Journal Articles
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Item The complexities of assessing volcanic hazards along the Cameroon Volcanic Line using spatial distribution of monogenetic volcanoes(Elsevier B V, Amsterdam, 2022-07) Schmidt C; Laag C; Whitehead M; Profe J; Tongwa Aka F; Hasegawa T; Kereszturi GVolcanic eruptions represent hazards for local communities and infrastructure. Monogenetic volcanoes (usually) erupt only once, and then volcanic activity moves to another location, making quantitative assessment of eruptive hazards challenging. Spatio-temporal patterns in the occurrence of these eruptions may provide valuable information on locations more likely to host future eruptions within monogenetic volcanic fields. While the eruption histories of many stratovolcanoes along the Cameroon Volcanic Line (CVL) are relatively well studied, only fragmentary data exist on the distribution and timing of this region's extensive monogenetic volcanism (scoria cones, tuff rings, maars). Here, we present for the first time a catalog of monogenetic vents on the CVL. These were identified by their characteristic morphologies using field knowledge, the global SRTM Digital Elevation Model (30 m resolution), and satellite imagery. More than ~1100 scoria cones and 50 maars/tuff rings were identified and divided into eight monogenetic volcanic fields based on the visual assessment of clustering and geological information. Spatial analyses show a large range of areal densities between the volcanic fields from >0.2 km−2 to 0.02 km−2 from the southwest towards the northeast. This finding is in general agreement with previous observations, indicating closely spaced and smaller edifices typical of fissure-fed eruptions on the flanks of Bioko and Mt. Cameroon in the southwest, and a more focused plumbing system resulting in larger edifices of lower spatial density towards the northeast. Spatial patterns were smoothed via kernel density estimates (KDE) using the Summed Asymptotic Mean Squared Error (SAMSE) bandwidth estimator, the results of which may provide an uncertainty range for a first-order hazard assessment of vent opening probability along the CVL. Due to the scarce chronological data and the complex structural controls across the region, it was not possible to estimate the number of vents formed during the same eruptive events. Similarly, the percentage of hidden (buried, eroded) vents could not be assessed with any acceptable statistical certainty. Furthermore, the impact of different approaches (convex hull, minimum area rectangle and ellipse, KDE isopaches) to define volcanic field boundaries on the spatial distribution of vents was tested. While the KDE boundary definition appears to reflect the structure of a monogenetic volcanic field better than other approaches, no ideal boundary definition was found. Finally, the dimension of scoria cones (approximated by their basal diameters) across the CVL was contrasted to the specific geodynamic setting. This region presents a complex problem for volcanic hazard analysis that cannot be solved through basic statistical methods and, thus, provides a potential testbed for novel, multi-disciplinary approaches.Item EPMA maps unveil the actual chemical variations and crystallization sequence of pyroxenes and plagioclase solidified from a basaltic liquid at variable cooling rates(Elsevier B V, 2023-12-05) Gennaro E; Radica F; Iezzi G; Vetere F; Nazzari M; Zellmer GF; Scarlato P; Romano CCrystal-chemical variations of pyroxene (px) and plagioclase (plg) have been analysed by X-ray electron microprobe (EPMA) mapping to quantify their actual chemical dispersions. These phases were experimentally crystallised from a basaltic liquid (B100, MORB from Iceland) at cooling rates of 1, 7, 60 and 180 ◦C/h from 1300 ◦C down to 800 ◦C. Experiments were run at ambient conditions applying defined temperature paths mirroring characteristic cooling rates from innermost to outermost portions of metre- to centimetre-thick lavas, dikes and bombs emplaced under submarine to subaerial conditions. As the cooling rate increases from 1 to 180 ◦C/h, the run-products become progressively enriched in pyroxene and depleted in plagioclase, while spinel is invariably low (few area%) and glass is significant only at 180 ◦C/h. An increase of cooling rate generally leads to enrichment of Al2O3 and depletion of MgO in px, while the opposite behaviour is observed for plg; these trends are mirrored by calculated cations (apfu: atom per formula unit) and components. Average variations as a function of cooling rate are similar to those already observed through classical analysis performed by single point EPMA. However, the actual chemical distributions of CaO versus MgO, Al2O3 and FeOtot oxides unveil the presence of a wider range in pyroxene chemistry. In particular, one px (px-1, CaO-rich, diopsidic type) is present at all the applied cooling rates; a very low CaO-px (px-2, pigeonite or orthopyroxene type) is detected at 1 ◦C/h; and, finally, once more population of px (px-3, CaO-poor diopsidic type) appears at 60 and 180 ◦C/h. By contrast, plg analyses yield invariably identical compositions. Textural variations as a function of cooling rate and geo thermometric estimations indicate that px-1 crystallised at high-T (or low ΔT), while plg mainly grew in the residual melt produced by the saturation of px. If only textures were evaluated, this order of segregation would like remain unrecognised since px at low cooling rates is smaller than plg. The abundance of phases, their crystal chemical features, and their order of segregation can be regarded through a theoretical framework of a time temperature-transformation (TTT) diagram. The most significant chemical variations are displayed by MgO and Al2O3 for both px and plg, which faithfully capture the evolution of cooling conditions. The chemical compositions of px-1 is close to the thermodynamic equilibrium only at 1 ◦C/h. As the cooling rates increase, the px chemistry indicates disequilibrium conditions. Finally, this study shows that as ΔT/Δt increases, the most abundant px (px-1) and plg are forced towards compositions that become progressively closer to those of the parental liquid.Item MAAR conference in Hungary(Massey University., 2005-01-01) Martin, Ulrike; Nemeth, KarolyNo abstract availableItem Interaction between lava lakes and pyroclastic sequences in phreatomagmatic volcanoes: Haláp and Badacsony, Western Hungary.(Massey University., 2002-01-01) Martin, Ulrike; Nemeth, KarolyNo abstract availableItem Exploding lakes in Vanuatu: ''Surtseyan-style'' eruptions witnessed on Ambae Island(International Union of Geological Sciences, 2006) Nemeth K; Cronin SJ; Charley D; Harrison M; Garae EAfter a long silence, Lake Vui on Ambae Island burst into spectacular life on November 28, 2005, disrupting the lives of the 10,000 inhabitants on this sleepy tropical island the SW Pacific. "Surtseyan-style" explosions burst through the Island's summit lake waters, forming a new tuff-cone, and threatening to form deadly lahars or volcanic floods. Such eruptions are rarely well observed, and these fleeting opportunities provide a chance to match volcanic processes with rock-sequences common in the geologic record...Item Depositional record of a Pliocene nested multivent maar complex at Fekete-hegy, Pannonian Basin, western Hungary.(Massey University., 2002-01-01) Martin, Ulrike; Nemeth, Karoly; Auer, Andreas; Breitkreutz, Christoph; Csillag, GaborNo abstract available