ИСТИНА

The work describes a simple physical model that provides a theoretical justification for a single origin of the different types of volcanic eruptions. The leading mechanism of this model is that a vertical column of magma under certain conditions starts to be erupted by the critical action of water vapor fluid contained by the crust under high pressure and high temperature. This fundamental mechanism of imbalance between liquid and gas was first described as a mechanism of geyser eruption (Nechayev 2012a). A generalization of this mechanism in case of volcanic eruptions is developed in (Nechayev 2012b). We assume that the acceleration of magma and its eruption results from a pressure difference between magma and fluid in the contact zone at depths exceeding 1 km. Water vapor as a fluid can be found in the subduction zones where the oceanic crust with the sedimentary layer saturated by water is pushing under the continental crust delivering water fluid in the zone of active volcanism. Water vapor fluid in the supercritical state behaves as an ideal gas. The greater the volume of fluid the smaller the decrease of its pressure during the expansion. If the fluid penetrates the magma conduit and its volume exceeds a certain critical value, the fluid starts to push magma as a piston. The critical volume is equal to γShf, where γ is the adiabatic coefficient of water vapor (γ=1,4), S is the section of the magmatic conduit, hf is the bedding depth of fluid layer. The greater the volume of superheated water fluid and the distance separating it from the magma chamber, the higher may be the eruption power. During volcanic eruption the fluid does work and expands, its pressure and density decrease, the eruption ends.. To be repeated and to form a stratovolcano the eruption need some time to restore the critical volume of the fluid. This can occur due to the diffusion of fluid from the periphery. Perhaps it is just the diffusion time which determines the interval between eruptions. After the possible depletion of fluid layer the activity of the volcano may be interrupted. But everything will change when at great depth under the magma chamber a new fluid layer is activated. Its warming to supercritical temperatures may require tens of thousands of years. However this long activation may be followed by a powerful eruption that can devastate the total magma chamber and cause the caldera formation. After the release of magma and pyroclastic material the superheated water fluid can escape. Flying through the conduit at supersonic speed, it tears off from the walls tiny particles of melt and turns into a deadly scorching cloud. Obviously, the release of the superheated water fluid is the catastrophic finale of Plinian eruption, the final, which was preceded by a long eruption of pyroclastics and devastation of magma chamber. All these events result from the excess of considered critical conditions and described imbalance in the "magma-crust-fluid" system. a) Nechayev A. About the mechanism of geyser eruption. . arXiv:1204.1560v1 b) Nechayev A. About the mechanism of volcanic eruption. . arXiv:1208.