Basaltic Eruptions Research Articles

The 1928 eruption of Mount Etna (Italy): Reconstructing lava flow evolution and the destruction and recovery of the town of Mascali

Mount Etna in Sicily (Italy) shows >2500years of interactions between volcanic eruptions and human activity, and these are well documented in historical sources. During the last 400years, flank eruptions have had major impacts on the urban fabric of the Etna region, especially in 1651–54, 1669, 1923 and 1928, and it is the last of these which is the focus of this paper. A detailed field and historical reconstruction of the 1928 eruption is presented which allows three themes to be discussed: the evolution of the flow field, lava volume and average magma discharge rate trend; the eruption's human impact, particularly the destruction of the town of Mascali; and the recovery of the region with re-construction of Mascali in a new location. Detailed mapping of lava flows allowed the following dimensions to be calculated: total area, 4.38×106m2; maximum length, 9.4km; volume, 52.91±5.21×106m3 and an average effusion rate of 38.5m3s−1. Time-averaged discharged rates are calculated allowing the reconstruction of their temporal variations during the course of the eruption evidencing a high maximum effusion rate of 374m3s−1. These trends, in particular with regard to the Lower Fissure main phase of the eruption, are in accordance with the ‘idealized discharge model’ of Wadge (1981), proposed for basaltic eruptions driven by de-pressurization of magma sources, mainly through reservoir relaxation (i.e. elastic contraction of a magma body). The eruption took place when Italy was governed by Mussolini and the fascist party. The State response both, during and in the immediate aftermath of the eruption and in the years that followed during which Mascali was reconstructed, was impressive. This masked a less benign legacy, however, that can be traced for several subsequent decades of using responses to natural catastrophes to manufacture State prestige by reacting to, rather than planning for, disasters.

Grain shape of basaltic ash populations: implications for fragmentation

Here, we introduce a new quantitative method to produce grain shape data of bulk samples of volcanic ash, and we correlate the bulk average grain shape with magma fragmentation mechanisms. The method is based on automatic shape analysis of 2D projection ash grains in the size range 125–63 μm. Loose bulk samples from the deposits of six different basaltic eruptions were analyzed, and 20,000 shape measurements for each were obtained within ~45 min using the Particle Insight™ dynamic shape analyzer (PIdsa). We used principal component analysis on a reference grain dataset to show that circularity, rectangularity, form factor, and elongation best discriminate between the grain shapes when combined. The grain population data show that the studied eruptive environments produce nearly the same range of grain shapes, although to different extents. Our new shape index (the regularity index (RI)) places an eruption on a spectrum between phreatomagmatic and dry magmatic fragmentation. Almost vesicle-free Surtseyan ash has an RI of 0.207 ± 0.002 (2σ), whereas vesiculated Hawaiian ash has an RI of 0.134 ± 0.001 (2σ). These two samples define the end-member RI, while two subglacial, one lacustrine, and another submarine ash sample show intermediate RIs of 0.168 ± 0.002 (2σ), 0.175 ± 0.002 (2σ), 0.187 ± 0.002 (2σ), and 0.191 ± 0.002 (2σ), respectively. The systematic change in RI between wet and dry eruptions suggests that the RI can be used to assess the relative roles of magmatic vs. phreatomagmatic fragmentation. We infer that both magmatic and phreatomagmatic fragmentation processes played a role in the subglacial eruptions.

Large-volume lava flows fed by a deep magmatic reservoir at Ağrı Dağı (Ararat) volcano, Eastern Turkey

Agri Dagi (Ararat), whilst being the tallest volcano in Turkey, is largely understudied. Two predominant peaks, Greater and Lesser Agri, make up the main edifice, which has been built during four main phases. The most recent phase consisted of two volcanic eruptions. The respective surface area and volume of the first volcanic eruption were estimated at 96 km2 and 3.2 km3, whereas those of the second eruption were much smaller with the surface area and volume estimated at 25 km2 and 0.6 km3. It is unusual for stratovolcanoes to produce basaltic eruptions of more than 3 km3, although these and larger volumes are not uncommon in flood basalt-type eruptions. Large basaltic eruptions from stratovolcanoes normally require volcano-tectonic forcing (e.g. subsidence of collapse caldera and graben). However, there is no evidence for such volcano-tectonic forcing, during the most recent eruptions at Agri Dagi (Ararat), and therefore, their comparatively large volume basaltic lavas need to be explained in a different way. Here, we present an analytical method for calculating the source volume needed to supply magma to the eruptions at Agri Dagi. We found that the lava flow of 3.2 km3 was likely fed by a very large magma reservoir (∼13,000 km3), while the second flow of 0.6 km3 was fed by a reservoir of a much smaller effective size (or ∼2000 km3). ‘Effective size’ depends on what fraction of the reservoir participates in the eruption. We propose that the entire reservoir supplied magma to the larger eruption, but only one of its compartments (about one fifth of the total volume of the reservoir) supplied magma to the smaller eruption. Although seismic tomography indicates a magma reservoir at great depths (>20–30 km) below the Agri Dagi volcano, geochemical constraints on some of the later-formed rocks suggest an interaction between a shallow chamber (at 8–10-km depth) and the deep reservoir approximately 0.5 Ma. We provide numerical models whose results indicate that dykes injected from the lateral margins of the deep-seated reservoir are more likely to reach the surface directly rather than replenish the shallow magma chamber, suggesting also that the compartment for the second eruption was at the margin of the reservoir.

Uplift history of the Jiaodong Peninsula, eastern North China Craton: implications for lithosphere thinning and gold mineralization

AbstractThe link between lithosphere thinning and formation of world-class gold deposits is well established in the Jiaodong Peninsula within the eastern North China Craton (NCC). However, the timing of initiation and duration of the lithospheric thinning process as well as the depth of formation of the mineralization remain uncertain. Since these parameters are fundamental to formulate exploration strategies, in this study we perform fission track (FT) analysis on zircon and apatite grains in Late Mesozoic granitoid samples from the Jiaodong Peninsula and provide new constraints for the mode and duration of lithospheric evolution and mineralization depth. The zircon FT ages range from 64.3 to 90.9 Ma and those of apatite show a range of 32.8–50.9 Ma. The data collectively display age peaks at ~60–80 and ~30–50 Ma. Reverse modelling of the apatite FT results indicates rapid crustal uplift during ~30–80 Ma in the Jiaodong Peninsula. This period coincides with the timing of maximal sedimentation in the neighboring basins and voluminous basaltic eruptions in the eastern NCC. We suggest that the Jiaodong Peninsula has experienced two stages of crust uplift in the Late Cretaceous and Paleogene as a consequence of the continuing lithosphere thinning, together with the surrounding basins, forming the horst–graben system in the eastern NCC. The Late Mesozoic granitoids are the main wall rocks for gold deposits in Jiaodong, and thus the crust denudation history gathered from the FT data suggest that the gold mineralization formed at depths ofc.6–11 km.

Baby, it's cold outside: Climate model simulations of the effects of the asteroid impact at the end of the Cretaceous

AbstractSixty‐six million years ago, the end‐Cretaceous mass extinction ended the reign of the dinosaurs. Flood basalt eruptions and an asteroid impact are widely discussed causes, yet their contributions remain debated. Modeling the environmental changes after the Chicxulub impact can shed light on this question. Existing studies, however, focused on the effect of dust or used one‐dimensional, noncoupled atmosphere models. Here we explore the longer‐lasting cooling due to sulfate aerosols using a coupled climate model. Depending on aerosol stratospheric residence time, global annual mean surface air temperature decreased by at least 26°C, with 3 to 16 years subfreezing temperatures and a recovery time larger than 30 years. The surface cooling triggered vigorous ocean mixing which could have resulted in a plankton bloom due to upwelling of nutrients. These dramatic environmental changes suggest a pivotal role of the impact in the end‐Cretaceous extinction.

Atmospheric outgassing and native-iron formation during carbonaceous sediment–basalt melt interactions

Organic carbon-rich sediment assimilation by basaltic magmas leads to enhanced emission of greenhouse gases during continental flood basalt eruptions. A collateral effect of these interactions is the generation of low oxygen fugacities (fO2) (below the iron-wüstite [IW] buffer curve) during magmatic crystallization, resulting in the precipitation of native-iron. The occurrence of native-iron bearing terrestrial basaltic rocks are rare, having been identified at three locations: Siberia, West Greenland, and Central Germany. We report the first combined study of Re–Os isotopes, highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re), and trace-element abundances for these three occurrences, in addition to host sediments at West Greenland. To quantify the amount of crustal assimilation experienced by the magmas, we present combined crystallization and assimilation models, together with fractional crystallization models, to assess how relative abundances of the HSE have been modified during crystallization. The radiogenic osmium isotopic compositions (γOsinitial +15 to +193) of mafic igneous samples are consistent with assimilation of old high Re/Os crustal contaminants with radiogenic 187Os/188Os, whereas the HSE inter-element fractionations (Pd/Os 2 to >10,000) suggest that some Siberian samples underwent an early stage of sulfide removal.Metalliferous samples from the Siberian intrusions of Khungtukun and Dzhaltul (associated with the Siberian flood basalts) yield internal 187Re–187Os ages of 266±83Ma and 249±50Ma, respectively, reflecting late-Permian emplacement ages. These results imply that crustal assimilation took place prior to crystallization of native-Fe. In contrast, metalliferous samples from Disko Island and Bühl (associated with the West Greenland flood basalts, and the Central European Volcanic Province, respectively) have trends in 187Re/188Os–187Os/188Os space corresponding to apparent ages older than their reported crystallization ages. These anomalous ages probably reflect concurrent assimilation of high Re/Os, radiogenic 187Os crust during crystallization of native-Fe, consistent with the character of local West Greenland sediments. In all three locations, calculations of combined assimilation of crustal sediments and fractional crystallization indicate between 1–7% assimilation can account for the Os-isotope systematics. In the case of Siberian samples, incompatible trace-element abundances indicate that lower crustal assimilation may have also occurred, consistent with the suggestion that crustal assimilation took place prior to native-Fe precipitation. The extent of local crustal contamination at Siberia, West Greenland, and Bühl necessitates that significant quantities of CH4, CO, CO2, SO2 and H2O were released during assimilation of carbonaceous sediments. Consequently, carbonaceous sediment–basalt melt interactions have collateral effects on total gas output from flood basalt volcanism into the atmosphere. However, the amount of carbonaceous sediment contamination experienced by melts forming the Khungtukun and Dzhaltul intrusions alone, cannot explain the major C-isotope excursions at the Permo–Triassic mass-extinction event. Instead, further unsampled intrusions that also experienced significant carbonaceous sediment–melt interactions would be required. Enhanced greenhouse gas-emission during the Permo–Triassic mass extinction may have been facilitated by a combination of mantle melting and carbonaceous sediment–melt interactions, together with other proposed mechanisms, including wildfires, or by microbial metabolic exhalation.

Paleozoic mafic-intermediate intrusions (320–287 Ma) in the Kalatongke area, southern Altai, NW China: Products of protracted magmatism in a convergent tectonic setting

Numerous small mafic-intermediate intrusions are present in the Kalatongke area in the southern part of the Paleozoic Altai Orogenic Belt, NW China. Previous studies reveal that most of these intrusions were emplaced at ∼287Ma, broadly coeval with the eruption of alkaline flood basalts at ⩽282Ma in the Tarim Craton. The similar ages have led some researchers to believe that the Tarim flood basalts and the Kalatongke mafic-intermediate intrusions are related to the same mantle plume. New and existing geochronological and geochemical data for the mafic-intermediate intrusions in the Kalatongke area together do not support such interpretation. Most of the intrusions in this small area (4.5×2km) were emplaced at ∼287Ma but our new data reveal that older intrusive rocks with zircon U-Pb ages from 290±1.5 to 320±2Ma are also present. The temporal and spatial distribution of these intrusions does not show a hotspot track as expected by the mantle plume model. The intrusive rocks have similar Nd-Hf isotope compositions (εNd=3–9, εHf=15–19) and are all characterized by light REE enrichments relative to heavy REE, plus pronounced negative Nb-Ta anomalies. The oldest intrusion (∼320Ma) also shows negative Zr-Hf anomalies, which are common in arc basalts but absent in continental flood basalts. The results of mixing calculations based on Sr-Nd-Hf isotopes and selected trace elements (Th, Nb and Yb) indicate that the parental magmas for these intrusions were all enriched in Th and depleted in Nb prior to crustal contamination, similar to the magmas generated in a convergent tectonic setting from subduction to post-subduction elsewhere in the world. The results from this study remind us that temporal correlation is not a reliable tool to determine the size of a mantle plume.

Using 40Ar/39Ar ages of intercalated silicic tuffs to date flood basalts: Precise ages for Steens Basalt Member of the Columbia River Basalt Group

To establish causality between flood basalt eruptions and extinction events and global environmental effects recorded by isotopic excursions in marine sediments, highly accurate and precise ages for the flood basalts are required. But flood basalts are intrinsically difficult to date. We illustrate how 40Ar/39Ar feldspar ages for silicic tuffs intercalated with and overlying sections of Steens Basalt, the earliest lavas of the Middle Miocene Columbia River Basalt Group in the northwestern United States, provide high-precision ages that, for the first time, make it possible to resolve age differences with stratigraphic position within a section of these flood lavas. The stratigraphically lowest rhyolitic tuff, a fall deposit, yielded an age of 16.592±±0.028 Ma (FCs = 28.02 Ma), and the uppermost, the alkali rhyolite ignimbrite Tuff of Oregon Canyon, is 16.468±±0.014 Ma. The argon and stratigraphic data indicate that Steens Basalt eruptions occurred from ∼16.64 to 16.43 Ma in the southern end of its distribution. We estimate that the Steens Mountain geomagnetic reversal occurred at 16.496±±0.028 Ma (±0.18 Ma total error). Our estimates of the timing for initiation of volcanism and volumetric eruptive rates do not seem to support volcanic forcing by the initial stages of Columbia River Basalt Group eruptions as an explanation for the abrupt warming and carbonate dissolution at the beginning of the Miocene Climatic Optimum.

Early Permian Qiangtang flood basalts, northern Tibet, China: A mantle plume that disintegrated northern Gondwana?

The Qiangtang flood basalt province (QFBP) was recently recognized in northern Tibet, China. It erupted during the Sakmarian–Kungurian and covered an area of >6.84×105km2, reaching a maximum thickness of 1.5–2.0km. The Qiangtang basalts have ocean-island basalt (OIB)-type trace element patterns, with enrichments of light rare-earth elements (LREEs) and Ti–Nb–Ta relative to similarly incompatible elements, as well as exhibiting no Eu-anomalies. The basalts show medium to high Sr and Pb isotopic ratios, and consistently positive εNd(t). Strong correlations between Mg index, and major and trace elements indicate that the basalts have experienced fractional crystallization of clinopyroxene and olivine, and may have been generated by partial melting of an OIB-type mantle plume source with negligible crustal contamination. Similar early Permian basalts are also present in the Lhasa terrane, South Tibet–northern India, the Sanjiang area, and the Tarim basin. These basalts have an estimated original volume of >7.51×105km3, spread over an area of >9.84×105km2. We propose that the large-scale eruption of these flood basalts was related to a Sakmarian–Kungurian mantle plume that possibly initiated the disintegration of northern Gondwana and the formation of the Tethys oceans.

Role of syn-eruptive plagioclase disequilibrium crystallization in basaltic magma ascent dynamics.

Timescales of magma ascent in conduit models are typically assumed to be much longer than crystallization and gas exsolution for basaltic eruptions. However, it is now recognized that basaltic magmas may rise fast enough for disequilibrium processes to play a key role on the ascent dynamics. The quantification of the characteristic times for crystallization and exsolution processes are fundamental to our understanding of such disequilibria and ascent dynamics. Here we use observations from Mount Etna's 2001 eruption and a magma ascent model to constrain timescales for crystallization and exsolution processes. Our results show that plagioclase reaches equilibrium in 1–2 h, whereas ascent times were <1 h. Using these new constraints on disequilibrium plagioclase crystallization we also reproduce observed crystal abundances for different basaltic eruptions. The strong relation between magma ascent rate and disequilibrium crystallization and exsolution plays a key role in controlling eruption dynamics in basaltic volcanism.

Volatiles and the tempo of flood basalt magmatism

Individual flood basalt lavas often exceed 103 km3 in volume, and many such lavas erupt during emplacement of flood basalt provinces. The large volume of individual flood basalt lavas implies correspondingly large magma reservoirs within or at the base of the crust. To erupt, some fraction of this magma must become buoyant and overpressure must be sufficient to encourage failure and dike propagation. The overpressure associated with a new injection of magma is inversely proportional to the total reservoir volume, and as a large magma body heats the surrounding rocks thermally activated creep will relax isotropic overpressure more rapidly. Here, we examine the viability of buoyancy overpressure as a trigger for continental flood basalt eruptions. We employ a new one-dimensional model that combines volatile exsolution, bubble growth and rise, assimilation, and permeable fluid escape from Moho-depth and crustal chambers. We investigate the temporal evolution of degassing and the eruptibility of magmas using the Siberian Traps flood basalts as a test case. We suggest that the volatile inventory set during mantle melting and redistributed via bubble motion controls ascent of magma into and through the crust, thereby regulating the tempo of flood basalt magmatism. Volatile-rich melts from low degrees of partial melting of the mantle are buoyant and erupt to the surface with little staging or crustal interaction. Melts with moderate volatile budgets accumulate in large, mostly molten magma chambers at the Moho or in the lower crust. These large magma bodies may remain buoyant and poised to erupt—triggered by volatile-rich recharge or external stresses—for ∼106 yr. If and when such chambers fail, enormous volumes of magma can ascend into the upper crust, staging at shallow levels and initiating substantial assimilation that contributes to pulses of large-volume flood basalt eruption. Our model further predicts that the Siberian Traps may have released 1019–1020 g of CO2 during a number of brief (∼104 yr) pulses, providing a plausible trigger for warming and ocean acidification during the end-Permian mass extinction. The assimilation of carbon-rich crustal rocks strongly enhances both flood basalt eruptibility and CO2 release, and the tempo of eruptions influences the environmental effects of CO2, SO2, and halogen degassing. The eruptive dynamics of flood basalts are thus inextricably linked with their environmental consequences.

Effect of melt/mantle interactions on MORB chemistry at the easternmost Southwest Indian Ridge (61°–67°E)

AbstractThe easternmost part of the Southwest Indian Ridge (61°–67°E) is an end‐member of the global ridge system in terms of very low magma supply. As such, it is a good laboratory to investigate the effect of melt/mantle interactions on the composition of erupted basalts: for a given volume of erupted basaltic melt, the volume of reacted mantle is potentially greater than at more magmatically robust ridges. We analyzed major, trace element and isotopic compositions in three groups of rocks: plagioclase‐bearing ultramafic and gabbroic rocks dredged in nearly amagmatic spreading corridors; basalts from the sparse volcanic cover of these corridors (“ultramafic seafloor basalts”); and basalts dredged from the intervening, more volcanically active domains (“volcanic seafloor basalts”). Ultramafic seafloor basalts have significantly lower CaO and Al2O3 contents at a given MgO than most volcanic seafloor basalts. We propose that both types of basalts are derived from similar parental melts, but that the ultramafic seafloor basalts are more affected by reactions between these parent melts and the mantle rocks in the lithosphere below the ridge. We infer that these reactions occur in the walls of conduits that allow the aggregated melts extracted from the melting mantle to rise through the axial lithosphere and to the eruption sites. The principal effect of these reactions is to enrich the asthenospheric melts in MgO through olivine dissolution. This effect is not expected to be as noticeable, but could still play a role in basalt petrogenesis at more magmatic regions of the global slow‐spreading MOR system.

Origin of temporal compositional trends in monogenetic vent eruptions: Insights from the crystal cargo in the Papoose Canyon sequence, Big Pine Volcanic Field, CA

Many monogenetic vents display systematic temporal–compositional variations over the course of eruption. Previous studies have proposed that these trends may reflect variable degrees of crustal assimilation, or melting and mixing of heterogeneous mantle source(s). Discrimination between these two endmember hypotheses is critical for understanding the plumbing systems of monogenetic volcanoes, which pose a significant volcanic hazard in many areas. In this study, we examine the Papoose Canyon (PC) monogenetic vent in the Big Pine Volcanic Field (BPVF), which had been well characterized for temporal–compositional variations in erupted basalts. We present new major and trace element and Sr–Nd–Pb-O isotopic data from the PC “crystal cargo” (phenocrysts and xenoliths). Comparison of “crystal cargo” and host basalt provides new constraints on the history of magma storage, fractionation, and crustal contamination that are obscured in the bulk basalts due to pre- and syn-eruptive magma mixing processes.The abundances of phenocrysts and ultramafic xenoliths in the PC sequence decrease up-section. Olivine and clinopyroxene phenocrysts span a wide range of Mg# (77–89). The majority of phenocrysts are more evolved than olivine or clinopyroxene in equilibrium with their host basalts (Mg#=68–71, equilibrium Fo≈85–89). In addition, the ultramafic xenoliths display cumulate textures. Olivine and clinopyroxene from ultramafic xenoliths have Mg# (73–87) similar to the phenocrysts, and lower than typical mantle peridotites. Sr–Nd–Pb isotope compositions of the xenoliths are similar to early PC basalts. Finally, many clinopyroxene phenocrysts and clinopyroxene in xenoliths have trace element abundances in equilibrium with melts that are more enriched than the erupted basalts. These features suggest that the phenocrysts and xenoliths derive from melt that is more fractionated and enriched than erupted PC basalts. Pressure constraints suggest phenocrysts and ultramafic xenoliths crystallized at ∼5–7 kbar, corresponding to mid-crust depths. Correlations between HFSE depletion and Sr–Nd–Pb isotopic compositions, high δ18O values in olivines, and radiogenic Os isotopic compositions in whole rocks also suggest incorporation of a crustally contaminated component.We propose that the phenocrysts and ultramafic xenoliths derive from melts that ponded and fractionated and assimilated continental crust, possibly in mid-crustal sills. These melts were drained and mixed with more primitive melts as the eruption began, and the temporal–compositional trends and decreasing crystal phase abundances reflect gradual deflation and exhaustion of these sills as the eruption progressed. The isotopic variations in the PC sequence span much of the compositional range observed in the BPVF. Evidence for variable crustal contamination of PC basalts suggests that much of the isotopic variation observed in the BPVF may also reflect crustal contamination rather than mantle source heterogeneity as previously proposed. In addition, evidence of pre-eruptive magma ponding and fractionation, if applicable to other monogenetic vents, may have significant implications for monitoring and hazard assessment of monogenetic volcano fields.

Insights into the lithosphere to asthenosphere melting transition in northeast Africa: Evidence from the Tertiary volcanism in middle Egypt

Throughout the northeast African and Arabian plates, small-volume basaltic volcanism has erupted sporadically over the last ~35Myr. While magmatic activity associated with the Afar plume and subsequent rifting of the Red Sea, Gulf of Aden and East African rift provide a mechanism for melt generation in those areas, the origin of melt elsewhere in the region has remained enigmatic. We have identified two types of intraplate basalt eruptions from middle Egypt, which we suggest were generated by distinct melting processes: sub-alkaline, hypersthene-normative basalts and basaltic andesites (2.8–4.9wt% total alkalis); and nepheline-normative alkali basalts and trachybasaltic flows (4.9–5.5wt% total alkalis). Estimations of the primary melt compositions suggest about twice as much fractionation in the sub-alkaline basalts (F=32–58%, dominantly clinopyroxene and olivine removal) in comparison to the alkaline basalts (F=18–30%, dominated by clinopyroxene removal). Application of the geochemical model HAMMS1 generates results which suggest the alkaline basalts are likely derived from low degrees of partial melting (F<2%) of a pyroxenite-bearing mantle source at moderate potential temperatures (TP) ~1386–1449°C, and pressures >3.7GPa. These conditions require melting at or below the base of lithospheric mantle. In contrast, the sub-alkaline basalts exhibit negative K-anomalies for all fractionation-corrected melts, suggesting a source with a K-bearing phase (likely amphibole), located within lithospheric mantle, constrained by the stability of amphibole to be <3GPa and 1170°C. We suggest that the enrichment in the lithospheric mantle is best explained by the percolation of fluids/or hydrous melts either from the Late Neo-Proterozoic subduction zone during accretion the Arabian-Nubian Shield, or from an old mantle plume that impinged the base of the lithosphere during the Phanerozoic. Destabilization of these enriched metasomes was likely a function of diffuse stretching associated with the opening of the Red Sea at ca. 30Ma, thermal perturbation from a slightly warmer asthenosphere, and the passage of asthenospheric melts through the lithospheric mantle. The strong correlation between these small-volume lavas and regional structural lineaments (e.g., Baharyia NE-SW Fault), and smaller NW-SE fractures associated with the Red Sea system, suggests that the eruption of lavas in the region is controlled by melt focusing into the lithosphere and favorable stress conditions in the upper crust. Thus, while the potential for melt generation may be present beneath much of north-east Africa, the rarity at which volcanism is discovered at the surface reflects what may be an unusual convergence of circumstances.

Homogeneous bubble nucleation in H2O- and H2O-CO2-bearing basaltic melts: Results of high temperature decompression experiments

High pressure and temperature decompression experiments were conducted to provide experimental information on the conditions of homogeneous bubble nucleation in basaltic melts. Experiments were performed on H2O- and H2O-CO2-bearing natural melts from Stromboli. Three starting volatile compositions were investigated: series #1 (4.91wt% H2O, no CO2), series #2 (2.37–2.45wt% H2O, 901–1011ppm CO2) and series #3 (0.80–1.09wt% H2O, 840–923ppm CO2). The volatile-bearing glasses were first synthesized at 1200°C and 200MPa, and second continuously decompressed in the pressure range 150–25MPa and rapidly quenched. A fast decompression rate of 78kPa/s (or 3m/s) was applied to limit the water loss from the glass cylinder and the formation of a H2O-depleted rim. Post-decompression glasses were characterized texturally by X-ray microtomography. The results demonstrate that homogenous bubble nucleation requires supersaturation pressures (difference between saturation pressure and pressure at which homogeneous bubble nucleation is observed, ∆PHoN) ≤50–100MPa. ∆PHoN varies with the dissolved CO2 concentration, from ≪50MPa (no CO2, series #1) to ≤50MPa (872±45ppm CO2, series #3) to <100MPa (973±63ppm CO2, series #2). In series #1 melts, homogeneous bubble nucleation occurs as two distinct events, the first at high pressure (200<P<150MPa) and the second at low pressure (50<P<25MPa), just below the fragmentation level. In contrast, homogenous nucleation in series #2 and #3 melts is a continuous process. As well, chemical near-equilibrium degassing occurs in the series #1 melts, unlike in the series #2 and #3 melts which retain high CO2 concentrations even for higher vesicularities (up to 23% at 25MPa). Thus, our experimental observations underline a significant effect of CO2 on the physical mechanisms of bubble vesiculation in basaltic melts. Our experimental decompression textures either reproduce or approach the characteristics of explosive basaltic eruptions, in terms of vesicularity, bubble shapes, sizes and number densities. Unimodal, exponential to power law bubble size distributions were encountered and correlated with the different melt series, textural characteristics and types of degassing.

Magma decompression rates during explosive eruptions of Kīlauea volcano, Hawaii, recorded by melt embayments

The decompression rate of magma as it ascends during volcanic eruptions is an important but poorly constrained parameter that controls many of the processes that influence eruptive behavior. In this study, we quantify decompression rates for basaltic magmas using volatile diffusion in olivine-hosted melt tubes (embayments) for three contrasting eruptions of Kīlauea volcano, Hawaii. Incomplete exsolution of H2O, CO2, and S from the embayment melts during eruptive ascent creates diffusion profiles that can be measured using microanalytical techniques, and then modeled to infer the average decompression rate. We obtain average rates of ~0.05–0.45 MPa s−1 for eruptions ranging from Hawaiian style fountains to basaltic subplinian, with the more intense eruptions having higher rates. The ascent timescales for these magmas vary from around ~5 to ~36 min from depths of ~2 to ~4 km, respectively. Decompression-exsolution models based on the embayment data also allow for an estimate of the mass fraction of pre-existing exsolved volatiles within the magma body. In the eruptions studied, this varies from 0.1 to 3.2 wt% but does not appear to be the key control on eruptive intensity. Our results do not support a direct link between the concentration of pre-eruptive volatiles and eruptive intensity; rather, they suggest that for these eruptions, decompression rates are proportional to independent estimates of mass discharge rate. Although the intensity of eruptions is defined by the discharge rate, based on the currently available dataset of embayment analyses, it does not appear to scale linearly with average decompression rate. This study demonstrates the utility of the embayment method for providing quantitative constraints on magma ascent during explosive basaltic eruptions.

Role of mechanical erosion in controlling the effusion rate of basaltic eruptions

AbstractIn many basaltic eruptions, observations show that the effusion rate of magma has a typical dependence on time: the effusion rate curves show first a period of increasing and later a decreasing phase by a maximum value. We present a model to explain this behavior by the emptying of a magma reservoir through a vertical cylindrical conduit with elliptical cross section, coupled with the its widening due to mechanical erosion, produced by the magma flow. The model can reproduce the observed dependence on time of effusion rate in basaltic eruptions. Eruption duration and the maximum value of effusion rate depend on the size of magma chamber, on lava viscosity and strongly on erosion rate per unit traction.

Yakchi chert–volcanogenic Formation—fragment of the Jurassic accretionary prism in the Central Sikhote-Аlin, Russian Far East

The Yakchi chert–volcanogenic formation is differentiated at the base of the stratigraphic succession in the Khor-Tormasu subzone of the Central Sikhote-clin structural–formational zone or the Samarka terrane of the Jurassic accretionary prism. The paper considers the results of biostratigraphic study of its deposits and petrogeochemical studies of its basalts. A tectonically disrupted sequence of the Yakchi Formation is restored on the basis of fossil conodonts and radiolarians, and its Late cermian–Middle Jurassic age is determined. The authors interpret the resulting stratigraphic succession in terms of changing depositional settings on the moving oceanic plate and recognize events of the ocean history recorded in it. Chert accumulated on the oceanic plate in pelagic canthalassa/caleopacifica from the Late cermian through to the Middle Jurassic. Deposition of siliceous claystone in the Late cermian–Early Triassic reflects the decline in productivity of radiolarians and a long anoxic event in Panthalassa. Chert accumulation resumed in the Triassic and persisted in the Jurassic, and it was interrupted by the eruption of basalts of different nature. Formation of the Middle–Late Triassic oceanic intraplate basalts likely occurred on the thick and old oceanic lithosphere and that of the Jurassic basalts on the thin and newly created lithosphere. In the Middle Jurassic, chert accumulation was replaced by accumulation of tuffaceous siltstone at a subduction zone along the csian continental margin. The middle Bathonian–early Callovian age of this siltstone closely predates accretion of the Yakchi Formation. The materials of the upper layer of the oceanic plate that formed over 100 million years in different parts of the ocean and on the lithospheric fragments of different ages were accreted to the continental margin. The bulk of the accreted material consists of oceanic intraplate basalts, i.e., fragments of volcanic edifices on the oceanic floor. accretion of this western part of the Khor-Tormasu subzone occurred concurrently with accretion of the southeastern part of the Samarka subzone in Primorye, which clarifies the paleotectonic zonation of the Central Sikhote-Alin accretionary prism. The cataclastic gabbroids and granitoids, as well as the clastic rocks with shallow-marine fossils in the Khor-Tormasu subzone, are considered as possible analogues of the Okrainka-Sergeevka allochthonous complex.

Balloon-borne measurement of the aerosol size distribution from an Icelandic flood basalt eruption

We present in situ balloon-borne measurements of aerosols in a volcanic plume made during the Holuhraun eruption (Iceland) in January 2015. The balloon flight intercepted a young plume at 8 km distance downwind from the crater, where the plume is ∼15 min of age. The balloon carried a novel miniature optical particle counter LOAC (Light Optical Aerosol Counter) which measures particle number concentration and size distribution in the plume, alongside a meteorological payload. We discuss the possibility of calculating particle flux by combining LOAC data with measurements of sulfur dioxide flux by ground-based UV spectrometer (DOAS).The balloon passed through the plume at altitude range of 2.0–3.1 km above sea level (a.s.l.). The plume top height was determined as 2.7–3.1 km a.s.l., which is in good agreement with data from Infrared Atmospheric Sounding Interferometer (IASI) satellite. Two distinct plume layers were detected, a non-condensed lower layer (300 m thickness) and a condensed upper layer (800 m thickness). The lower layer was characterized by a lognormal size distribution of fine particles (0.2 μm diameter) and a secondary, coarser mode (2.3 μm diameter), with a total particle number concentration of around 100 cm−3 in the 0.2–100 μm detection range. The upper layer was dominated by particle centered on 20 μm in diameter as well as containing a finer mode (2 μm diameter). The total particle number concentration in the upper plume layer was an order of magnitude higher than in the lower layer.We demonstrate that intercepting a volcanic plume with a meteorological balloon carrying LOAC is an efficient method to characterize volcanic aerosol properties. During future volcanic eruptions, balloon-borne measurements could be carried out easily and rapidly over a large spatial area in order to better characterize the evolution of the particle size distribution and particle number concentrations in a volcanic plume.

Oblique rift opening revealed by reoccurring magma injection in central Iceland.

Extension deficit builds up over centuries at divergent plate boundaries and is recurrently removed during rifting events, accompanied by magma intrusions and transient metre-scale deformation. However, information on transient near-field deformation has rarely been captured, hindering progress in understanding rifting mechanisms and evolution. Here we show new evidence of oblique rift opening during a rifting event influenced by pre-existing fractures and two centuries of extension deficit accumulation. This event originated from the Bárðarbunga caldera and led to the largest basaltic eruption in Iceland in >200 years. The results show that the opening was initially accompanied by left-lateral shear that ceased with increasing opening. Our results imply that pre-existing fractures play a key role in controlling oblique rift opening at divergent plate boundaries.