Journal Articles

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    Pitfalls in erosion level calculation based on remnants of maar and diatreme volcanoes (Les pièges de la reconstitution des topographies d'érosion initiales fondée sur les vestiges des maars et diatrèmes volcaniques)
    (Massey University., 2007-01-01) Nemeth, Karoly; Martin, Ulrike; Csillag, Gabor
    Abstract Erosion estimates based on geometrical dimension measurements of eroded maar/diatreme volcanoes are useful methods to determine syn-volcanic surface level and syn-volcanic bedrock stratigraphy. However, such considerations on volcanic architecture should only be employed as a first-order approach to determine the state of erosion. We demonstrate, on both young and eroded maar/diatreme volcanoes, that establishing the volcanic facies architecture gives vital information on the environment in which the volcano erupted. In 'soft' rocks, maar volcanoes are broad and underlain by 'champagne glass'-shaped diatremes. In contrast, the crater wall of maar volcanoes that erupted through "hard rocks" will be steep, filled with lacustrine volcaniclastic deposits and underlain by deep diatremes. Résumé L'estimation de l'érosion fondée sur la géométrie des volcans de type maar-diatrème est une méthode applicable à la reconstruction de la surface syn-volcanique et de la stratigraphie de la série sous-jacente. Toutefois les considérations relatives à l'architecture volcanique doivent seulement être utilisées comme une première approche pour déterminer le niveau initial de la surface aujourd'hui érodée. Nous démontrons que la détermination des faciès volcaniques, tant dans les maars-diatrèmes récents que dans les systèmes érodés de ce type, donne des informations fondamentales sur la nature des roches encaissantes au sein desquelles l'éruption volcanique s'est produite. Dans les roches encaissantes meubles, les maars sont larges avec un diatrème sous-jacent en forme de « coupe de champagne». Au contraire, au sein de roches plus résistantes, les cratères de maars sont entourés de parois raides et bien souvent remplis de dépôts lacustres d'origine volcanoclastique qui masquent des diatrèmes profonds.
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    Cenozoic diatreme field in Chubut (Argentina) as evidence of phreatomagmatic volcanism accompanied with extensive Patagonian plateau basalt volcanism?
    (International Union of Geological Sciences, 2007) Nemeth K; Martin U; Haller MJ; Alric VI
    In Patagonia, Argentina, at the northern border of the Patagonian Cenozoic mafic plateau lava fields, newly discovered diatremes stand about 100 m above the surrounding plain. 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 erosional 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 for the extensive lava fields with phreatomagmatism facilitating magma rise with effective opening of fissures before major lava effusions.
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    Mio/Pliocene phreatomagmatic volcanism in the Western Pannonian Basin
    (Massey University., 2004-01-01) Martin, Ulrike; Nemeth, Karoly
    No abstract available
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    Syn- and post-eruptive erosion, gully formation, and morphological evolution of a tephra ring in tropical climate erupted in 1913 in West Ambrym, Vanuatu
    (Elsevier, 2007) Nemeth K; Cronin SJ
    Syn- and post-eruptive erosion of volcanic cones plays an important role in mass redistribution of tephra over short periods. Descriptions of the early stages of erosion of tephra from monogenetic volcanic cones are rare, particularly those with a well-constrained timing of events. In spite of this lack of data, cone morphologies and erosion features are commonly used for long-term erosion-rate calculations and relative age determinations in volcanic fields. This paper offers new observations which suggest differing constraints on the timing of erosion of a tephra ring may be operating than those conventionally cited. In 1913 a tephra ring was formed as part of an eruption in west Ambrym Island, Vanuatu and is now exposed along a continuous 2.5 km long coastal section. The ring surrounds an oval shaped depression filled by water. It is composed of a succession of a phreatomagmatic fall and base-surge beds, interbedded with thin scoriaceous lapilli units. Toward the outer edges of the ring, base-surge beds are gradually replaced in the succession by fine ash-dominated debris flows and hyperconcentrated flow deposits. The inter-fingering of phreatomagmatic deposits with syn-volcanic reworked volcaniclastic sediments indicates that an ongoing remobilisation of freshly deposited tephra was already occurring during the eruption. Gullies cut into the un-weathered tephra are up to 4 m deep and commonly have c. 1 m of debris flow deposit fill in their bases. There is no indication of weathering, vegetation fragments or soil development between the gully bases and the basal debris flow fills. Gully walls are steep and superficial fans of collapsed sediment are common. Most gullies are heavily vegetated although some active (ephemeral) channels occur. These observations suggest that the majority of the erosion of such tephra rings in tropical climates takes place directly during eruption and possibly for only a period of days to weeks afterward. After establishment of the gully network, tephra remobilisation is concentrated only within them. Therefore the shape of the erosion-modified volcanic landform is predominantly developed shortly after the eruption ceases. This observation indicates that gully erosion morphology may not necessarily relate to age of such a landform. Different intensities of erosion during eruption (related to water supply or rainfall) are probably the major influence on gully spacing, modal depth and form. Longer-term post-eruption processes that could be indicators of relative age may include internal gully deepening (below basal debris flow fill sediments) and possibly widening and side-slope lowering due to undercutting and side-collapse. © 2006 Elsevier B.V. All rights reserved.
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    High level sill and dyke intrusions initiated from rapidly buried mafic lava flows in scoria cones of Tongoa, Vanuatu (New Hebrides), South Pacific
    (Massey University., 2006-01-01) Nemeth, Karoly; White, James D. L.
    Scoria cones are generally considered to grow rapidly in days to weeks or months. During their growth lava flows may be fed onto the cone surface from lava-lake breaches, or form by coalescence of spatter; such flows are preserved interbedded with scoria lapilli and ash beds. On Tongoa, an island of the Vanuatu volcanic arc in the South Pacific, a series of scoria cones developed during the Holocene, forming a widespread monogenetic volcanic field. Half sections of scoria cones along the coast expose complex interior architecture cone architectures. On the western side of Tongoa Island a scoria cone remnant with steeply crater-ward dipping beds of scoria ash and lapilli contains various dm-to-m thick lava flows, which are connected by irregular dikes cutting obliquely across the beds of the cone. The lava flows are coherent igneous bodies with well-developed flow top and basal breccias. The lavas interbedded with the cone-forming layers are part of a larger (up to 7 m thick) body that is connected to dykes and sills of irregular geometries that intrude the cone's pyroclastic layers. This 3D relationship suggests that the lava flows were buried quickly under the accumulating scoriaceous deposits. This allowed subsequent escape of magma from the fluid interiors of flows, with the magma then squeezed upward or laterally into the accumulating pyroclastic pile. Movement of the pile above the partly mobile lava, and potential destabilisation during intrusion into the pile of lava squeezed from the flows, may signal the onset of localised cone failures, and could be implicated in development of major cone breaches (e.g. Paricutin).
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    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, Gabor
    No abstract available