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 paper proposes a formalized technique of probabilistic forecast by applying statistical estimation of seismicity level and a number of additional functions that characterize the predictive situation. The possibilities of this method are illustrated by the example of seismic activation before the eruptions of Bezymianny Volcano (Klyuchevskaya group, Kamchatka).
The initial data is the catalog of earthquakes of the Klyuchevskaya group of volcanoes during 1999-2014, compiled by the Kamchatka Branch of the Geophysical Survey, RAS. During this time there were 21 eruptions of Bezymianny Volcano. The precursor is defined as a threshold function associated with the current seismicity level and its characteristic form before the eruption.
The values of parameters characterizing the precursor are given including validity, reliability and efficiency, calculated by two methods. It is shown that the reliability of the precursor decreases with the increase in threshold values, and its validity increases.
The reliability of the method is 0.38–0.95, i.e., from 38 % to 95 % of eruptions had the precursor depending on the threshold level; and the validity is 0.3–0.6, i.e., from 30 % to 60 % of the identified precursors are realized also depending on the threshold level. Values of efficiency confirm the non-random nature of the precursor appearance.
The method includes determination of the probability of the forecast realization. The nomogram for probabilities depending on the duration of the forecast and the values of the prognostic parameter is designed.

Abstract—This study is concerned with the petrographic, mineralogic, and geochemical features in the K-high basaltic trachyandesites that were discharged by the 2012–2013 parasitic eruption on Ploskii Tolbachik Volcano. These K-high basaltic trachyandesites exhibit some obvious characteristics that testify to their suprasubduction origin. They are deeply differentiated rocks with strongly fractionated plagioclase.A study of the Sr, Nd, and Pb radiogenic isotope ratios in the K-high basaltic trachyandesites provided evidence of their mantle origin and of the fact that the crust has exerted no influence on their compositions. We performed a comparative analysis of the ratios of the concentrations for some incoherent elements in the K-high basaltic trachyandesites, as well as in intraplate, riftogenic, and island-arc moderate potassium basalts and basaltic andesites in relation to the concentrations of these elements in the primitive mantle. The geochemical features of these K-high basaltic trachyandesites classify them as belonging to the suprasubduction subalkaline formation of the potassium series.