rocks worldwide. This variety is reflected in this issue. The issue brings together studies on different aspects of basaltic magmatism. Thus, this volume contains petrological and geochemical studies spanning from studies of mantle peridotites to those on volcanic rocks as well as papers presenting geophysical data and interdisciplinary interpretation

Cenozoic diatreme field in Chubut (Argentina) as an evidence of phreatomagmatic volcanism accompanied with extensive Patagonian plateau basalt volcanism?

In Patagonia, Argentina, at the northern border of the Patagonian mafic Cenozoic plateau lava fields newly discovered diatremes stand about 100 m above the surrounding plane. These diatremes document phreatomagmatic episodes associated with the formation of the volcanic fields. The identified pyroclastic and intrusive rocks are exposed lower diatremes of former phreatomagmatic volcanoes and their feeding dyke systems. These remotely located erosion remnants cut through Paleozoic granitoids and Jurassic/Cretaceous alternating siliciclastic continental successions that are relatively easily eroded. Plateau lava fields are generally located a few hundreds of metres above the highest level of the present tops of the preserved diatremes suggesting a complex erosional history and potential interrelationships between the newly identified diatremes and the surrounding lava fields. Uprising magma from the underlying feeder dyke into the diatreme root zone intruded the clastic debris in the diatremes, inflated them and mingled with the debris to form subterranean peperite. The significance of identifying diatremes in Patagonia are twofold: 1) in the syn-eruptive paleoenvironment, water was available in various “soft-sediments”, commonly porous, media aquifer sources, and 2) the identified abundant diatremes that form diatreme fields are good source candidates of the extensive lava fields with phreatomagmatism facilitating magma rise with effective opening of fissures before major lava effusions.Fil: Németh, Karoly. Geological Institute of Hungary. Department of Mapping; Hungría. Massey University; Nueva ZelandaFil: Ulrike, Martin. Geo-Zentrum an der KTB; AlemaniaFil: Haller, Miguel Jorge F.. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Nacional Patagónico; Argentina. Universidad Nacional de la Patagonia "San Juan Bosco"; ArgentinaFil: Alric, Viviana Ines. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Nacional Patagónico; Argentina. Universidad Nacional de la Patagonia "San Juan Bosco"; Argentin

Prompt gamma aktivációs analitikai vizsgálatok vulkáni kőzeteken a Balaton-felvidéktől Észak-Patagóniáig

The importance of the light elements in geochemistry has been growing because the analytical techniques for measuring them went through a great improvement. Boron is an incompatible and fluid-mobile element possessing two isotopes with greatly different abundances in nature, and also the abundance of boron greatly differs in different reservoirs. These facts make boron an important tracer of the recycled materials is subduction zones. In the subducting slab most of the boron is concentrated in the oceanic crust and sediments, but most of it leaves the slab close to the suture zone of the subduction. However, some minerals can retain boron and carry it further down into the mantle, even reaching depths of the lower mantle and taking part in deep recycling. The most suitable analytical techniques for B concentration measurements, is a non-destructive nuclear method of prompt gamma activation analysis (PGAA). This article resumes conclusions based on the B data measured in volcanic rocks from a back-arc basin setting from the Balaton-Highland and North-Patagonia. We emphasize that the origin of the alkaline basalt magmas in these regions are connected to extension influenced decompression-melting and asthenospheric upwelling. The B/Sm ratio of the examined rocks from Balaton-Highland and Crater Basalt volcanic fields overlap and are higher than those from Rio Genoa. The greater amount of fluid-mobile boron in the alkaline basalts from the Balaton-Highland compared to the North-Patagonian samples, indicate higher value of crustal thinning due to extension in the Pannonian basin.A geokémia tudományága a geológia és a kémia módszereit ötvözve alakult ki. A geokémia a kémiai elemek vizsgálatán keresztül kutatja a geológiai folyamatokat, a Föld kialakulásának körülményeit próbálja feltárni, követve a Föld mintegy 4,5 milliárd éves fejlődését, egészen ajelenleg is zajló folyamatokig. Ennek a ma is dinamikusan fejlődő tudományágnak az előretörését jelentős részben a technikai fejlődés, az analitikai módszerek finomodása határozza meg. Kezdetben csupán a fókomponenseket tudták kimutatni, majd idővel a nyomelem-koncentrációk pontos mérésére is alkalmas módszereket fejlesztettek. Az egyre nagyobb mennyiségű geokémiai adat statisztikai feldolgozása általános geokémiai következtetésekre adott lehetőséget. A nyomelemek pontosabb kémiai meghatározása révén már kvantitatív petrogenetikai modellszámításokat lehetett végezni. A nyomelem-analízis és a radiogén izotóp-geokémia együttes használatával pedig egy új kutatási terület, a Föld kémiai fej lődését vizsgáló kémiai geodinamika született meg. Kihívást már csak a nehezen mérhető nemesgázok és könnyű elemek koncentrációjának pontos meghatározása, valamint azok izotóparányainak mérése jelent. A bór koncentrációjának pontos mérésére is csak az utóbbi évtizedekben nyílt lehetőség. Ezt követően rajzolódott ki a bómak mint fluid-mobilis könnyű elemnek a geokémiai jelentősége, ami a ftuidumok körforgásának vizsgálatát és a geotektonikai környezetek elkülönítését illeti.Fil: Gméling, Katalin. MTA Izotópkutató Intézet Nukleáris Kutatások Osztály; HungríaFil: Pécskay, Zoltán. MTA Atommagkutató lntézet; HungríaFil: Haller, Miguel Jorge F.. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Nacional Patagónico; Argentina. Universidad Nacional de la Patagonia "San Juan Bosco"; ArgentinaFil: Massaferro, Gabriela Isabel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Nacional Patagónico; Argentina. Universidad Nacional de la Patagonia "San Juan Bosco"; ArgentinaFil: Németh, Karoly. Volcanic Risk Solutions; Nueva Zeland

Inverted volcanic relief: Its importance in illustrating geological change and its geoheritage potential

co-auteur étrangerInternational audienceWe describe volcanic inverted relief sites around the world, making a comparative analysis of thosemost significant sites found fromliterature and our own search on imagery and global topographic maps. Over fifty significant areas of volcanic inverted relief were found. The comparative analysis is based on geoscience values defined by the main geological and landscape elements that define inverted relief. This subjective analysis is open and can be verified and extended if other significant sites emerge, thus forming the basis of a future, exhaustive global comparison of this important geomorphological feature. Invertedreliefoccurswhen valleys transformto ridges due to differential erosion of relatively resistant valley-fill, and weaker slope lithologies. It is found in various geological settings, and it is very common in volcanic terrains, especially monogenetic volcanic fields, where most examples are inverted lava flows. Relief inversion provides a clear indication of slowgeological changes and landscape evolution through erosion and can be thought of in popular terms as a geological clock. Volcanic inverted relief was recognised in the 18th - 19th centuries in the Chaîne des Puys (Auvergne, France), and used as evidence to first support plutonism by Nicolas Desmarest and then support uniformitarianism by George Poulett Scrope. We review the geological and geomorphological features of volcanic inverted relief world-wide, with an emphasis on the classical Auvergne.We explore how volcanic relief inversion chart geological changes, and their value for studying geological systems and landscape evolution. With our comparative analysis we can propose sites with the greatest geoheritage potential for representing inverted relief globally and suggest how this can be valued as geoheritage. As volcanic inverted relief is an important sub-set of all inverted relief, and is generally associated with important surface, volcanic and tectonic processes, and is often ongoing, it can be an important geoheritage component in natural sites.We suggest that it should should be present in the International Union of Geological Sciences (IUGS) Global Geosite list, can be a component of geosites in UNESCO Global Geoparks. It is also a feature for geological criteria (viii) in UNESCO World Heritage sites, where it fulfils all the requirements being both amajor geomorphological feature and a fingerprint of significant geological processes in Earth evolution