Title: Mechanisms of hydrovolcanic pyroclast formation: Grain-size, scanning electron microscopy, and experimental studies
Abstract: 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 (SEM) reveal information on the formation of hydrovolcanic pyroclasts. Strombolian explosions result from limited water interaction with magma and the pyroclasts produced are dominantly 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 μm 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 μm in diameter. Types 3 and 4 are typical of fine ash. Type 5 pyroclasts characterize ash less than 100 μm 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 pyroclast shapes. Stress-wave fracturing (cavitation) of the melt results from high pressure and temperature gradients at the melt/water interface. Simultaneous brittle fracture and quenching produces Type 1 pyroclasts. Type 2 develops smooth fused surfaces due to turbulent mixing with water after fracture and before quenching. Fluid instabilities promote turbulent mixing of melt and water and produce fine ash. This kind of fragmentation occurs during high-energy explosions. The increased melt surface area due to fine fragmentation promotes high-efficiency heat exchange between the melt and water. Shapes of resulting pyroclasts are determined by maximum surface area (Type 3) and surface tension effects (Type 4). Type 5 pyroclasts result from nearly simultaneous vesicle burst and melt/water fragmentation.
Publication Year: 1983
Publication Date: 1983-09-01
Language: en
Type: article
Indexed In: ['crossref']
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Cited By Count: 452
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