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Classification of ignimbrites and their eruptions
No full textThe term “ignimbrite” probably encompasses the one of the largest ranges of deposit types on Earth, associated with the partial to total collapse of explosive eruption columns feeding pyroclastic density currents. Surprisingly, there is no quantified classification scheme for ignimbrite types, as there is for fallout deposits, and this is a remarkable deficiency of modern volcanology. This has so far prevented the identification of standardized descriptors for ignimbrites and the improvement of methods for the documentation of their characteristics, such as happened for fallout deposits, building on the classification scheme proposed by Walker in 1973. Despite some earlier attempts, ignimbrite types do not conform to eruption style nomenclature. In this paper, we explore and discuss descriptors for a classification scheme based on the correlation of runout, areal extent, aspect ratio and volume from a compiled database comprising 92 ignimbrites, which then allows current understanding of pyroclastic flow dynamics to be considered. We refer to single ignimbrite outflow units, i.e. emplaced without significant breaks in their sedimentation, in extra-caldera settings and forming individual cooling units, irrespective of internal lithofacies architecture. Our main finding is that ignimbrites show remarkable power-law relationship between dispersal area/equivalent runout and bulk volume. Runout is directly related to increasing mass flow rate feeding the pyroclastic current. Volume is related to the magnitude of the flow event. We therefore propose that by measuring first order field observables such as bulk volume and dispersal area provides the opportunity to evaluate magnitude and intensity of related pyroclastic currents and, for large eruptions dominated by ignimbrites, of the eruption. Based on the relationships identified we propose that ignimbrites that originated from the collapse of single point-source eruption columns, usually smaller than 1 km3, are named “Vulcanian ignimbrites” and “Plinian ignimbrites” depending on the style of the eruption they are associated with. Larger ignimbrites that originated from caldera-forming eruptions along ring-fault fissure vents should be regarded as related to a separate eruption style - with respect to the common Hawaiian-Plinian trend -, where the effect of increased mass flow rate due to ring-fissure vents is dominant and controls the dynamics of the resulting collapsing fountains and pyroclastic flows, irrespective of the kind of eruption style that preceded the onset of the caldera collapse. These are named “caldera-forming ignimbrites” and are further subdivided into small, intermediate, large and super, based on their increasing erupted volume
The implications of spatter, pumice and lithic clast rich proximal co-ignimbrite lag breccias on the dynamics of caldera forming eruptions: the 151 ka Sutri eruption, Vico Volcano, Central Italy
No full textThe 151 ka Sutri Formation, erupted from the Vico Volcano, Central Italy, represents a caldera forming eruptive sequence consisting of associated pyroclastic fall and ignimbrite deposits and voluminous coarse, proximal lithic, spatter and pumice-rich breccia facies.
After an initial limited Plinian fallout phase (Sutri A) and a partial eruption column collapse event that generated a small volume pyroclastic flow (Sutri B), the later stages of the Sutri eruption produced a southerly-dispersed lithic clast-rich breccia (Sutri C) that grades vertically into a fines poor ignimbrite (Sutri D). Sutri C is interpreted to be a co-ignimbrite lag breccia involving an element of lateral flow produced during vent widening. Vent widening in the south presumably occurred in response to vent wall rock instabilities and increased magma discharge rate during the early Plinian phase that destroyed a large portion of the southern side of the pre-existing Vico stratovolcano/shield edifice. The vertical gradation into an overlying fines poor ignimbrite (Sutri D) reflects changing source conditions at the vent, a progressive increase in magma discharge rate and cessation of vent-widening activity.
A succeeding, more extensive and complex association of contemporaneous proximal lithic-rich and spatter-rich breccias that are distributed radially around the present day caldera (Sutri E4) are also considered. No stratigraphic relationship exists between the Sutri E4 breccia facies and the lower breccia facies (Sutri C) due to limited outcrop. Three subdivisions of Sutri E4 are recognised based on field analysis, grainsize studies and petrographic characteristics. All occur at the same stratigraphic level radially around the caldera and are distinguished as separate facies from Sutri C and D based on componentry: Sutri E4 (sbx) — spatter-rich breccia (north–east); Sutri E4 (lsbx) — boulder size lithic clast-rich breccia with associated spatter component (southeast–south) and Sutri E4 (flsbx) — spatter-rich breccia with associated lithic clast component (southwest–west). Mixtures of dense, boulder size lithic clasts, spatter and pumice clasts were most likely ejected up along a widening conduit in the north as well as through newly created, radially distributed fissures formed by steadily increasing magma discharge rate in response to progressive collapse of the magma chamber roof into the chamber. Eruption column collapse occurred and lithic, spatter and pumice rich co-ignimbrite breccias were emplaced radially around the vent (Sutri E4 (sbx, lsbx) signalling the onset of caldera collapse, shortly followed after by emplacement of the Tufo rosso a scorie nere (Sutri E4 (flsbx) and E5) and final caldera collapse.
This study is concerned with the mechanism responsible for mixing of large variably vesiculated spatter, highly vesiculated pumice and abundant lithic clasts preserved in both these proximal breccias (i.e. in the conduit and/or eruption column or during transport and emplacement) and considers the implications for the eruption styles produced.
The juvenile clast types are mixed in each breccia facies together with voluminous quantities of conduit wall rock lithic clasts. We concluded that they were generated contemporaneously within a single volcanic conduit that likely widened over time into concentrically distributed fissures. The degree of vesiculation and resulting morphology of the juvenile clasts likely reflect variations in magma rise rate whereby fast magma rise rate or faster than bubble nucleation and coalescence produced spatter and slower magma rise rates promoted bubble growth and coalescence producing pumice
Syn-Depositional deformation of a substrate produced by the shear force of a pyroclastic density current: An example from the Cimino Ignimbrite, Northern Lazio, Italia
No full textSubstrate deformation by pyroclastic density currents is very sparsely described in the literature. The rare occurrence of syn-depositional substrate deformation suggests that special circumstances are required to transmit shear from the base of a pyroclastic density current into the deposited ignimbrite and the substrate. One example of a substrate deformed by a pyroclastic density current is found at the base of the Pleistocene ignimbrite at Monte Cimino, central Italy. A series of reverse faults that offsets the basal contact were produced by the shear force of the pyroclastic current during deposition of the ignimbrite. The faults formed on the vent-facing side of a palaeo-slope that strikes sub-parallel to the flow direction of the pyroclastic current. Fault offsets suggest motion was parallel to the flow direction of the pyroclastic current, rather than down-slope. We propose that these faults resulted from fluctuations in the shear force of the pyroclastic density current as it was channelled down a palaeovalley. The lower flow boundary, which separated the deposited ignimbrite and the substrate from the moving pyroclastic density current, momentarily stepped down into the substrate, so that the upper 0.5m of the substrate and about 1.5m of the deposited ignimbrite became incorporated into the current. This momentary coupling of the current and the substrate induced reverse faulting in the substrate and the deposited portion of the ignimbrite. Deposition appears to have been ongoing during the formation of these faults, as well as afterward. Following the formation of the faults, the lower flow boundary seems to have been quickly re-established above the faults (approximately 1.5m above the base of the ignimbrite), allowing deposition to continue without further deformation of the substrate
Facies architecture and origin of a submarine rhyolitic lava flow-dome complex,Ponza,Italy
No full textThe Villa Senni Eruption Unit (Colli Albani Volcano, Italy): the basal surge-fallout deposit and the proximal co-ignimbrite breccia facies.
No full textThe Villa Senni Eruption Unit is a complex, tephri-foiditic, caldera-forming, large volume ignimbrite succession from the Colli Albani Volcano, Italy, emplaced at about 350 ka. The total estimated volume of products is approximately 50 cubic kilometres. The eruption unit internal stratigraphy is consists of a widely dispersed thin phreatomagmatic ash-surge deposit at the base followed by a scoria fallout deposit dispersed to the east of the volcano. It is up to 120 cm thick in proximal areas, and thins laterally, with a dispersal area that is subplinian. The overlying succession is the bulk of the deposit and is made up of a lower ignimbrite unit, known as the Tufo Lionato, and an upper ignimbrite unit at the top, known as Pozzolanelle, or Villa Senni. Both the lower and upper ignimbrite units show a proximal co-ignimbrite facies made of a coarse lithic and spatter rich breccia which grades laterally and vertically into the standard ignimbrite facies. While the depositional facies of the basal surge-fallout and overlying ignimbrite succession match those of common felsic large volume ignimbrites, the pyroclasts of the Villa Senni Eruption Unit show a moderate vesicularity and ductile surface deformation suggesting a relatively low viscosity of the magma in accordance with the low silica content of its composition. This work addresses the issue of the possible mechanisms that may contribute to the large volume basaltic explosivity of the Villa Senni magma
Emplacement processes of the mafic Villa Senni Eruption Unit (VSEU) ignimbrite succession, Colli Albani volcano, Italy
No full textThermal state and implications for eruptive styles of the intra-Plinian and climactic ignimbrites of the 4.6 ka Fogo A eruption sequence, São Miguel, Azores
No full textThe 4.6 ka Fogo A Plinian eruption was a caldera-forming volcanic event on São Miguel Island, Azores. The deposit succession is very complex, composed of a thick trachytic Plinian fallout deposit interstratified with two intra-Plinian ignimbrites (named “pink ignimbrite” and “black ignimbrite” sequentially). The succession ends with a main ignimbrite (named “dark brown ignimbrite”), which represents the deposit of complete collapse of the eruption column and the end of the eruption. In this work, emplacement temperatures of the three ignimbrites are estimated by study of partial thermal remanent magnetization (pTRM) of lithic clasts. A total of 140 oriented lithic clasts were collected from 15 localities distributed along the northern and southern flanks of Fogo volcano. The paleomagnetic data reveal different emplacement temperatures and thermal histories that were experienced by each ignimbrite. The results indicate the presence of five different paleomagnetic behaviours that suggest emplacement temperatures of 350–400 °C for the first (pink) intra-Plinian ignimbrite, temperatures higher than 580–600 °C for the second (black) intra-Plinian ignimbrite and 250–370 °C for the last (dark brown) climactic ignimbrite. The thermal history experienced by each pyroclastic flow and its ignimbrite deposit was also assessed by the use of the magnetite-ilmenite geothermometer to determine the pre-eruptive magma temperature (estimated to be around 900 °C). We interpret the different emplacement temperatures of the Fogo A ignimbrites as being due to a combination of factors. These include (i) collapse from different heights of the eruption column and the resultant different amounts of air entrainment into the gas-particle mixture, (ii) variable content of lithic clasts and (iii) different types of juvenile clasts in the ignimbrites
The significance of breccias in the Monte Cimino volcanic system, central Italy: the evolution of a silicic lava dome complex from a series of cryptodomes to a pyroclastic flow generating volcano.
No full textA variety of breccias record the transformation of the rhyodacite Cimini dome complex from an early intrusive phase to later explosive eruptions. The character of the breccias provides insight into the eruptive styles and the surface expression of the dome complex through this transition to explosive eruptions. The 1.330 Ma crystal-rich rhyodacite lava domes were initially emplaced as cryptodomes into unconsolidated sediments, forming a peperitic breccia carapace around concentrically-banded lava domes. Continued intrusion and uplift combined to produce an emergent dome building phase of volcanism. The early-formed peperitic breccias appear to have been remobilised as debris flows from the margins of the rising edifice. These breccias roughly encircle the domes, and represent a low talus apron, which was then covered by sediment-poor rock fall or debris avalanche deposits. This phase of dome growth appears to have been non-explosive, as most of the breccias were driven by gravity. A series of block-and-ash flow deposits on the plains surrounding the domes document a more explosive phase of lava dome activity. The lava comprising the block-and-ash flow deposits has a somewhat different mineral assemblage and matrix character from any of the remaining lava domes, and may represent the destruction of a particular dome. The destruction of this dome may have depressurised the conduit and led to more explosive eruptions that formed the overlying welded ignimbrites at 1.30 Ma. Block-and-ash flow volcanism has not been previously recognized in this district, and provides a temporal and genetic link between the cryptodomes and the welded ignimbrites
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