THE DYNAMICS OF EXPLOSIVE VOLCANIC-ERUPTIONS

Explosive volcanic eruptions involve the ejection of dense mixtures of ash and gas from a volcanic vent at high speed and pressure. This mix ture is generated as liquid magma rises from a crustal magma chamber a nd decompresses, exsolving water vapor. As gas is exsolved, the mixtur e 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 m ixture of ash and volatiles ascends to the volcanic vent. On eruption from the vent, the material decompresses, either into a volcanic crate r or directly into the atmosphere. In the case of free decompression, the mixture typically has a high speed, while decompression in a crate r can lead to either very low or very high eruption speeds. After deco mpression, the hot, dense mixture begins to entrain and heat ambient a ir, thereby lowering the mixture density, but it also decelerates unde r gravity. If the eruption velocity is sufficiently high, then the mat erial can become buoyant and will generate a buoyant ash plume, called a Plinian eruption column, which rises above the vent. In contrast, i f 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 mixtu re into potential energy through entrainment and heating of ambient ai r. The height of rise of such eruption columns depends upon the erupti on rate, the stratification of the atmosphere, the degree of thermal d isequilibrium between the particles and the air, and the amount of wat er vapor in the atmosphere. Dense, hot ash flows, generated by collaps ing fountains, transport ash and clasts laterally from the vent, sedim enting many of the larger clasts and entraining air en route. As a res ult, the density of the mixture may fall below that of the atmosphere, and the finer-grained solid material may thereby become buoyant and r ise from the flow. The distance it travels increases with both the clo ud mass and the mean particle size. The ensuing buoyant ash plume, cal led a coignimbrite eruption column, may have a source several kilomete rs from the original volcanic vent. Once the thermal energy of an erup tion column has become exhausted, the ash intrudes laterally into the atmosphere. Ultimately, the cloud is swept downwind, where sedimentati on of ash leads to fall deposits over hundreds of kilometers from the volcano.