Conduit processes during the July–August 2001 explosive activity of Mt. Etna (Italy): inferences from glass chemistry and crystal size distribution of ash particles

Explosive activity at Mt. Etna from July 19 to August 7, 2001, provides a good case study to investigate the causes of the transitions between style of basaltic explosion. In this period, a new vent, located at 2550 m above sea level on the southern flank of the volcano, exhibited three types of activities that followed one another: initial ash and steam explosions with the emission of radial jets, of hydromagmatic origin; intermediate fire fountaining and Strombolian explosions, due to magma vesiculation; and finally, sustained to pulsing ash explosions, caused by overpressurization of the degassed and cooling top of the magma column. The activities produced two end-members of juvenile ash in the size range 0.4–0.1 mm: (1) brown, fluidal- to irregular-shaped, vesicular sideromelane glass particles, and (2) microcrystalline, blocky, poorly vesicular tachylite particles. Component analysis of the ash reveals a gradual decrease in the abundance of sideromelane, replaced by tachylite, in the transition from the Strombolian to the final ash explosion activity. Dense blocks with irregular, variable surface textures also characterize the products of the late pulsing ash explosions. Petrographic, chemical, and crystal size distribution analyses, together with morphological evidences, indicate that sideromelane quenched earlier than tachylite during the final stage of magma evolution. In fact, the groundmass of tachylite formed by subsequent crystallization of magma, possibly at lower temperature and under different degassing conditions. We hypothesize that sideromelane formed in the central part of the volcanic conduit, where the buoyant rise of gas bubbles caused a higher magma ascent velocity, which did not allow time for vesicle to escape or collapse before fragmentation. Conversely, tachylite crystallized at the margins of the conduit, where slow-moving magma accumulated, temperature was lower, and vesicle collapsed, forming a network of cracks favorable to permeable gas flow. Reduced magma emission rate at the end of the Strombolian phase caused an increase in the thickness of the peripheral degassed magma zone, until it formed a plug at the top of the conduit, and activity gradually shifted to pulsing ash explosions. These were driven by repeated explosions of the overpressurized plug, in a small-scale, vulcanian-like, explosive process. We suggest that the relative abundance of sideromelane and tachylite ash particles in basaltic explosion products may provide information on the evolution of velocity gradients within magma flux.