New data on petrography, chemical and mineral composition of Pleistocene- Holocene basaltic lavas from shield volcanoes and scoria cones of Uksichan volcanic field (the Central Kamchatka volcanic belt, Kamchatka peninsula) show that they originate from a common magma source and parental magma, but with different degree and types of fractional crystallization. The computer simulation evidences that the lavas from late shield volcanoes were formed during isobaric crystallization (H2O ~ 2 wet %, ƒО2 – +1.2 NNO) under a pressure of 5 to 1 kbar with a step of 1 kbar. Such physical conditions correspond to relatively slow rising of magma chambers from depth of ~ 15 km to ~ 3 km. Scoria cones were generated during polibaric fractional crystallization (H2O ~ 2.6 wet. %, ƒО2 – +1.1 NNO) with decompression speed of 0.25 kbar / % crystallization. This type of differentiation assumes the relatively fast melt transport without a long storage in the crust magma chambers. Petrochemical indicators of two differentiation types are behavior of CaO and Al2O3.

A multidisciplinary study of samples that were dredged during cruises of the R/V Vulkanolog was continued in order to interpret results from a survey of the anomalous magnetic field on the Pacific seafloor. We acquired data on the main ferromagnetic carrier and the nature of magnetization. The naturally occurring magnetic parameters and petromagnetic properties of the rocks that make up the edifices of the Sofu underwater volcanic cluster at the Izu-Bonin island arc have been studied. We provide the results of thermomagnetic, petrologic, and microprobe studies of the dredged samples. It is shown that the main magnetic mineral carriers of natural remanent magnetization are unaltered titanomagnetite and oxidized titanomagnetite. The differentiation of rocks over values of natural remanent magnetization was due to different crystallization conditions for titanomagnetite, while that over magnetic susceptibility was controlled by the magnetic grain size. The structural, mineralogic, and petrographic features of the samples show that they were taken from rocks that were discharged during different phases of eruptions.

This is the first petrophysical study of extrusive rocks (dacites to andesites) discharged by Bezymianny Volcano. We provide a comparative description of properties for extrusive rocks in accordance with identified age groups. We show the dynamics in the variation of extrusive rock properties in relation to their ages, with the result that the older a rock the higher are its density, strength, and elastic parameters. Rocks petrophysical features are compared between extrusive domes and lava flows. We argue for petrophysical properties to be applicable for deriving more accurate results for the genesis of rocks having similar petrophysical properties, in particular, rocks of extrusive and effusive origin.