Browsing by Author "Martin U"

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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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    Compositional variation during monogenetic volcano growth and its implications for magma supply to continental volcanic fields
    (Massey University., 2003) Nemeth K; White JD; Reay A; Martin U
    Individual volcanoes of continental monogenetic volcanic fields are generally presumed to erupt single magma batches during brief eruptions. Nevertheless, in two unrelated volcanic fields (the Waipiata volcanic field, New Zealand, and the Miocene-Pliocene volcanic field in western Hungary), we have identified pronounced and systematic compositional differences among products of individual volcanoes. We infer that this indicates a two-stage process of magma supply for these volcanoes. Each volcano records: (1) intrusion of a basanitic parent magma to lower- to mid-crustal levels and its subsequent fractionation to form a tephritic residual melt; (2) subsequent transection of this reservoir by a second batch of basanitic melt, with tephrite rising to the surface at the head of the propagating basanite dyke. Eruption at the surface then yields initial tephrite, typically erupted as pyroclasts, followed by eruption and shallow intrusion of basanite from deeper in the dyke. By analogy with similar tephrite-basanite eruptions along rift zones of intraplate ocean-island volcanoes, we infer that fractionation to tephrite would have required decades to centuries. We conclude that the two studied continental monogenetic volcanic fields demonstrate a consistent history of early magmatic injections that fail to reach the surface, followed by capture and partial eruption of their evolved residues in the course of separate and significantly later injections of basanite that extend to the surface and erupt. This systematic behaviour probably reflects the difficulty of bringing small volumes of dense, primitive magma to the surface from mantle source regions. Ascent through continental crust is aided by the presence in the dyke head of buoyant tephrite captured during transection of the earlier-emplaced melt bodies.
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    Eroded porous-media aquifer controlled hydrovolcanic centers in the South Lake Balaton region, Hungary: The Boglar volcano
    (Akad�miai Kiad�, 1999) Nemeth K; Martin U; Philippe M
    The volcanic centers next to Balatonboglar township represent 3.5 Ma old products of post-extensional alkaline basaltic volcanism in the Pannonian Basin (eastern Central Europe). They are small, eroded volcanic centers located on the southern shore of Lake Balaton and genetically related to the Bakony-Balaton Highland Volcanic Field eruptive centers. The relatively small area (500 m x 500 m) contains at least 2 eruptive centers, which are probably related to each other and have built up a complex volcano, called the Boglar Volcano. The volcanic rocks overlie the older Pannonian clastic sedimentary sequence and represent the topographic highs in this area. The areas of lower elevation around the eruptive centers are covered by Pleistocene to Holocene swamp, lake and river clastic sediments, which strongly suggest intense erosion during the last few million years. All volcanic rocks around Balatonboglar are volcaniclastic. There is no evidence of lava flow occurrence. The volcaniclastic sediments have been divided into two lithofacies associations. The largest amount of volcaniclastic rocks is located in the center of the local hills and has been interpreted as a phreatomagmatic crater fill lapilli tuff. They contain large amphibole megacrysts and small olivine crystals. The second lithofacies association is interpreted as lahar deposits. This sequence contains an unusually large amount of fossil tree trunks, which are identified as Abies species. Within a small area in the western hills small outcrops show evidence of maar-lake clastic sediment occurrence. On the hilltops debris shows intimate interaction processes between clastic sediments and basaltic melt. We interpret this to mean that the eruptive centers of Boglar Volcano were formed under subaerial conditions, with explosions fueled by intensive interaction between water-saturated Pannonian sand and uprising basaltic magma.
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    Late miocene paleo-geomorphology of the bakony-balaton highland volcanic field (Hungary) using physical volcanology data
    (Gebruder Borntraeger, 1999) Nemeth K; Martin U
    A new view is presented of the Bakony-Balaton Highland Volcanic Field (BBHVF), Hungary, active in late Miocene and built up of ca. 100 mostly alkaline basaltic eruptive centers, scoria cones, tuff rings, maar volcanic complexes and shield volcanoes. A detailed map shows the physical volcanology of the monogenetic volcanic field. In areas where thick Pannonian Sandstone beds build up the pre-volcanic strata normal maar volcanic centers have formed with usually thick late magmatic infill in the maar basins. In areas, where relatively thin Pannonian Sandstone beds resting on thick Mesozoic or Paleozoic fracture-controlled, karstwater-bearing aquifer, large unusual maar volcanic sequences appear (Tihany type maar volcanoes). In the northern part of the field large former scoria cones and shield volcanoes give evidence for a smaller impact of the ground and surface water causing phreatomagmatic explosive activity. The Tihany type maar volcanic centers are usually filled by thick maar lake deposits, building up Gilbert type gravelly, scoria rich deltas in the northern side of the maar basins, suggesting a mostly north to south fluvial system in the pre-volcanic surface. Calculating paleosurface elevation for the eruptive centers, two paleo-geomorphology maps are drawn for a younger (4-2.8 Ma) and an older (7.54-4 Ma) scenario. The erosion rate of the volcanic field is estimated to vary between 96 m/Ma and 18 m/Ma. In the western site of BBHVF the erosion rate is higher (more than 60 m/Ma, Tapolca Basin), and an average 50 m/Ma in the center and eastern side.
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    Pitfalls in erosion level calculation based on remnants of maar and diatreme volcanoes
    (Groupe Francais de Geomorphologie, 2007) Nemeth K; Martin U; Csillag G
    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.