Two powerful eruptions of Quizapu vent on Cerro Azul Volcano, Chile are used as examples to discuss
the problem of effusive eruptions of magmas having high preeruptive volatile concentrations. A physicochemical
mechanism is proposed for magma degassing, with the volatiles being lost before coming to the surface.
The model is based on the interaction of magmas residing in chambers at different depths and on the difference
between the solubility of water in the melt and the water equilibrium concentration in a magma body
having a considerable vertical extent. The shallower chamber can accumulate the volatiles released from the
magma that is supplied from the deeper chamber. An explanation is provided of the dramatic differences in the
character of the 1846–1847 and 1932 eruptions, which had identical chemical–petrographic magma compositions.

The 1991 eruption of Avacha volcano resulted in a lava plug inside its crater, making high-temperature fumaroles available for sampling. At present, there are two high-temperature fumarolic fields: the Eastern (up to 665 ◦ C) and the Western (up to 840 ◦ C), both associated with a fissure in the lava plug caused by a weak 2001 explosion. The paper presents chemical and isotopic compositions (H-O-C-S) of the directly sampled fumaroles over the period 2013 – 2023, mainly from the Eastern field. We revealed seasonal variations of water isotopic composition and concentrations of some components of the gas. High-temperature gases from Avacha volcano are characterized by chemical and isotopic compositions typical for volcanoes in subduction zones, but with a slightly increased content of H2O, a reduced content of HCl. A relatively high concentration of methane is noted in the gases of low-temperature field. Methane in high-temperature gas with δ13C(CH4) = 16.8 ‰ has abiogenic origin. For high-temperature gases, their redox state (H2/H2O and CO/CO2) is controlled mainly by the sulfur gas buffer (H2S/SO2); methane is not chemically equilibrated. The molar ratio C/S ~ 1 is typical for volcanoes in the Kuril-Kamchatka Arc. The measured fumarolic temperatures at the Eastern field are descending over time from 626 ◦ C in 2013 to 410 ◦ C in 2023. The apparent equilibrium temperatures calculated for reactions that include CO, CO2, H2, H2O, H2S and SO2 are generally higher than the measured temperatures and do not show the descending trend. However, calculated equilibrium temperatures for the H2O-CO-CO2-CH4 system are very close to the measured temperatures. Two periods of the increased seismic activity which occurred from 2013 to 2023, in November 2014 – January 2015 and October – December 2019, correlated with changes in the morphology and gas flow rates at the Western fumarolic field.

Previous data are summarized and new evidence is presented on the Avacha eruptions of 1926-1927, 1938, and 1945. The last eruption of January 1991 is described. The dynamics of the Avacha eruptive activity is considered for a period of 1737-1991. The eruptions are classified into different types. The type and size of a future event are predicted and the related hazard is assessed. It is recommended that the southwestern and southern sectors of the Avacha surrounding should be declared forbidden for residential or industrial construction because of a high volcanic hazard. -Journal summary

Some of the previous views on the style of the Avacha eruptions during 1737-1909 are revised on the basis of new data obtained by the authors. The types of eruptions, their geological and geomorphological effects, and the related volcanic hazards are reassessed. All eruptions were explosive events, except for the 1894-1895 extrusive-explosive eruption. The eruptions of 1737, 1779, and 1827 are classified as large, the others, as mild or medium-size events. -from Journal summary

Remnants of five catastrophic lahars have been discovered, described, and dated by the carbon-14 method. They occurred during eruptions of Avacha (violent explosions with voluminous juvenile pyroclastics) and Koryakskiy (large fissure lava flows): 3500 to 3200 14C years ago or 1900-1500 years B.C. These lahars were much higher in vigor, hazard, and effect on the environment than the lahars generated by the historic eruptions of these volcanoes. -from Journal summary

During the 2012-2013 we have measured SO2 on Kamchatka volcanoes (Gorely, Mutnovsky, Kizimen, Tolbachik, Karymsky, Avachinsky) using DOAS (differential optical absorption spectroscopy). Mobile-DOAS, on a base of USB2000+, has been used as an instrument. The goal of this work was to estimate SO2 emission by Kamchatka volcanoes with the different types of activity. Mutnovsky and Avachinsky during the measurements period passively degassed with SO2 emission ~ 480 t/d and 210 t/d, respectively. Gorely volcano was very active, with intensive vapor-gas activity with gas discharge rate 800-1200 t/d. During the measurements at Karymsky volcano there were relatively weak explosive events (ash plum rose up to 0.5 km above the crater) with 5-10 minutes periodicity. For this time, SO2 discharge rate was ~350-400 t/d. Due to the remoteness and difficulties for accessibility of Kizimen volcano, the measurements were done only once – on October 15th, 2012. 5 traverses have been done above the gas plume. SO2 emission was ~ 700 t/d. On Tolbachik fissure eruption we have measured SO2 emission repeatedly from January until August 2013. The intensive effusion of the lava flows (basaltic andesite by composition) and frequent explosions in the crater of the cinder cone were characteristic features of this eruption. The measured gas emission was from ~1500-2200 t/d in January until 600-800 t/d in August 2013. All measurements were made not permanently, but to the extent possible. Therefore, it is difficult to make detailed conclusions on the SO2 emission on these volcanoes. Nevertheless, this research may become a starting point for the development of the system of the constant monitoring of volcanic gases emission by the active volcanoes of Kamchatka.

Estimation of the sulfur dioxide emission by Kamchatka volcanoes using differential optical absorption spectroscopy.

The geochemical variations of magmas across and along supra-subduction zones (SSZ) have been commonly attributed to profound changes in the phase and chemical compositions of the mantle source and subduction-derived melt and fluid fluxes, as well as the physical parameters (e.g. depth, temperature, oxygen fugacity etc) of slab dehydration, mineral breakdown and melting. Here we test the variability of the Late Quaternary primitive magmas in the southern and northern parts of the meridionally oriented Eastern Volcanic Belt (EVB) of Kamchatka, with a slab depth varying from 60 to 160 km. Eight high-Mg (Mg# > 60 mol%) basalts were characterized for major, trace and platinum-group element (PGE) abundances, as well as the compositions of olivine phenocrysts and olivine-hosted spinel inclusions. The basalts in our study are geochemically typical of SSZ magmas and contain similar liquidus assemblages of forsteritic olivine (Mg# 78–92 mol%), low-Ti Cr-spinel and clinopyroxene. Although the absolute abundances of major and trace elements, and their ratios, in the basalts fluctuate to some extent, the observed variability cannot be correlated with any of considered parameters in the geometry of the Kamchatka SSZ and conditions of melting. This unexpected result led to the evaluation of the platinum-group element (PGE) systematics against the lithophile and chalcophile trace element geochemistry and the compositions of phenocrysts. Total whole-rock PGE content varies from 2.3 to 11.7 ppb, whereas the normalized PGE concentration patterns are typical for supra-subduction zones magmas and broadly similar in all studied samples. They are enriched in Rh, Pd and Pt relative to mid-ocean ridge basalts (MORB) and have nearly identical concentrations of Ir-group PGE. The only parameter that correlates well with PGE contents is the average Mg# of olivine phenocrysts from 84 to 90.3 mol%. This is interpreted to result from minor cryptic fractionation of sulfide melt, together with primitive olivine, in low-to-mid crustal conditions. Negative Ru anomalies on chondrite-normalized diagrams correspond to the Fe2+/Fe3+ ratios in spinel (a proxy for magma redox conditions), which reflects a replacement of monosulfide solid solution by laurite in the mantle wedge during oxidation.