Water plays an extremely important role in volcanism: it acts as an evacuator of viscous melts in a variety of ways, which is ensured by the presence of relevant properties of its phase states, which successively changing with the fall of the environmental parameters. In this sense, the supercritical (fluid) state of water is especially significant. The paper provides a summary of fluid properties that are unique in many ways. The properties determine the relationship between water fluid and silicate melt, which in turn explains the cause of volcanic phenomena and the course of eruptions: explosions of different power, the emergence of the so-called fluidized mass, scorching clouds, landslides and breakthroughs on the slopes, the formation of ignimbrites, as well as the mechanism of gas transport to the foot of volcanoes. Both by role and quantity, water is the main volcanic substance, which together with the silicate melt constitutes magma.

Features of the dynamics of volcanic eruptions for the first time are explained as a consequence of the properties of high-temperature phases of water and their transitions. Some properties of water in supercritical (fluid) condition are given. The latter determine the nature of the relationship between water and melt in the volcanic process, which in turn explains the explosions of different power, the rapidity of scorching clouds, the transfer of gases to the foot of volcanoes, breakouts and landslides on the slopes, the occurrence of ash, pumice, pseudoliquefied mass, as well as possibly ignimbrits and volcanic glass. It shows the impossibility of penetration of cold water into the active volcanic channel, as well as their high (before the change of phase) heating in the conditions of the crust.

The paper presents geological and geochemical characteristics of Cretaceous guyots of Marcus-Necker intraplate rise in the Pacific Ocean. Comparison of guyots located in different parts of the extended system revealed significant differences in their geochemical characteristics, which suggest lateral and (or) vertical mantle heterogeneity in the region. The studied volcanic rocks belong to the alkaline series. In some cases, they exhibit low values of Na2O/K2O. This volcanism is not typical for most of intraplate ocean rises. No consistency is observed between volcanic rock composition of guyots, their age and (or) geographic position. A complex pattern of magnetic anomaly lineations and irregular location of seamounts of different age cause difficulties in applying hot spot model to explain the formation of this intraplate magmatic province. The location of seamounts in the structure of the rise system and the entire system in the structure of the Pacific Ocean does not contradict the formation of fault zones, which initiate melting of upper mantle geochemically inhomogeneous substrate and lead to the formation of intraplate rises. Deep fluid flows may cause a large-scale melting of the upper mantle.

SIGMA-3D software package of structural interpretation is an effective tool for the automated three-dimensional modeling while studying magnetic anomalies created by underwater volcanoes. With the help of this software package submarine volcanoes Grigorieva, Krylatka, and Yubileiny, located in the Kurile island arc, have been studied.

Five major explosive eruptions discharging pyroclastic volumes of more than 1 km^3 have been identified in the Holocene eruptive history of Avacha Volcano. All of these were Plinian-type eruptions, three (of andesitic composition) occurring 7150, 5500 and 4500 14C B.P. Two eruptions (3500 and 3280 14C B.P.) that supplied andesitic basalts preceded or started the formation of Young Cone on Avacha Volcano. We have reconstructed the dynamics and parameters of the eruptions, assessed their geologic, geomorphic and ecologic effects, and provided a long-term forecast.

This paper discusses two catastrophic Plinian-type eruptions which occurred close enough in time (IIAV1 -3500 and IIAV3 - 3280 14C yrs B.P.) on Avacha Volcano and initiated the activity of its Young Cone. We studied the stratigraphy of the ejecta, reconstructed their chronology and parameters, assessed their environmental impact. The ejecta of both of these eruptions were dominated by tephra whose volume is >3 and >1.1 km3, respectively. The eruptive columns rose as high as 21-28 km. The IIAV1 ash layer can be followed for 300 km northeast of the volcano, the ashfall area enclosed within the 1 cm isopach being about 50000 km2. Both eruptions were accompanied by pyroclastic flows, surges, and catastro3phic lahars. The juvenile3pyroclastics is basaltic andesite. By the values of total discharge volume (>3.6 km3 for IIAV1 and >1.21 km3 for IIAV3), these eruptions must be among the largest to have occurred during the eruptive history of the Young Cone.