Ключевская группа вулканов (КГВ) располагается в северной части Центральной Камчатской депрессии (ЦКД) и является одной из самых крупных и наиболее активных вулканических структур на Камчатке и в мире. В южной части КГВ находится позднеплейстоцен-голоценовый Толбачинский массив, который извергался неоднократно в течение голоцена и исторического времени.
Первые исторические описания активности Толбачинского массива были выполнены русским исследователем Степаном Крашенинниковым, сделавшим записи об извержении Толбачика 1739 года. Позднее, в течение 20-го столетия, исследователи изучали и публиковали данные о многочисленных доисторических и исторических извержениях как на вершине вулкана Плоский Толбачик, так и в зоне моногенных конусов на юго-юго-западном фланге массива. В основном эти работы были посвящены динамике извержений и постэруптивным изменениям, морфологии вершинной части вулкана.
Всемирную известность вулкан Толбачик приобрел только после Большого трещинного Толбачинского извержения 1975–1976 гг. (БТТИ), когда в течение полутора лет на южном склоне вулкана сформировались четыре новых моногенных конуса и связанные с ними лавовые поля. Это извержение излило на поверхность высоко-Mg и высоко-Al базальты с общим объемом продуктов извержения (лава и тефра) 2,2 км3, что сделало его одним из крупнейших извержений XX века. В период между БТТИ и извержением 2012–2013 гг. множество публикаций в российских и международных журналах представляли данные об извержении Толбачика 1975–1976 гг. Благодаря этому к настоящему времени большинство существующей информации по Толбачинскому вулканическому массиву относится к извержению 1975–1976 гг. В то же время другие части этого грандиозного массива и прилегающие к нему моногенные конусы и лавовые поля изучались в гораздо меньшей степени.
В той же зоне моногенных конусов, что и извержение 1975–1976 гг., в 2012 г. началось новое крупное извержение – Трещинное Толбачинское извержение имени 50-летия ИВиС (ТТИ-50). Задача настоящего раздела – дать обзор накопленных знаний и существующих гипотез по геологии, тектонике, петрологии, геохимии, геофизике и истории развития Толбачинского вулканического массива к началу ТТИ-50.

High-precision electron microprobe analyses were obtained on olivine grains from Klyuchevskoy, Shiveluch and Gorely volcanoes in the Kamchatka Arc; Irazu, Platanar and Barva volcanoes of the Central American Arc; and mid-ocean ridge basalt (MORB) from the Siqueiros Transform. Calcium contents of these subduction zone olivines are lower than those for olivines from modern MORB, Archean komatiite and Hawaii. A role for magmatic H2O is likely for subduction zone olivines, and we have explored the suggestion of earlier workers that it has affected the partitioning of CaO between olivine and silicate melt. We provide a provisional calibration of DCaO Ol/L as a function of magmatic MgO and H2O, based on nominally anhydrous experiments and minimally degassed H2O contents of olivine-hosted melt inclusions. Application of our geohygrometer typically yields 3–4 wt % magmatic H2O at the Kamchatka and Central American arcs for olivines having 1000 ppm Ca, which agrees with H2O maxima from melt inclusion studies; Cerro Negro and Shiveluch volcanoes are exceptions, with about 6% H2O. High-precision electron microprobe analyses with 10–20 lm spatial resolution on some olivine grains from Klyuchevskoy and Shiveluch show a decrease in Ca content from the core centers to the rim contacts, and a sharp increase in Ca in olivine rims. We suggest that the zoning of Ca in olivine from subduction zone lavas may provide the first petrological record of temporal changes that occur during hydration of the mantle wedge and dehydration during ascent, and we predict olivine H2O contents that can be tested by secondary ionization mass spectrometry analysis.

A series of large caldera-forming eruptions (361–38 ka) transformed Gorely volcano, southern Kamchatka Peninsula, from a shield-type system dominated by fractional crystallization processes to a composite volcanic center, exhibiting geochemical evidence of magma mixing. Old Gorely, an early shield volcano (700–361 ka), was followed by Young Gorely eruptions. Calc-alkaline high magnesium basalt to rhyolite lavas have been erupted from Gorely volcano since the Pleistocene. Fractional crystallization dominated evolution of the Old Gorely magmas, whereas magma mixing is more prominent in the Young Gorely eruptive products. The role of rechargeevacuation processes in Gorely magma evolution is negligible (a closed magmatic system); however, crustal rock assimilation plays a significant role for the evolved magmas. Most Gorely magmas differentiate in a shallow magmatic system at pressures up to 300 MPa, ∼3 wt% H2O, and oxygen fugacity of ∼QFM + 1.5 log units. Magma temperatures of 1123–1218 °C were measured using aluminum distribution between olivine and spinel in Old and Young Gorely basalts. The crystallization sequence of major minerals for Old Gorely was as follows: olivine and spinel (Ol + Sp) for mafic compositions (more than 5 wt% of MgO); clinopyroxene and plagioclase crystallized at ∼5 wt% of MgO (Ol +Cpx + Plag) and magnetite at ∼3.5 wt% of MgO (Ol + Cpx + Plag +Mt). We show that the shallow magma chamber evolution of Old Gorely occurs under conditions of decompression and degassing. We find that the caldera-forming eruption(s) modified the magma plumbing geometry. This led to a change in the dominant magma evolution process from fractional crystallization to magma mixing. We further suggest that disruption of the magma chamber and accompanying change in differentiation process have the potential to transform a shield volcanic system to that of composite cone on a global scale.

Kamchatka and the Kuril Islands are home to 36 active volcanoes with yearly explosive eruptions that eject ash to heights of 8 to 15 km above sea level, posing hazards to jet planes. In order to reduce the risk of planes colliding with ash clouds in the north Pacific, the KVERT team affiliated with the Institute of Volcanology and Seismology of the Far East Branch of the Russian Academy of Sciences (IV&S FEB RAS) has conducted daily satellite-based monitoring of Kamchatka volcanoes since 2002. Specialists at the IV&S FEB RAS, Space Research Institute of the Russian Academy of Sciences (SRI RAS), the Computing Center of the Far East Branch of the Russian Academy of Sciences (CC FEB RAS), and the Far East Planeta Center of Space Hydrometeorology Research (FEPC SHR) have developed, introduced into practice, and were continuing to refine the VolSatView information system for Monitoring of Volcanic Activity in Kamchatka and on the Kuril Islands during the 2011–2015 period. This system enables integrated processing of various satellite data, as well as of weather and land-based information for continuous monitoring and investigation of volcanic activity in the Kuril–Kamchatka region. No other information system worldwide offers the abilities that the Vol-SatView has for studies of volcanoes. This paper shows the main abilities of the application of VolSatView for routine monitoring and retrospective analysis of volcanic activity in Kamchatka and on the Kuril Islands.

Compositional zonation in olivine phenocrysts and diffusion modelling have been used in the last ten years to estimate magma residence times and the duration of magma ascent. The fundamental assumption is that mixing with newly injected magma into a reservoir triggers diffusional exchange between mafic olivine crystals and more evolved magma and that this magma mixing eventually triggers eruption. If depth of mixing is known, this translates to ascent rates of magmas to the surface. We applied this approach to a series of different arc basalt lavas from Kamchatka to constrain the rates of magma ascent and magma resident in what is one of the most active subduction zones in the world that is also dominated by an abundance of unusually mafic magmas. Our sample collection cover the principal modes of arc magmatism in Kamchatka: from different volcanic complexes (stratovolcano, dikes, summit eruptions, monogenetic cones), of different age (from Late-Pleistocene to Holocene and recent eruptions), from different magmatic regimes (long-lived volcanoes vs. monogenetic eruptions) and different major element composition (from basalt to basaltic andesite of different geochemical character including LILE enrichments). We analyzed and modelled zonation profiles for a range of elements with different diffusivities (e.g. Mg-Fe, Ca, Ni, Mn, Cr) to assess the role of variable diffusivities as a function of major and trace elements in the olivines from different P-T conditions. First data were obtained on samples from the Klyuchevskoy, Shiveluch and Tolbachik, including recent most eruption in 2012/2013. These data show that for some samples the zonation patterns are much more complex than is usually observed: high-Mg olivines at different volcanoes have very different zonation patterns, including normally, reversely zoned grains or even show highly complex repetitive zonation that indicate large compositional changes in the surrounding magma at very short time scales (years). Thus in some Kamchatka basalts, we observe unusual Mg-Fe zonations that are linked to complex mixing, possibly resorption and subsequent crystal growth processes that are generally not preserved due to fast diffusion of Mg-Fe. Based on a first assessment of our measured profiles, the values for diffusion times in Fo-rich olivines (88 to 92% Fo) vary from only a few months to years and thus magma ascent from deep magma sources must have been fast.