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Variations in eruptive style and depositional processes associated with explosive, phonolitic composition, caldera-forming eruptions: The 151 ka Sutri eruption, Vico Caldera, central Italy
No full textVico Volcano in central Italy, experienced a complex eruptive history (419–95 ka). The Vico volcanic edifice was constructed by voluminous (50 km3) leucitite composition effusive lava flows and minor explosive activity over a 50 ka period (305–258 ka). The summit of the edifice was destroyed during the Sutri eruption (151 ka) that resulted in the formation of the 8 km caldera depression.
A revised stratigraphy for the eruptive products produced by the Sutri eruption (Sutri Formation) is proposed, based on extensive field observations, detailed stratigraphic logging and petrographic analysis; the Sutri A unit consists of a fallout deposit of limited dispersal; the Sutri B unit consists of a small volume, variably welded pumice-rich ignimbrite; the Sutri C unit is a lithic and spatter clast-rich co-ignimbrite lag-flow breccia; the Sutri D unit is a fines-depleted pumice-rich ignimbrite; Sutri (E1–E3) subunits consist of spatter clast-rich ignimbrite; Sutri E4 (sbx) is a spatter clast-rich co-ignimbrite lag-flow breccia; the Sutri E4 (lsbx) subunit is a lithic, pumice and spatter clast-rich co-ignimbrite lag-flow breccia; the climactic Sutri E5 unit is a variably zeolitised spatter and pumice-rich ignimbrite (with lithic-rich base; Sutri E4 flsbx) to the west).
The reconstruction of the Sutri eruption leading up to and including formation of the Vico Caldera based on the new stratigraphy of the Sutri Formation, consists of three distinct phases:
Phase 1—Initial Plinian activity produced a fallout deposit (Sutri A) deposited to the south, followed by partial eruption column collapse that generated a pyroclastic flow directed to the south/southeast (Sutri B). Vent widening in response to vent-wall rock instabilities and increased magma discharge rate overloaded the eruption column with dense, lithic debris resulting in southward collapse and deposition of a thick co-ignimbrite lag breccia in the deflation zone (Sutri C). A change in clast supply at vent from lithic clast-dominated to pumice clast-dominated produced a fines-depleted ignimbrite, Sutri D (plbx) prior to cessation of activity in the south.
Phase 2—Shortly following the initial Plinian phase or partially overlapping, decompression of the magma chamber in response to partial magma chamber roof block collapse promoted explosive fragmentation of magma at the base of the conduit (as pumice) and led to the opening of a new vent in the north. Sustained mixing of highly vesiculated, grey pumice clasts and dense, poorly vesiculated, black spatter clasts occurred both in the conduit and in the eruption column and subsequent column collapse produced a small volume pyroclastic flow directed to the north, dominated by dense, black spatter clasts (Sutri E1–E3).
Phase 3—Sustained eruptive activity continued in the north and increased magma discharge rate likely then resulted in great instability of the magma chamber roof and surrounding wall rocks of the progressively draining magma chamber. The eruption column collapsed radially depositing vast quantities of lithic clasts, spatter clasts and some pumice clasts into the deflation zone as co-ignimbrite lag-flow breccias (Sutri E4 sbx, lsbx), signalling the onset of caldera collapse. Eruption column collapse also produced a radially distributed, pyroclastic flow that deposited a moderate volume ignimbrite (Sutri E5) with a lithic-rich base (Sutri E4 flsbx) at proximal localities in the west. Final caldera collapse occurred shortly following emplacement of Sutri E5 presumably in response to evacuation of the magma chamber and final collapse of the magma chamber roof.
We infer from the eruption dynamics that progressive collapse of the magma chamber roof occurred culminating in final climactic and post-eruptive caldera collapse
Excursion Field Book- 4th International Workshop on Collapse Calderas September 23-29, 2012, Vulsini Calderas, Italy
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The eruption, pyroclastic flow behaviour, and caldera in-filling processes of the extremely large volume (1290 km3), intra- to extra-caldera, Permian Ora (Ignimbrite) Formation, Southern Alps, Italy
No full textColumnar jointing in vapor phase altered Cerro Galan Ignimbrite, Paycuqui, Argentina
No full textAbstract Columnar jointing is thought to occur primarily
in lavas and welded pyroclastic flow deposits. However, the
non-welded Cerro Galán Ignimbrite at Paycuqui, Argentina,
contains well-developed columnar joints that are instead
due to high-temperature vapor-phase alteration of the
deposit, where devitrification and vapor-phase crystalliza-
tion have increased the density and cohesion of the upper
half of the section. Thermal remanent magnetization
analyses of entrained lithic clasts indicate high emplace-
ment temperatures, above 630°C, but the lack of welding
textures indicates temperatures below the glass transition
temperature. In order to remain below the glass transition at
630°C, the minimum cooling rate prior to deposition was
3.0 10–8.5 102°C/min (depending on the experimental
data used for comparison). Alternatively, if the deposit was
emplaced above the glass transition temperature, conduc-
tive cooling alone was insufficient to prevent welding.
Crack patterns (average, 4.5 sides to each polygon) and
column diameters (average, 75 cm) are consistent with
relatively rapid cooling, where advective heat loss due to
vapor fluxing increases cooling over simple conductive
heat transfer. The presence of regularly spaced, complex
radiating joint patterns is consistent with fumarolic gas rise,
where volatiles originated in the valley-confined drainage
system below. Joint spacing is a proxy for cooling rates and
is controlled by depositional thickness/valley width. We
suggest that the formation of joints in high-temperature,
non-welded deposits is aided by the presence of underlying
external water, where vapor transfer causes crystallization
in pore spaces, densifies the deposit, and helps prevent
welding
Hydrothermal breccia textures and processes: Lisca Bianca Islet, Panarea, Eolian Islands, Italy
No full textPanarea is a largely submarine to partly subaerial Quaternary lava dome complex-stratovolcano with a long- lived, active, shallow hydrothermal system, located in the Aeolian Islands volcanic arc of southern Italy. The emergent top of the volcano forms a small archipelago, made up of calc-alkaline basaltic andesite to rhyolite lava domes (ca. 150–20 ka). We document the facies outcropping on Lisca Bianca islet, Panarea archipelago, based on grain size, clast fabric, and degree of hydrothermal alteration, identifying coherent facies, boulder breccia facies, cobble breccia facies, pebble breccia facies, and pervasively altered andesite facies (alunite-mar- casite-sulfur). The breccias all have ubiquitous jigsaw-fit clast textures, and are variably hydrothermally altered. The breccias are interpreted as hydrothermal breccias and are distinguished from primary volcanic facies based on their distinguishing characteristics. The breccias formed through a cyclical process, involving the following: stage 1: progressive build-up of fluid pressure toward the level of the tensile strength of the host andesite; stage 2: incipient fracturing of the andesite when fluid pressure approaches and then exceeds the tensile strength of the andesite under critical fracturing conditions; stage 3: pervasive fracturing of the host andesite, leading to an increase in permeability as a network of fractures develops; stage 4: declining pressure, with fluid flow rates that lead to infilling and sealing of fractures by natroalunite, thereby reducing permeability, leading to pro- gressive build-up of fluid pressure again, and the beginning of a new cycle
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Volcanological constraints on the post-emplacement zeolitisation of ignimbrites and geoarchaeological implications for Etruscan tomb construction (6th – 3rd century B.C) in the Tufo Rosso a Scorie Nere, Vico Caldera, Central Italy
No full textWe examine the role of physical volcanological processes including eruption style (magmatic versus phreatomagmatic) as well as transport and depositional processes (pyroclastic fall versus pyroclastic flow) in promoting the ideal hydrologic conditions necessary for large scale, homogeneous, post-emplacement zeolitisation of ignimbrites, associated with caldera forming eruptions. We consider the Tufo Rosso a Scorie Nere (TRSN) of Vico Caldera (151 ka), in central Italy. The TRSN exhibits pervasive, homogenous alteration of high alkali tephriphonolitic and phonolitic glass to zeolite minerals (chabazite and phillipsite) in all regions of the study area and at all stratigraphic levels with the exception of the basal 1 m.
Based on detailed lithological studies, we propose that a large geothermal field around the vent area was destroyed during the closing stages of the Sutri eruption. Subsequent incorporation and entrapment of superheated geothermal fluids into the ensuing pyroclastic flow during eruption column collapse greatly influenced the emplacement temperature and provided the necessary water required for post-emplacement zeolitisation of the TRSN. We suggest that the absence of zeolitisation at the base of the ignimbrite is directly related to transport conditions reflecting cooler regions in the lower portions of the deposit where the flow came into contact with the underlying substrate.
We also consider the geoarchaeological implications of enhanced strength and cohesiveness provided by the zeolite rock framework on Etruscan tomb location and burial architecture in the Vico region. The TRSN contains literally hundreds of hypogeum-style Etruscan tombs at a number of sites across the study area. This study focuses on two sites in particular, the Norchia Necropoli and the San Guiliano Necropoli. Considering the enhanced mechanical properties of zeolitised ignimbrites we infer that physically the TRSN would still have been a relatively soft rock, suitable for the carving of tombs. However, we infer the increased strength and cohesiveness provided by the zeolite framework enhanced the conservation potential of these tombs, preserving them for over two thousand years
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