Quench fragmentation is a non-explosive process that occurs when molten magma is super-cooled to glass upon contact with ambient water. This occurs when coherent lavas are erupted subaqueously, when ...they flow into water, when magma intrudes into water-saturated sediments, and when magma and water interact explosively during phreatomagmatism. Quench fragmentation also occurs alongside explosive phreatomagmatic activity. Although products of quench fragmentation (hyaloclastite sensu stricto) have been discussed qualitatively in the volcanological literature, compared to explosive fragmentation processes very little is known about the exact dynamics of quench fragmentation of magma and how this relates to the rheology and physical properties of volcanic glass. Based on literature from materials engineering, we present a detailed review of the processes by which glass forms, the properties of glass, and the fracture mechanics that cause it to fragment non-explosively. We also consider how this can be applied to understanding the dynamics behind the volcanological processes of in-situ glass fragmentation during quenching in wet environments and phreatomagmatism. Important parameters for the occurrence of quench fragmentation are the temperature difference between the magma and the ambient water and how much the ambient water is superheated above its Leidenfrost temperature. The geometry of the lava or magma intrusion, the thermal conductivity and the thermal expansion are also of great importance. The resistance of the magma against fragmentation can be increased with the presence of crystals provided the thermal expansion of the crystals does not greatly exceed that of the glass; vesicles have the opposite effect, unless the magma is highly vesicular. This overview then provides a solid basis for further quantitative study of quench fragmentation and hyaloclastite formation.
Ash particles have unique morphologies and shapes that are characteristic of certain fracture mechanisms and can be used to identify the type of fragmentation such as magmatic brittle or ductile ...fragmentation and phreatomagmatic molten fuel coolant interaction type fragmentation. Identifying these two different fragmentation processes is especially important in complex volcanic systems where both fragmentation processes can occur. In this study, image parameter analysis and statistical parameter analysis are used to compare ash particles from standardised magma fragmentation experiments with natural ash particles from the Pleistocene Lake Purrumbete maar, southeastern Australia. The pyroclastic Lake Purrumbete maar sequence containsvarious deposit types that show evidence for changing eruption conditions, therefore it is of a main interest to determine the main fragmentation mechanism that formed these deposits. A comparison with experimental ash particles revealed that Lake Purrumbete ash particles show significant differences from experimental samples of magmatic brittle type fragmentation, whereas they show no significant differences from phreatomagmatic molten fuel coolant interaction type fragmentation samples, indicating a predominance of phreatomagmatic fragmentation during the eruption of Lake Purrumbete. The experiments further show that pre-existing stresses also influence the particle shape and may be the reason for the absence of a significant similarity between most of the particle populations.
•Comparison of natural and experimental ash particles using statistical Image Parameter Analysis (IPA).•The shape of the ash particles is predominantly influenced by the fragmentation mechanism.•Pre-stresses within the magma body before fragmentation cause small variations in the particle shape.•Despite evidence for changes in the eruption style, all natural ash particles were formed by phreatomagmatic fragmentation.
The Permian Ora Formation (277–274Ma) preserves the products of the Ora caldera ‘super-eruption’, Northern Italy. The stratigraphic architecture of the exceptionally well preserved intra-caldera ...succession provides evidence for caldera collapse at the onset of the eruption, a multiple discharge point, fissure eruption style, and progressive, incremental caldera in-filling by numerous pyroclastic flow pulses within the caldera. The ignimbrites of the Ora Formation are voluminous (>1290km3), crystal-rich (~25 to 55%), and ubiquitously welded. The Ora Formation has been divided into four members (a–d), which also define the principal eruption phases. The eruption proceeded in four main stages: (1) early caldera collapse and vent opening, producing locally distributed, basal co-ignimbrite lithic breccia (member a); (2) vent clearing, which produced the eutaxitic, lithic-rich ignimbrite and minor thin ground and ash-cloud surge deposits (member b); (3) waxing and steady eruption, which produced the dominant eutaxitic, coarse-crystal-rich ignimbrite, with local lithic-rich and fine-crystal-rich ignimbrite and minor surge deposits (member c); and (4) waning eruption, recorded by the eutaxitic, fine-crystal-rich ignimbrite, with local lithic-rich ignimbrite deposits (member d).
The incremental filling and late-stage outpouring of pyroclastic material from the caldera is recorded by vertical and lateral lithofacies deposit variation and some correlation between stratigraphic sections. These findings reveal a structure to the outwardly monotonous, >1300m thick, intra-caldera fill and thinner (<230m) outflow successions. These data together with the gradational contacts between the main ignimbrite lithofacies, support the hypothesis that pyroclastic material was erupted from multiple source regions in various parts of the caldera, during quasi-steady, low eruption column collapse and pyroclastic flow forming events. Field study revealed the absence of a Plinian fallout deposit, suggesting a lack of a high, buoyant, Plinian precursor eruption phase. This caldera was initiated immediately by a low collapsing column phase, producing the main, thick ignimbrite succession. Simultaneously, catastrophic volcano-tectonic caldera collapse and decompression of the magma chamber occurred, facilitated by the regional extensional environment in the Permian and pre-existing crustal weaknesses. The Ora pyroclastic flow system is suggested as having been a hot and poorly expanded, high particle concentration, granular density current. The confined nature of the majority of the erupted products to the intra-caldera setting, reduced the formation of the full array of facies commonly expected in ignimbrites in extra-caldera settings.
•One of the first detailed analyses of the emplacement processes of intra-caldera ignimbrites•Cross section through the intra-caldera stratigraphy preserved•The Ora Formation preserves the complete eruptive stratigraphy of the Ora caldera.•Reconstruction of the Ora eruption reveals four eruptive phases with no significant time breaks.•We establish the presence of multiple depositional pulses within the larger eruptive package.
Lake Purrumbete maar is located in the intraplate, monogenetic Newer Volcanics Province in southeastern Australia. The extremely large crater of 3000m in diameter formed on an intersection of two ...fault lines and comprises at least three coalesced vents. The evolution of these vents is controlled by the interaction of the tectonic setting and the properties of both hard and soft rock aquifers. Lithics in the maar deposits originate from country rock formations less than 300m deep, indicating that the large size of the crater cannot only be the result of the downwards migration of the explosion foci in a single vent. Vertical crater walls and primary inward dipping beds evidence that the original size of the crater has been largely preserved. Detailed mapping of the facies distributions, the direction of transport of base surges and pyroclastic flows, and the distribution of ballistic block fields, form the basis for the reconstruction of the complex eruption history,which is characterised by alternations of the eruption style between relatively dry and wet phreatomagmatic conditions, and migration of the vent location along tectonic structures. Three temporally separated eruption phases are recognised, each starting at the same crater located directly at the intersection of two local fault lines. Activity then moved quickly to different locations. A significant volcanic hiatus between two of the three phases shows that the magmatic system was reactivated. The enlargement of especially the main crater by both lateral and vertical growth led to the interception of the individual craters and the formation of the large circular crater. Lake Purrumbete maar is an excellent example of how complicated the evolution of large, seemingly simple, circular maar volcanoes can be, and raises the question if these systems are actually monogenetic.
► Reconstruction of the evolution of a very large circular maar structure ► Formation by coalescence of multiple shallow craters, not deep excavation ► The evolution of the maar was strongly influenced by the substrate and the tectonic setting. ► Multiple eruption phases, with at least one significant time break
New results for the Colli Albani volcano (Roma, Italy) surveyed for the Geological Map of Italy at 1
:
50,000 scale (CARG Project), integrated with previous data, provide insights on caldera ...evolution. The Colli Albani, a quiescent volcano, became active at ∼600 ka. Eruptive compositions are consistently mafic (<
50% SiO
2); nevertheless, morphology and the dominantly explosive eruptive style match those of felsic calderas. The volcano is composite, containing multiple superposed edifices or lithosomes. The oldest edifice (Vulcano Laziale (VL), ca. 600–350 ka) is a 1600 km
2 plateau of low aspect ignimbrites (VEI 5–7) with a central caldera. After the last large eruption (>
50 km
3 deposits), forming the Villa Senni Eruption Unit ignimbrites at ca. 355 ka, two edifices were built within the caldera: (1) The horseshoe-shaped Tuscolano-Artemisio (TA) composite edifice (or lithosome) consists of coalescing, peri-caldera, fissure-related scoriae cones interbedded with lava flows; the fissure system forms two segments controlled by regional fractures; (2) The steep-sided Faete stratovolcano (949 m a.s.l.) filled the caldera. The TA and Faete lithosomes partly interfinger and were emplaced at ∼350–260 ka. Their products indicate reduced eruption rates relative to the VL period and a change to effusive and mildly explosive eruptions. The most recent and still active phase of phreatomagmatic activity formed overlapping maars and tuff cones along the western and northern slopes of the volcano, collectively named Via dei Laghi composite lithosome. The Colli Albani caldera is poly-phase: (1) a piecemeal caldera is associated with large volume ignimbrites of the VL edifice; the present shape of the caldera is related to the Villa Senni eruption; 2) the TA composite edifice, erupted from peripheral-caldera fissures, is unrelated to explosive phases of caldera collapse: the TA final products cover a morphologically stable caldera wall. The peripheral fractures feeding the TA composite edifice are interpreted as volcano-tectonic structures activated during the late stage downsag of the caldera. Reduced eruption rates during the TA and the Faete stages (10
−
1
km
3/1 ka respect to >
10
0 km
3/1 ka for the VL edifice) suggest a reduced recharge of the magma chamber that may have induced prolonged deflation and downsagging of the caldera floor and the opening of outward dipping peripheral fractures. By this interpretation, the TA edifice represents the surface expression of ring dykes at depth. The absence of similar fissure-structures along the western caldera rim may relate to the deep geometry of the ring-faults dipping inward in those areas and therefore not favourably oriented for magma intrusion during a period of general subsidence. By contrast, the following and still active phreatomagmatic phase, that has emplaced the Via dei Laghi composite edifice, is located right on the western side of the caldera, and may therefore relate to resurgent conditions. Classical petrological and PERs (Pearce Elements Ratios) analyses indicate that lavas are co-genetic and show a differentiation trend up through stratigraphy driven by crystal fractionation of the
lc-
cpx paragenesis, and by assimilation of upper crust, consituted by up to 6000 m thick up-thrusted Mesozoic–Cenozoic carbonatic successions.
Geophysical modelling techniques are applied to examine and compare the subsurface morphology of maar volcanoes within the Newer Volcanics Province to better understand their eruptive histories and ...the hazards associated with future eruptions within the province. The maar volcanoes under investigation include the Ecklin and Anakie maars, and the Red Rock and Mount Leura Volcanic Complexes, which vary in their complexity, morphology, eruptive styles and host rock type. The Ecklin and Anakie maars display relatively simple geophysical signatures. Long wavelength gravity lows with corresponding magnetic highs are observed across the craters and were reproduced' during modelling with the presence of a shallow maar–diatreme at Anakie and two coalesced diatremes containing denser central vents at Ecklin. Red Rock and Mount Leura have more complex geophysical signatures, consisting of short wavelength gravity and magnetic highs superimposed on longer wavelength gravity lows. These anomalies are reproduced during modelling with coalesced ‘bowl shaped’ diatremes containing dykes and magma ponds. The complex diatreme geometries revealed from forward and inverse modelling suggest that the eruption histories of these volcanoes are more complex than their morphology would suggest. Multiple coalesced diatreme structures indicate an eruption involving vent migration, while preserved dykes within the diatreme suggest short-lived fluctuations between phreatomagmatic and magmatic eruption styles. The geometry of the diatremes is consistent with maars hosted in a soft-substrate, which likely contributed to the migration of vents observed at Ecklin, Red Rock, and Mt Leura. The shallow diatreme observed within the Anakie maar is attributed to a short-lived eruption and low water content within the granitic host rock.
•Potential field modelling is applied to image the geometry of maar–diatremes.•Diatremes are broad and shallow and consist of multiple coalesced vents.•Juvenile rich zones, dykes and magma ponds are identified in the geophysical models.•Results suggest eruptions involving vent migration and fluctuating eruptive styles.
The 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 Pleistocene Lake Purrumbete maar eruption sequence of the Newer Volcanics Province of southeastern Australia records stratigraphic, temporal and geochemical variations that cannot be explained by ...crustal processes, such as fractional crystallisation and crustal contamination. Samples from the Lake Purrumbete maar have distinct trace element trends that correlate with stratigraphic height, with decreasing incompatible element concentrations from the bottom of the sequence to the top, indicating that the most enriched melts were ejected during the earliest eruption stages, with the least enriched melts erupted in the final stages. Variations in rare earth element ratios indicate that these melts were formed by dynamic melting of garnet lherzolite mantle material, consistent with other volcanic centres within the basaltic cones subprovince of the Newer Volcanics Province. The geochemistry of the erupted units at Lake Purrumbete also record minor differentiation of the melts by olivine fractionation, although these processes alone cannot explain the observed geochemical variations in the eruption sequence. Further differentiation of the melts by different processes such as deep clinopyroxene fractionation or assimilation of metasomatised lithosphere may be responsible for the observed geochemical trends at Lake Purrumbete. This shows the complexity of the plumbing system of monogenetic volcanoes, but also shows the opportunities that these volcanoes provide to study these processes in detail, as the geochemical variations that are preserved in this type of volcanism by fast magma ascent through the crust, as evidenced by the presence of fresh mantle xenoliths, predominantly reflect processes that occurred in the mantle rather than in the crust.
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