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The dynamics of explosive volcanic eruptions


  • Andrew W. Woods


Explosive volcanic eruptions involve the ejection of dense mixtures of ash and gas from a volcanic vent at high speed and pressure. This mixture is generated as liquid magma rises from a crustal magma chamber and decompresses, exsolving water vapor. As gas is exsolved, the mixture inflates, accelerates, and becomes foam-like. Once the liquid films around the bubbles are unable to spread as rapidly as the bubbles are expanding through decompression, the films rupture, and a fragmented mixture of ash and volatiles ascends to the volcanic vent. On eruption from the vent, the material decompresses, either into a volcanic crater or directly into the atmosphere. In the case of free decompression, the mixture typically has a high speed, while decompression in a crater can lead to either very low or very high eruption speeds. After decompression, the hot, dense mixture begins to entrain and heat ambient air, thereby lowering the mixture density, but it also decelerates under gravity. If the eruption velocity is sufficiently high, then the material can become buoyant and will generate a buoyant ash plume, called a Plinian eruption column, which rises above the vent. In contrast, if the eruption velocity is small or the mass flux is very large, then the material will typically collapse back toward the Earth and form a dense, laterally spreading flow. Buoyant eruption columns are able to transport the material high into the atmosphere, since they provide an efficient means of converting the initial thermal energy of the mixture into potential energy through entrainment and heating of ambient air. The height of rise of such eruption columns depends upon the eruption rate, the stratification of the atmosphere, the degree of thermal disequilibrium between the particles and the air, and the amount of water vapor in the atmosphere. Dense, hot ash flows, generated by collapsing fountains, transport ash and clasts laterally from the vent, sedimenting many of the larger clasts and entraining air en route. As a result, the density of the mixture may fall below that of the atmosphere, and the finer-grained solid material may thereby become buoyant and rise from the flow. The distance it travels increases with both the cloud mass and the mean particle size. The ensuing buoyant ash plume, called a coignimbrite eruption column, may have a source several kilometers from the original volcanic vent. Once the thermal energy of an eruption column has become exhausted, the ash intrudes laterally into the atmosphere. Ultimately, the cloud is swept downwind, where sedimentation of ash leads to fall deposits over hundreds of kilometers from the volcano.

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