Explosive Volcanism

Abstract

Wohletz, K.H., 1983. Mechanisms of hydrovolcanic pyroclast formation: grain-size, scanning electron microscopy, and experimental studies. Pyroclasts produced by explosive magma/water interactions are of various sizes and shapes. Data from analysis of over 200 samples of hydrovolcanic ash are interpreted by comparison with experimentally produced ash. Grain size and scanning electron microscopy (S~) reveal information on the formation of hydrovolcanic pyroclasts. Strombolian explosions result from limited water interaction with magma and the pyroclasts produced are dominant3~v centimeter-sized. With increasing water interaction, hydrovolcanism increases in explosivity to Surtseyan and Vulcanian activity. These eruptions produce millimeter-to micron-sized pyroclasts. The abundance of fine ash (<63 ~m diameter) increases from 5 to over 30 percent as water interaction reaches an explosive maximum. This maximum occurs with interactions of virtually equal volumes of melt and water. Five dominant pyroclast shape-types, determined by SEM, result from hydrovolcanic fragmentation: (1) blocky and equant; (2) vesicular and irregular with smooth surfaces; (3) moss-like and convoluted; (4) spherical or drop-like; and (5) plate-like. Types 1 and 2 dominate pyroclasts greater than 100 um in diameter. Types 3 and 4 are typical of fine ash. Type 5 pyroclasts characterize ash less than lO0 um in diameter resulting from hydrovolcanic fragmentation after strong vesiculation. Fragmentation mechanisms observed in experimental melt/water interactions result from vapor-film generation, expansion, and collapse. Fragments of congealed melt are products of several alternative mechanisms including stress-wave cavitation, detonation waves, and fluid instability mixing. All result in rapid heat transfer. These mechanisms can explain the five observed