Experimental modeling of the explosion mechanism of basaltic magmas

Processes in the feeders of basaltic volcanoes during Strombolian-type eruptions were examined with the use of a complex apparatus for modeling basaltic eruptions (CAMBE), which was designed and manufactured by the authors for this purpose. The experimental setup consists of modeling and registering units and has a height of 18 m. It was designed with regard for the geometric dimensions of a natural feeding volcanic system: the ratio of the inner diameter of the feeder to its height is approximately 1: 1000. CAMBE was the first modeling equipment making possible passing a flow of gas-saturated liquid through the conduit, which allowed us to study the nucleation of gas bubbles, their growth, coalescence, transformations of the gas structures, and the kinetics of the gas phase. The experiments were carried out in a manner that made it possible to eliminate effects of structural barriers and fluctuations in the liquid flow velocity. As a result of the experiments, a new (previously unknown) regime in the flow of two-phase systems through a vertical conduit was discovered: the cluster regime, which is characterized by systematically alternating dense accumulations of gas bubbles (bubble clusters) and liquid devoid of a free gas phase. It is demonstrated that the liquid, bubbly, cluster, and slug regimes systematically grade into one another and are polymorphic modifications of gas-saturated liquids moving through vertical conduits. Our data led us to propose a new model for the gas-hydrodynamic movement of magmatic melt through the conduit of a basaltic volcano: depending on the gas-hydrodynamic regime in the volcanic vent, various types of eruptive activity (up to explosions) may take place. The analyses of basaltic magma explosions allowed us to describe them from a new standpoint and recognize the following four major modes of their manifestations at the surface: (1) weak ash explosions early during the cluster regime, (2) strong ash explosions during the mature cluster regime, (3) bomb explosions during the slug regime, (4) bomb grading to ash explosions during the slug regime associated with trains of small bubbles.