Island arc volcanism involving differentiated magmas (andesites to rhyolites) is one of the most dreaded types due to the violence of the eruptions and our difficulty to predict the eruption time, style, and damage extent. Andesitic stratovolcanoes commonly produce effusive growth of lava domes, whose collapse generate low-energy C-PDC, and highly explosive events generating pumice C-PDC. Effusive and explosive stages sometimes occur during the same eruption, as illustrated by the P1 eruption dated to ~ 1350 AD at Mt Pelée, Martinique, FWI (Boudon and Balcone-Boissard 2021) and the 1530 AD eruption of la Soufrière of Guadeloupe (Boudon et al. 2008). In some rare cases, flank collapse following the growth of a lava cryptodome in one flank of the volcano can trigger laterally-directed blasts that generate extensive high-energy diluted pyroclastic density currents (D-PDC), as illustrated for instance by the 1980 eruption at Mt St Helens, WA, USA (Voigt et al. 1981) and the 1956 Bezymianny eruption, Kamchatka, Russia (Belousov 1996; Belousov et al. 2007). Several eruptions show the sequence of flank collapse, laterally-directed blast, Plinian column and/or deposition of pumice C-PDC, terminated by lava dome growth, such as the 1980 eruption at Mt St Helens (Voigt et al. 1981) and the 1956 Bezymianny eruption (Belousov 1996; Belousov et al. 2007). At Mt St Helens in 1980, the combination of direct observations of the eruption with the study of the erupted products allowed major advances in the understanding of the volcanic processes. A better understanding of these devastating eruptions is necessary to progress towards a better evaluation of the risk assessment, which is particularly important for densely populated areas.
Although located in a remote area with limited risks of casualties, Bezymianny’s 1956 climactic eruption is an opportunity to investigate the mechanisms and timescales that lead to flank collapses and laterally-directed blast explosions, pumiceous explosive phases, and lava-dome growths, which can be used for other volcanic systems for which risk assessment is highly critical. To this aim, we propose two companion papers: the present one (part I) is dedicated to a refinement of the plumbing system beneath Bezymianny’s edifice through petrological constraints, and a second one (part II; Ostorero et al. subm.) focuses on the timescales of the reactivation processes before Bezymianny’s 1956 climactic eruption.
The 1956 eruption of Bezymianny volcano
Geological setting
Bezymianny is an andesitic stratovolcano located in the Central Kamchatka Depression that results from the Pacific oceanic plate subducting beneath the Okhotsk oceanic plate at a rate of 8 cm/year (DeMets, 1992). Bezymianny belongs to the Klyuchevskoy group of volcanoes that includes the presently active Bezymianny, Tolbachik, and Klyuchevskoy volcanoes (with approximate altitudes of 2900, 3700, and 4700 m, respectively). Yet, these three volcanoes have different compositions, eruptive styles, and feeding systems, as demonstrated by seismic tomography (Koulakov et al. 2017). Indeed, seismic tomography and thermal remote sensing studies suggested that the explosive Bezymianny volcano is fed through a dispersed system of crustal reservoirs, where felsic magma fractionates from mafic magma bodies, ascending to the upper crust from where eruptions of andesitic magmas are sourced (Koulakov et al. 2017, 2021; Coppola et al. 2021).
Eruptive chronology
After 1000 years of dormancy, Bezymianny awakened in September 1955 with increasing seismic activity (hypocenters estimated at depth less than 5 km; Gorshkov and Bogoyavlenskaya 1965; Belousov et al. 2007). On October 22nd 1955, the first historical eruption of Bezymianny began with an explosive phreatic emission of steam and ash (Gorshkov 1959), followed by lava dome growth in the new crater (seen for the first time on 25 January 1956) and a slow uplift (~ 100 m) of the southeastern slope of the cone interpreted as magma emplacement in a form of a cryptodome (Belousov and Belousova 1998). The volume of the accumulated cryptodome magma was estimated to 0.15–0.20 km3 (Belousov et al. 2007). The flank of the Bezymianny cone destabilized by the cryptodome intrusion gravitationally collapsed on 30 March 1956, first generating a debris avalanche of 0.5 km3, followed by a laterally-directed blast (called blast hereafter) of a volume of 0.2–0.4 km3 resulting from the depressurization of the cryptodome. The blast led to formation of a devastating D-PDC that covered an area of ~ 500 km² (Belousov and Belousova 1998; Belousov et al. 2007; Fig. 1a_Field). The depressurization of the cryptodome, and progressively of the conduit-magma reservoir feeding system, induced by the edifice collapse led to eruption of the blast D-PDC and the post-blast pumice C-PDC generated by an eruptive cloud of 30–40 km high that immediately collapsed gravitationally into widespread pumiceous C-PDCs totaling 0.5 km3 DRE (Belousov 1996; Turner et al. 2013). The total volume of emitted magma during the climactic phase of the eruption was on the order of 1 km3 DRE (Belousov 1996). The 1956 eruption was followed by intermittent lava dome growth and periodic collapses into block-and-ash C-PDCs until now (Turner et al. 2013; Davydova et al. 2022). Since 1977, there have been a few explosive eruptions almost every year, making Bezymianny one of the most active volcanoes in the world since 1956.
Petrology of the erupted products
The 1956 erupted materials are calc-alkaline andesites (60–62 wt% SiO2, 18–19 wt% Al2O3, 6–7 wt% CaO, 3–4 wt% Na2O, and 1–2 wt% K2O; Belousov 1996; Neill et al. 2010; Shcherbakov et al. 2013; Davydova et al., 2022) that comprise various lithologies differing in i) their vesicularity, with the clasts from the blast being on average less vesiculated than the pumices from the post-blast C-PDCs (Belousov et al. 2007) (Fig. 1b,c_Field), and ii) the crystallinity of the groundmasses, with the clasts from the blast being more crystallized than the post-blast C-PDC pumices (Neill et al. 2010). For the blast material, there is a negative correlation between clast vesicularity and groundmass crystallinity (Neill et al. 2010). All juvenile clasts are composed of ~ 35 vol% phenocrysts made up of 21 vol% plagioclase, 3–4 vol% amphibole (partly decomposed), 1–2 vol% Fe-Ti oxides, 1 vol% orthopyroxene, accessory minerals (such as clinopyroxene, Cu-Fe sulfides, and zircon) and ~ 75 vol% groundmass consisting of plagioclase, orthopyroxene, Fe-Ti oxides, silica phases, and a rhyolitic glass (Shcherbakov et al. 2013). Amphibole has bimodal composition (in Al and other elements), where all crystals are surrounded by reaction rims, evidencing a complex magmatic history (Plechov et al. 2008; Shcherbakov et al. 2013; Turner et al. 2013). Plagioclase phenocrysts are strongly zoned, with compositions ranging from 48 to 77 mol% anorthite (An48 − 77; Shcherbakov et al. 2013). Plagioclase microlites from the blast clasts range from An30 to An50 (Neill et al. 2010), but there are no compositional data for plagioclase from the post-blast C-PDC pumices. Orthopyroxene phenocrysts have homogeneous compositions from 62 to 64 mol% enstatite (En62 − 64; Shcherbakov et al. 2013). Fe-Ti oxides are titanomagnetite and rare ilmenite, with 60–70 mol% magnetite and 20–22 mol% ilmenite (Mt60 − 70 and Ilm20 − 22, respectively; Shcherbakov et al. 2013). Silica phase are ubiquitous in the post-blast C-PDC pumices and in the blast samples (Gorshkov and Bogoyavlenskaya 1965; Plechov et al. 2008). Glasses are rhyolitic, with matrix glasses showing SiO2 contents from 74 to 79 wt% and plagioclase-hosted melt inclusions showing 73–76 wt% SiO2 (Shcherbakov et al. 2013). From bulk H2O measurements (Karl-Fischer titration) and mass balance calculations for the blast samples, Neill et al. (2010) suggested glass H2O contents of up to 2 wt% in the less vesiculated clasts (density > 2000 kg/m3) and of about 1.0 ± 0.5 wt% in the more vesiculated material, which indicates solubility pressures of 10–20 MPa.
The magma plumbing system prior to the 1956 and recent eruptions
Several studies have estimated the 1956 pre-eruptive magma storage conditions and have mainly suggested two distinct reservoirs, yet depth or pressure and temperature estimates for these reservoirs differ, depending on the study. Using phenocryst compositions, Kadik et al. (1986) estimated a deep reservoir at pressures of 300–800 MPa (depths of 11–30 km recalculated with a rock density of 2700 kg/m3), temperatures of 875–930°C and > 6 wt% H2O dissolved in the melt. Plechov et al. (2008) also inferred a magma storage pressure range of 300–800 MPa calculated using the MELTS algorithm (Ghiorso and Sack 1995) and a magma storage temperature of 890 ± 20°C calculated from amphibole-plagioclase thermometry, yet re-heating to ~ 1005°C immediately prior to eruption causing amphibole breakdown at depths of > 500 MPa. Constraints from phase-equilibrium experiments by Almeev et al. (2013a, b) equally suggested magma storage and partial crystallization at a pressure of ~ 600 MPa (23 km depth) and temperatures of 800–920°C. Shcherbakov et al. (2013) suggested a deep reservoir, in which amphibole and plagioclase were stable, at ≥ 200 MPa (≥ 8 km depth), but at relatively low temperature of < 850°C determined based on phase equilibrium experiments. There is thus consensus for the existence of a deep reservoir, in which amphibole was stable, at pressures of about ≥ 200 MPa (~ 8 km) and possibly up to 800 MPa (~ 30 km) and temperatures between ~ 775 and 930°C.
A shallower reservoir has also been proposed by several studies. From amphibole destabilization rims and phase-equilibrium experiments starting with the 1956 eruption products, Shcherbakov et al. (2013) suggested that magma of the climactic phase of the 1956 eruption was transiently stored at a pressure of 50–100 MPa (~ 2–4 km depth) and at a temperature of 890–930°C for at least up to 40 days in the case of the cryptodome-forming and possibly as short as 2–14 days for the pumice-forming magma. The authors further suggested heating of the magma upon final ascent to ~ 950–1000°C, likely from latent heat release resulting from decompression-induced microlite crystallization. There is thus consensus for the existence of a shallow pre-eruptive reservoir, in which amphibole was not stable, at pressures somewhere between 30 and 100 MPa (1–4 km) and at magma temperatures of ~ 900–930°C.
Although the post-1956 eruptions mostly produced basaltic andesites (as opposed to andesites in 1956) and the magmas may not be stored as they were in 1956, recent seismic surveys suggested the existence of two levels of magma storage. From seismic tomography surveys below the Klyuchevskoy group of volcanoes, Thelen et al. (2010) and Fedotov et al. (2010) suggested a large and deep magma body at ≥ 7–10 km depth, that is at pressures of ≥ 200–300 MPa, for the 1956–2010 period of Bezymianny activity. From earthquake distribution centers, Thelen et al. (2010) proposed magma storage at 1.0-1.5 km depth, that is, at ~ 30–40 MPa. Combining seismic tomography, remote sensing, and petrological data, Koulakov et al. (2021) suggested that Bezymianny’s 2017 explosive eruption was controlled by the coexistence of magma and gas reservoirs located at depths of 2–3 km below the summit.
Characterizing the plumbing system beneath Bezymianny prior to 1956 is a prerequisite step for unravelling the processes and components driving Bezymianny’s eruptive dynamics and the basis for constraining the timescales of the reactivation processes before the 1956 eruption (Ostorero et al. subm.). A number of estimates already exist, on which we build, but none of the studies to date has petrologically characterized a large number of blast and pumice samples as we have, or an equally large number of phases in as much detail as we do here. To refine the pressure (depth) and temperature conditions of the shallow plumbing system beneath Bezymianny, we have performed a petrological study of the 1956 blast products and post-blast C-PDC pumices, with key constraints from amphibole destabilization rims and groundmass microlites. The proposed 1956 magma plumbing system beneath Bezymianny is then compared to the recent plumbing system beneath the volcano as well as other volcanic centers that have experienced sector collapse that generated a blast followed by a pumiceous explosive phase.