Journal of Volcanology and Seismology最新文献

This paper deals with the isotope composition of helium (³Не/⁴Не = R) in the groundwater of the Southern Baikal volcanic area (SBVA) and Southern Khangai volcanic area (SKhVA) during the Late Cenozoic Period. We have found differences in the behavior and value of that parameter. It was found that the differences in the concentrations of ³Не/⁴Не in the SBVA and the SKhVA resulted from mantle reservoirs that have different isotope compositions of helium. This confirms that the Late Cenozoic volcanism in the SBVA and SKhVA is controlled by mantle sources related to mantle plumes of the Central Asian hot mantle field.

This paper provides information on the 2022 eruptive activity of Ebeko Volcano. Phreatic explosions had been occurring in the crater lake from January 22 to June 13 due to water seepage through a plug in the upper part of the magma conduit with subsequent boiling. Vulcanian type explosions started since June 14 and dried the lake. The ash particle-size distribution changed toward smaller sizes. Petrographic, mineralogical, and geochemical studies of the tephra define this period as a phreatomagmatic eruption based on the presence of fresh juvenile material. Interaction between magma and waters of the Ebeko hydrothermal system results in its depletion in alkali and enrichment in silica. We hypothesize that the formation of amorphous water-bearing silica in the form of numerous segregations and its subsequent dehydration can favor the volcano’s explosive activity.

This study examines and compares 3D distributions of crustal and upper mantle density contrast with a set of geological and geophysical data for the heads of six plumes (Yellowstone, Emeishan, Cathaysia, Sea-of-Okhotsk, Maya–Selemdzha, and Indigirka–Kolyma plumes) down to 200 km depth. According to our data, the asthenospheric parts of the plumes have mushroom shapes, while the asthenospheric magmas are spreading out beneath the lithosphere bottom, less frequently beneath the crustal bottom. The plume heads become narrower at distances of 250–300 km from the central conduit to decrease to diameters of 200–300 km at depths of 100–120 km. In most of the cases, the plume lithospheric and crustal fragments are convex toward the ground surface. The uplifts are occasionally complicated with local depressions in the upper crust, which can be explained by subsidence of the domes of the structures above magma chambers in the subcrustal viscous layer and asthenosphere. Plumes are frequently associated with zones of lithosphere tension (rifts), resulting in linear zones of lower viscosity being mapped in the lower lithospheric and crustal cross sections of the plumes. The structural settings of the plumes under consideration here are controlled by boundaries of lithosphere plates and large segments of the second order. The identity of geometry and rheology in the plumes that were formed at different times (Triassic to Neogene), and in regions far removed from each other (Northeast Russia, Amur region, northwestern United States, South China, and Sea of Okhotsk), provide evidence of the universality of the tectonic settings that favor the penetration of mantle flows into the upper tectonic shells of the Earth. The foremost among these are tension zones in the lithosphere, especially areas where differently directed lithospheric faults intersect.

To explore an efficient approach refining seismomagnetic information through magnetic field survey technology, the author introduces the seismomagnetic monitoring system of the capital region in China. It includes field measurement techniques, data processing methods and application performance. The variation characteristics of the crustal magnetic field and seismomagnetic information in this area are systematically calculated and statistically analyzed based on the geomagnetic data products of the X, Y, and Z components. The results show that the mean amplitudes of X, Y and Z of the crustal magnetic field in the study area are 1.87, 1.90, and 1.39 nT, and the mean square deviations are 2.41, 2.48, and 1.94 nT, respectively. The epicenters of future large earthquakes will mostly lie near the “0” value line of crustal magnetic field variation with very low amplitude, which X, Y and Z elements are 18.7, 45.5 and 53.9% of the mean values in the survey area, respectively, and rise obviously after the earthquake. There is a high correlation between the location of the epicenter and the anomalous region of low spatial variation ratio of geomagnetic field secular variation.

We have studied the geological structure, material composition of the ores, and the age of the Nizhny Birkachan volcanogenic gold–silver deposit discovered recently. The ore bodies consist of veins and vein-streak zones of adularia-carbonate-quartz composition; they lie in granodiorite porphyries with U‒Pb zircon age (ID-TIMS) equal to 335 ± 2 Ma. The ores are low sulfide, low silver (Au/Ag = 1–2), with pyrite dominating the ore minerals. The Ag minerals are tennantite, Ag sulfide, native gold and silver, and hessite. From an ore vein we obtained an adularia-based ⁴⁰Ar/³⁹Ar age equal to 169 ± 4 Ma, which reflects the rejuvenation of the isotopic argon system after the emplacement of a dike of unaltered Jurassic basites that cuts through the ore body. The Nizhny Birkachan deposit has a geological structure and ore composition that are very similar to those of other Au-Ag deposits at the Kedon volcano-plutonic belt such as Kubaka and Birkachan; it was also formed in the age span 290–335 Ma.

Crustal magmatic structures possess pronounced resonant properties, which enable these structures to generate secondary seismic waves at their eigen frequencies. Strain data acquired with the help of a 75-meter laser interferometer were used to identify resonant modes and to estimate the parameters of the magmatic structures beneath the Elbrus Volcanic Center. Such resonant modes are unique features for each magmatic feature, and determine the size and physico-mechanical properties of an identified internal structure. The present study contains an analysis of a local feature that has manifested itself as a compact area in the form of weak seismic pulses that have been recorded in the Elbrus area using small-aperture seismic instruments operated by the Geophysical Survey of the Russian Academy of Sciences (GS RAS) in 2011. The results of these studies based on seismic and strain observations, as well as on the results of microseismic sounding, have shown the existence of a new peripheral shallow magma chamber 2.5‒3 km across in size as part of the Elbrus Volcanic Center.

This paper presents new results from geological-stratigraphic and isotope geochronological studies of young lavas in the northeast part of the Javakheti Highland, Lesser Caucasus, Georgia. We provide a first description of the Algeti complex-structured valley lava flow about 55 km in total length; no information on this feature is available in the geological literature. We show that young magmatism in the northeast part of the Javakheti region has been evolving in the time interval of 3.2–1.5 Ma B.P. Its earlier phases have produced the longest (up to 100 km) valley basaltic lava river of those known in the Lesser Caucasus (the Khrami flow) (3.19 ± 0.10 Ma B.P.). Subsequently (2.7–2.5 Ma B.P.), the eruptions continued to form extensive lava plateaus there (Tsalka, Gomareti, and other plateaus). In the Late Piacentian–Early Gelasian (2.7–2.0 Ma B.P.) the active vents in the northern Javakheti Range started to form the Algeti basaltoid valley flow, with this process lasting for ~1 Ma. The terminal phase of its formation (1.9–1.5 Ma B.P.) was probably related to eruptions of the volcanic cones in the area of Lake Tabatskuri. These data, along with the reconstruction of the history of young magmatism, enabled us to trace the main patterns in the generation of the present-day relief and the network of river valleys in the area of study in the Lesser Caucasus.

In this study, we combined the results of petrography [pheno-crystallinity (({{phi }_{{PC}}}))] and magma compositions (bulk and melt compositions) to calculate the magma viscosity (({{{{mu }}}_{{{text{eff}}}}})) of the lava flows and domes that erupted from Mount Ungaran, Central Java, Indonesia. The lava flows were characterized by slightly larger SiO₂ variations than those of lava domes, with a large overlap between each phase (46.7–57.8 and 53.2–59.8 wt % SiO₂, respectively). However, lava flows were typically less crystalline than the lava domes (average ({{phi }_{{PC}}}) of 33 and 40%, respectively). Because lava flows share an identical composition to lava domes and temperature is inversely proportional to SiO₂ content, it is inferred that magma composition and temperature did not play a substantial role in controlling magma viscosity. Instead, we found that pheno-crystallinity was the most important parameter. Specially, for a ±7% difference of pheno-crystallinity (at a given SiO₂), magma viscosity could differ by one order of magnitude, ultimately controlling lava morphology: high-viscosity magma (5.6–7.8 log Pa s) formed lava domes, whereas low-viscosity magma (4.6–6.6 log Pa s) produced lava flow. Moreover, we found that lava dome samples exhibited gentler phenocryst size distribution (CSD) slopes than lava flow samples (2.1–3.4 and 2.7–6.9, respectively). Because the CSD slope was inversely proportional to the magma residence time (CSD slope = –1/Gt), we suggest that lava dome formation, which requires a high magma viscosity, originates from a longer-lived and more crystalline magma, whereas lava flow with low magma viscosity originates from a young and less crystalline magma. Thus, in the case of mafic-to-intermediate magma, as in the present case, we think that the resultant lava morphology is strongly controlled by the abundance of phenocrysts and magma residence time.

Lava domes and lava flows are major manifestations of effusive volcanic eruptions. Less viscous lava tends to flow long distances depending on the volcanic slope topography, the eruption rate, and the viscosity of the erupted magma. When magma is highly viscous, its eruption to the surface leads to the formation of lava domes and their growth. The meshless smoothed particle hydrodynamics (SPH) method is used in this paper to simulate lava dynamics. We describe the SPH method and present a numerical algorithm to compute lava dynamics models. The numerical method is verified by solving a model of cylindrical dam-break fluid flow, and the modelled results are compared to the analytical solution of the axisymmetric thin-layer viscous current problem. The SPH method is applied to study three models of lava advancement along the volcanic slope, when the lava viscosity is constant, depends on time and on the volume fraction of crystals in the lava. Simulation results show characteristic features of lava flows, such as lava channel and tube formation, and lava domes, such as the formation of a highly viscous carapace versus a less viscous dome core. Finally, the simulation results and their dependence on a particle size in the SPH method are discussed.

We present an overview of the different seismic precursors of the Nyiragongo volcano eruption on May 22, 2021, as well as a statistical analysis of the seismic swarms recorded during the pre-eruptive period. The analysis of the seismic activity of Nyiragongo volcano during the pre-eruptive period shows that there was a particular seismicity composed of hybrid and/or volcano-tectonic type events that characterized the long-period seismic swarm recorded about a month before the May 2021 eruption. In February 2016, similar seismic activity was observed at Nyiragongo volcano, which led to the opening of a new vent within Nyiragongo's main crater on February 29, 2016, and which remained active until the May 2021 eruption. During the period from January 2016 to May 2021, the lava lake remained very active with significant fluctuation as evidenced by the Real Seismic Amplitude measurement at Rusayo and Kibati stations. The increase and stabilization of the lava lake to a higher level resulted in an increase and accumulation of stress on the flanks of the volcano since the lava lake had already reached a higher critical level. Whether in January 2016, November 2016 or April 2021, we show that this particular type of so-called hybrid seismic events recorded during a swarm at Nyiragongo volcano could lead to an eruption either inside the main crater (February 2016) or on the flanks in May 2021.