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  • 1
    Electronic Resource
    Electronic Resource
    Springer
    Bulletin of volcanology 51 (1989), S. 451-462 
    ISSN: 1432-0819
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract The vesicularity of juvenile clasts in pyroclastic deposits gives information on the relative timing of vesiculation and fragmentation, and on the role of magmatic volatiles versus external water in driving explosive eruptions. The vesicularity index and range are defined as the arithmetic mean and total spread of vesicularity values, respectively. Clast densities are measured for the 16–32 mm size fraction by water immersion techniques and converted to vesicularities using measured dense-rock equivalent densities. The techniques used are applied to four case studies involving magmas of widely varying viscosities and discharge rates: Kilauea Iki 1959 (basalt), Eifel tuff rings (basanite), Mayor Island cone-forming deposits (peralkaline rhyolite) and Taupo 1800 B.P. (calc-alkaline rhyolite). Previous theoretical studies suggested that a spectrum of clast vesicularities should be seen, depending on the magma viscosity, eruption rate, and the presence and timing of magma: water interaction. The new data are consistent with these predictions. In magmatic “dry” eruptions the vesicularity index lies uniformly in the range 70%–80% regardless of magma viscosity. For high viscosities and eruption rates the vesicularity ranges are narrow (〈 25%), but broaden to between 30% and 50% as the viscosity and eruption rates are lowered and the volatiles and magma can de-couple. In phreatomagmatic “wet” eruptions, widely varying clast vesicularities reflect complex variations in the relative timing of vesiculation and water-induced fragmentation. Magma:water interaction at an early stage greatly reduces the vesicularity indices (〈 40%) and broadens the ranges (as high as 80%), whereas late-stage interaction has only a minor effect on the index and broadens the range to a limited extent. Clast vesicularity represents a useful third parameter in addition to dispersal and fragmentation to characterise pyroclastic deposits.
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  • 2
    ISSN: 1432-0819
    Keywords: Key words Explosive volcanism ; Magmatic ; Phreatomagmatic ; Crater Hill ; Basalt
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract  A series of alternating phreatomagmatic ("wet") and magmatic ("dry") basaltic pyroclastic deposits forming the Crater Hill tuff ring in New Zealand contains one unit (M1) which can only be interpreted as the products of mixing of ejecta from simultaneous wet and dry explosions at different portions of a multiple vent system. The principal characteristics of M1 are (a) rapid lateral changes in the thicknesses of, and proportions in juvenile components in individual beds, and (b) wide ranges of juvenile clast densities in every sample. M1 appears to have been associated with an elongate source of highly variable and fluctuating magma : water ratios and magma discharge rates. This contrasts with the only other documented mixed (wet and dry) basaltic pyroclastic deposits where mixing from two point sources of quite different but stable character has been inferred.
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  • 3
    Publication Date: 2019-01-11
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 4
    ISSN: 1432-0819
    Keywords: explosive volcanism ; dome-building volcanism ; phreatomagmatic acticity ; fall deposits ; surge deposits ; rhyolite ; Maroa volcano
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract The 14 ka Puketarata eruption of Maroa caldera in Taupo Volcanic Zone was a dome-related event in which the bulk of the 0.25 km3 of eruption products were emplaced as phreatomagmatic fall and surge deposits. A rhyolitic dike encountered shallow groundwater during emplacement along a NE-trending normal fault, leading to shallow-seated explosions characterised by low to moderate water/magma ratios. The eruption products consist of two lava domes, a proximal tuff ring, three phreatic collapse craters, and a widespread fall deposit. The pyroclastic deposits contain dominantly dense juvenile clasts and few foreign lithics, and relate to very shallow-level disruption of the growing dome and its feeder dike with relatively little involvement of country rock. The distal fall deposit, representing 88% of the eruption products is, despite its uniform appearance and apparently subplinian dispersal, a composite feature equivalent to numerous discrete proximal phreatomagmatic lapilli fall layers, each deposited from a short-lived eruption column. The Puketarata products are subdivided into four units related to successive phases of:(A) shallow lava intrusion and initial dome growth; (B) rapid growth and destruction of dome lobes; (C) slower, sustained dome growth and restriction of explosive disruption to the dome margins; and (D) post-dome withdrawal of magma and crater-collapse. Phase D was phreatic, phases A and C had moderate water: magma ratios, and phase B a low water: magma ratio. Dome extrusion was most rapid during phase B, but so was destruction, and hence dome growth was largely accomplished during phase C. The Puketarata eruption illustrates how vent geometry and the presence of groundwater may control the style of silicic volcanism. Early activity was dominated by these external influences and sustained dome growth only followed after effective exclusion of external water from newly emplaced magma.
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  • 5
    Electronic Resource
    Electronic Resource
    Springer
    Bulletin of volcanology 50 (1988), S. 350-351 
    ISSN: 1432-0819
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
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  • 6
    ISSN: 1432-0819
    Keywords: dilute gravity current ; rain-flushed ash grain size ; grain morphology ; Hatepe Plinian deposit ; Taupo ; New Zealand
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract Two groups of poorly sorted ash-rich beds, previously interpreted as rain-flushed ashes, occur in the ca. AD 180 Hatepe Plinian pumice fall deposit at Taupo volcano, New Zealand. Two ash beds with similar dispersal patterns and an aggregate thickness of up to 13 cm make up the lowermost group (A). Group A beds extend 45 km north-east of the vent and cover 290 km2. In the southern part of the group A distribution area, a coarse ash to lapilli-size Plinian pumice bed (deposit B) separates the two group A beds. The scarcity of lapilli (material seen elsewhere from the still-depositing pumice fall) in group A beds indicates that they were rapidly transported and deposited. However, this rapid transportation and deposition did not produce cross-bedding, nor did it erode the underlying deposits. It is proposed that thick (〉600 m) but dilute gravity currents generated from the collapsing outer margin of the otherwise buoyant Hatepe Plinian eruption column deposited the group A beds. The upper ash beds (group C) consist of one to seven layers, attain an aggregate thickness of ≤35 cm, and vary considerably in thickness and number of beds with respect to distance from vent. Group C beds contain variable amounts of ash mixed with angular Plinian pumices and are genuine rain-flushed ashes. Several recent eruptions at other volcanoes (Ukinrek Maars, Vulcan, Rabaul, La Soufrère de Guadeloupe and Soufrière, St Vincent) have produced gravity currents similar in style, but much smaller than those envisaged for group A deposits. The overloaded margins of otherwise buoyant eruption plumes generated these gravity currents. Laboratory studies have produced experimental gravity current analogues. Hazards from dilute gravity currents are considerable but often overlooked, thus the recognition of gravity current deposits will contribute to more thorough volcanic hazard assessment of prehistoric eruption sequences.
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  • 7
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 385 (1997), S. 306-307 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Dade and Huppert1 present a model for emplacement of the Taupo ignimbrite (New Zealand) from a turbulent dilute pyroclastic current (0.3% solids by volume). This model contrasts sharply with that of a concentrated current (tens of per cent solids by volume) previously proposed by me2 to explain ...
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  • 8
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] The central Taupo Volcanic Zone (TVZ; Fig. 1) is the most productive Quaternary rhyolitic volcanic system on Earth. At least 34 voluminous ignimbrites (30-1,000 km3 bulk volume) have been erupted from eight caldera volcanoes"'12 in central TVZ since 1.6Myr. Several of the New Zealand ignimbrites ...
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  • 9
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 286 (1980), S. 912-912 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] WALKER ET AL. REPLY-We accept without reservation the relationships described by Fisher and others from the Lesser Antilles. We have long believed in the concept that pyroclastic flows sensu stricto and pyroclastic surges have fundamentally different transport mechanisms. What is in question, ...
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  • 10
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 283 (1980), S. 286-287 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] The young age of the Taupo and Rabaul ignimbrites, and their occurrence in humid areas where a luxuriant vegetation anchors and protects pyroclastic deposits very soon after they have formed, has resulted in their remarkably good structural preservation. These ignimbrites shown new features which ...
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