Novel interpretation for shift between eruptive styles in some volcanoes (original) (raw)
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Journal of Volcanology and Geothermal Research, 2007
The extrusion of viscous andesite lava forming domes can terminate in explosive activity. To understand the driving forces behind this behaviour, a study of an AD1655 eruption episode at Mt. Taranaki was carried out. We propose that simple changes in magmatic conditions of a single hydrous melt during ascent caused sudden changes in explosivity and gave rise to pumice with highly variable vesicularities and colour. Fractionation of hornblende + plagioclase + clinopyroxene + Fe-Ti oxide at the onset of magma ascent, and step-wise crystallisation of plagioclase ± clinopyroxene in different parts of a single melt within the conduit was controlled by rates of initial rise, capping by an impermeable lava dome, and differential rates of vesiculation and volatile exsolution. This resulted in a vertical stratification in the conduit, comprising a viscous, hypocrystalline lava cap, that overlay alternating zones of grey, brown and grey magma-foams. Horizontal gradients in geochemistry in the conduit are also indicated by different clast textures. The eruption consisted of an initial extrusive phase followed by three pulses of sub-plinian activity. Each phase or pulse, corresponded to individual layers within the conduit. Ejecta included block-and-ash flow deposits, three pyroclastic pumice-flow deposits of alternating grey, brown and grey pumice, as well as fallout deposits dominated by grey pumice. The brown magma foam contained more microlites, had a more-evolved matrix glass, and a higher temperature than the grey magma foams above and below. Its eruption destabilised the sub-plinian eruption column because it was more degassed. It fragmented less efficiently than the grey magma foams due to its lower viscosity, preventing pressure build-up in bubbles. Incomplete mixing at interfaces between brown and grey magma phases gave rise to banded pumices.
Bulletin of Volcanology, 2011
Triggering mechanisms of large silicic eruptions remain a critical unsolved problem. We address this question for the~2.08-Ma caldera-forming eruption of Cerro Galán volcano, Argentina, which produced distinct pumice populations of two colors: grey (5%) and white (95%) that we believe may hold clues to the onset of eruptive activity. We demonstrate that the color variations correspond to both textural and compositional variations between the clast types. Both pumice types have bulk compositions of high-K, high-silica dacite to low-silica rhyolite, but there are sufficient compositional differences (e.g.,~150 ppm lower Ba at equivalent SiO 2 content and 0.03 wt.% higher TiO 2 in white pumice than grey) to suggest that the two pumice populations are not related by simple fractionation. Trace element concentrations in crystals mimic bulk variations between clast types, with grey pumice containing elevated Ba, Cu, Pb, and Zn concentrations in both bulk samples (average Cu, Pb, and Zn concentrations are 27, 35, and 82 in grey pumice vs. 11, 19, and 60 in white pumice) and biotite phenocrysts and white pumice showing elevated Li concentrations in biotite and plagioclase phenocrysts. White and grey clasts are also texturally distinct: White pumice clasts contain abundant phenocrysts (44-57%), lack microlites, and have highly evolved groundmass glass compositions (76.4-79.6 wt.% SiO 2 ), whereas grey pumice clasts contain a lower percentage of phenocrysts/microphenocrysts (35-49%), have abundant microlites, and have less evolved groundmass glass compositions (69.4-73.8 wt.% SiO 2 ). There is also evidence for crystal transfer between magma producing white and grey pumice. Thin highly evolved melt rims surround some fragmental crystals in grey pumice clasts and appear to have come from magma that produced white pumice. Furthermore, based on crystal compositions, white bands within banded pumice contain crystals originating in grey magma. Finally, only grey pumice clasts form breadcrusted surface textures. We interpret these compositional and textural variations to indicate distinct magma batches, where grey pumice originated from an originally deeper, more volatile-rich dacite recharge magma that ascended through and mingled with the volumetrically dominant, more highly crystalline chamber that produced white pumice. Shortly before eruption, the grey pumice magma stalled within shallow fractures, forming a vanguard magma phase whose ascent may have provided a trigger for eruption of the highly crystalline rhyodacite magma. We suggest that in the case of the Cerro Galán eruption, grey pumice provides evidence not only for Editorial responsibility: J. Stix This paper constitutes part of a special issue: Cas RAF, Cashman K (eds) The Cerro Galan Ignimbrite and Caldera: characteristics and origins of a very large-volume ignimbrite and its magma system.
Dynamics of the ca. 4965yr 14C BP “Ochre Pumice” Plinian eruption of Popocatépetl volcano, México
Journal of Volcanology and Geothermal Research, 2010
The Ochre Pumice (OP) Plinian fallout was produced by Popocatépetl volcano in central Mexico, during a major Plinian eruption that occurred 4965 ± 65 14 C yr BP (3700 BC). The OP is part of the Ochre Pumice Sequence (OPS), that consists of surge, fall, and pyroclastic flow deposits. The OP Plinian fallout shows a bimodal grain size distribution with poor to moderate sorting that improves progressively towards the upper beds. The juvenile component is mainly pumice (N83 wt.%), whereas the accidental components consist of igneous, sedimentary, and metamorphic clasts from the walls of the magma chamber and/or conduit. The vesicularity of the pumice decreases from bottom to top of the stratigraphic sequence while the crystal and glass contents increase. This suggests increasing magma degassing prior and during the eruption. The chemical composition of the pumice varies insignificantly; at the base it is less evolved (SiO 2 = 61 wt.%) and it becomes slightly more silicic towards the top (SiO 2 = 63 wt.%). Pumice clasts have a crystallinity index that ranges between 10 and 25 vol.% and display a seriate texture with phenocrysts of euhedral plagioclase (Pl) + clinopyroxene (Cpx) + orthopyroxene (Opx) + olivine (Ol) ± oxide (Ox) ± apatite (Ap). The size of the maximum lithic (2.5 cm) at a horizontal distance of 19 km indicates an eruption column height of 37-41 km. The isopach map allows the recognition of a dispersal axis that points towards the NNE, where an area of~300 km 2 was covered by N6 cm of pumice and ash. A minimum volume for the OP Plinian fallout deposit was calculated using a minimum thickness of 6 cm found at distal outcrops, and was estimated at~4.9 km 3 which corresponds to 2 km 3 of dense rock equivalent (DRE). Stratigraphic relations indicate that the entire OPS was emplaced during a short time interval of a few days to weeks and consisted of three main different phases: phreato-magmatic, Plinian, and vulcanian. It started violently with the emplacement of surges and then culminated with the eruption of the OP fallout deposit. As the eruption advanced, discharge rates became more intermittent and the height of the column fluctuated and finally collapsed, generating pumice-and-ash flows that were emplaced around the volcano. This short but intense activity was followed during subsequent years by the growth of domes in the crater and the emplacement of block-and-ash-flows generated by vulcanian explosions.
Volcanic Eruptions: Cyclicity During Lava Dome Growth
Springer eBooks, 2009
Andesite Magma or volcanic rock is characterized by in-20 termediate SiO 2 concentration. Andesite magmas have 21 rheological properties that are intermediate between 22 basalt and rhyolite magmas. Silica content in andesites 23 ranges from approximately 52 to 66 weight percent. 24 Common minerals in andesite include plagioclase, am-25 phibole and pyroxene. Andesite is typically erupted at 26 temperatures between 800 to 1000°C Andesite is par-27 ticularly common in subduction zones, where tectonic 28 plates converge and water is introduced into the man-29 tle. 30 Basalt Magma or volcanic rock contains not more than 31 about 52% SiO 2 by weight. Basaltic magmas have 32 a low viscosity. Volcanic gases can escape easily with-33 out generating high eruption columns. Basalt is typi-34 cally erupted at temperatures between 1100 to 1250°C. 35 Basalt flows cover about 70% of the Earth's surface and 36 huge areas of the terrestrial planets and so are the most 37 important of all crustal igneous rocks. 38 Bingham liquid is a fluid that does not flow in response 39 to an applied stress until a critical yield stress is 40 reached. Above the critical yield stress, strain rate is 41 proportional to the applied stress, as in a Newtonian 42 fluid. 43 Bubbly flow A multi-phase flow regime, in which the gas phase appears as bubbles suspended in a continuous liquid phase. Conduit A channel, through which magma flows towards the Earth's surface. Volcanic conduits can commonly be approximately cylindrical and typically a few 10's meters across or bounded by near parallel sides in a magma-filled fracture. Conduits can be vertical or inclined. Crystallization Conversion, partial or total, of a silicate melt into crystals during solidification of magma. Degassing n. (degas v.) The process by which volatiles that are dissolved in silicate melts come out of solution in the form of bubbles. Open-and closed-system degassing can be distinguished. In the former, volatiles can be lost or gained by the system. In the latter, the total amount of volatiles in the bubbles and in solution in the magma is conserved. Differentiation The process of changing the chemical composition of magma by processes of crystallization accompanied by separation melts from crystals. Dome A steep-sided, commonly bulbous extrusion of lava or shallow intrusion (cryptodome). Domes are commonly, but not exclusively, composed of SiO 2rich magmas. In dome-forming eruptions the erupted magma is so viscous, or the discharge rate so slow, that lava accumulates very close to the vent region, rather than flowing away. Pyroclastic flows can be generated by collapse of lava domes. Recent eruptions producing lava domes include the 1995-2006 eruption of the Soufrière Hills volcano, Montserrat, and the 2004-2006 eruption of Mount St. Helens, USA. Dyke A sheet-like igneous intrusion, commonly vertical or near vertical, that cuts across pre-existing, older, geological structures. During magmatism, dykes transport magma toward the surface or laterally in fracturelike conduits. In the geologic record, dykes are preserved as sheet-like bodies of igneous rocks. Explosive eruption A volcanic eruption in which gas expansion tears the magma into numerous fragments with a wide range of sizes. The mixture of gas and entrained fragments flows upward and outward from volcanic vents at high speed into the atmosphere. Depending on the volume of erupted material, eruption intensity and sustainability, explosive eruptions are classified as Strombolian, Vulcanian, sub-Plinian, Plinian or Mega-Plinian; this order is approximately in the order of increasing intensity. Strombolian and Vulcanian eruptions involve very short-lived explosions. Please note that the pagination is not final; in the print version an entry will in general not start on a new page. Editor's or typesetter's annotations (will be removed before the final T E X run) 149 Volatile A component in a magmatic melt which can be 150 partitioned in the gas phase in significant amounts 151 during some stage of magma history. The most com-152 mon volatile in magmas is water vapor H 2 O, but there 153 are commonly also significant quantities of CO 2 , SO 2 154 and halogens.
Bulletin of Volcanology, 2021
Proximal deposits of small-volume trachytic eruptions are an under-studied record of eruption dynamics despite being common across a range of settings. The 59 ± 4 ka Echo Canyon deposits, Ascension Island, resulted from a small-volume explosive-effusive trachytic eruption. Variations in juvenile clast texture reveal changes in ascent dynamics and transitions in eruption style. Five dominant textural types are identified within the pumice lapilli population. Early Strombolian-Vulcanian eruption phases are typified by macro- and micro-vesicular equant clast types. Sheared clasts are most abundant at the eruption peak, transitioning to dense clasts in later phases due to shear-induced coalescence, outgassing and vesicle collapse. Melt densification and outgassing via tuffisite veins increased plume density, contributing to partial column collapse and the explosive-effusive transition. Bulk vesicularity distributions indicate a shift in dominant fragmentation mechanism during the erupti...
The Plinian Lower Pumice 2 eruption, Santorini, Greece: Magma evolution and volatile behaviour
Journal of Volcanology and Geothermal Research, 2009
The Plinian Lower Pumice 2 (LP2) eruption (172 ka) was one of the first major caldera-forming eruptions of the Santorini volcanic complex (Greece). The eruption shows some striking similarities to the caldera-forming Late Bronze Age (Minoan) eruption in terms of field, petrological and geochemical characteristics of its eruptive products, which are used to reveal the storage conditions of the LP2 magmas, pre-eruptive magmatic processes and the behaviour and degassing of volatiles prior to and during eruption.The LP2 eruption comprises four, predominantly rhyodacitic eruptive units (LP2-A, B, C, D). The lowermost unit of the Plinian LP2 deposits (LP2-A) consists of a basal phreatomagmatic bed (LP2-A1), which is overlain by three discrete pumice fall deposits (LP2-A2-1, A2-2, A3), the most prominent of which (LP2-A3) contains abundant, quench-textured scoriae that range in composition from basalt to basaltic andesite. The eruption proceeded with the deposition of pumice-rich pyroclastic flows (LP2-B) characterised by a lower, stratified and cross-bedded ignimbrite (LP2-B1) that may grade into a massive, non-welded ignimbrite (LP2-B2), a lithic-rich pumiceous breccia (LP2-C) and a co-ignimbrite lithic lag breccia (LP2-D).The main volume of rhyodacitic magma, which formed by fractionation of olivine, clinopyroxene, orthopyroxene, plagioclase, amphibole, Fe–Ti oxides, pyrrhotite and apatite from basaltic parental magmas and assimilation of crustal rocks, was held at mid-crustal levels (≤ 16 km depth), magmatic temperatures of 831 ± 12 °C and an oxygen fugacity slightly above the fayalite–magnetite–quartz (FMQ) oxygen buffer. Injection of ∼ 200 °C hotter mafic magma into the rhyodacitic reservoir and subsequent mingling and minor hybridisation with the resident magma helped to remobilise the rhyodacitic host magma and determined the final compositional range of the erupted products.Melt inclusion data show that sulphur concentrations were reduced to < 270 ppm in the rhyodacite, primarily due to partitioning of sulphur into pyrrhotite or, depending on temperature, a FeS-rich melt during magmatic differentiation at oxygen fugacities around the FMQ oxygen buffer. Sulphur concentrations in groundmass glasses of the LP2 pumices suggest that ∼ 43% of the remaining sulphur was released into the atmosphere during the LP2 eruption, the climatic effects of which are considered minor when compared to eruptions of more oxidised silicic arc magmas. Chlorine remained dissolved in the melt during magmatic differentiation prior to and during the LP2 eruption, indicating that chlorine emissions to the atmosphere were negligible.
Geochemistry, Geophysics, Geosystems, 2008
1] The Campi Flegrei (CF) caldera is one of the most dangerous quiescent volcanic systems in the world. Its activity mostly resulted in low-magnitude explosive eruptions, such as that of the Monte Nuovo tuff cone that represents the last eruptive event within the caldera (A.D. 1538). However, there have been more energetic Plinian events, e.g., the Agnano Monte Spina eruption (4.1 ka), and very highly explosive, caldera-forming eruptions, e.g., the Campanian Ignimbrite eruption (39 ka). Here, we integrate new and literature data on the groundmass texture and composition of pyroclastic products from the three above eruptions with the aim of unraveling how volatiles content, degassing mechanisms, and crystallization processes influence magma explosivity and eruption dynamics at CF. Previous studies indicate that the investigated rocks share similar major element bulk and phenocryst chemistry; also similar is the water content of their trapped melt inclusions. These observations suggest that the magmas feeding these eruptions had comparable physicochemical properties during storage in the shallow crust. However, our investigations indicate that the studied rocks differ in texture and composition of the groundmass and viscosity of the related magmas. We ascribe such differences to the variable style of volatile exsolution and outgassing from the melt, primarily in response to changes of the rate of magma ascent to the surface. We conclude that the magma ascent rate was the key parameter in driving explosive eruptions at CF, and we G 3
Bulletin of Volcanology, 2018
Plinian eruptions are characterized by high intensity and an overall steady character, and result in a stable convective column. The main processes controlling the dynamics of such steady and stable plume systems have been extensively investigated. Conversely, sub-Plinian eruptions are unsteady, as recorded by the large variability of the products and deposits. Our knowledge of the processes creating this unsteadiness on various timescales remains limited, and still requires more observations as well as theoretical and experimental investigation. Here, we focus on the sub-Plinian eruption of the Greenish Pumice (GP, 19,265 ± 105 BP), Mt. Somma-Vesuvius (Italy). On the basis of coupled geochemical and textural analyses of samples from the wellestablished stratigraphy of the GP deposits, we investigate volatiles (H 2 O, CO 2 , F, Cl) to better constrain the unsteady sub-Plinian eruptive style. This allows us to carry out a detailed study of the degassing processes in relation to the eruption dynamics. We find that degassing by open-system processes generally dominates throughout the entire eruption, but alternates with episodes of closed-system degassing. The fluctuating degassing regimes, responsible for the variable magma ascent rate within the conduit, are also responsible for the eruptive column instability. Volatile behavior is well correlated with textural heterogeneities of the eruptive products. Both reflect higher conduit heterogeneity than for Plinian eruptions, where we find a higher horizontal gradient in magma ascent velocity due to a smaller conduit diameter.