MOVIE OF AN ERUPTING VOLCANO
Dynamics of a Plinian Eruption
The following movie depicts a cross-sectional view an explosively erupting volcano, typical of a so-called Plinian eruption. A variety of pressure surfaces exist within the magma column beneath the erupting volcano, and within the eruption column above the volcano. The section below describes the nature of these pressure surfaces and explains the dynamic processes associated with the eruption model.
The dynamics of an erupting volcano is demonstrated in the following cross-section. Eruptions are fed from a magma column that exists directly above a magma chamber. The magma column contains two critical pressure surfaces that separate three magmatic regimes which different physical properties:
(1) a lower exsolution surface separates a non-vesiculated magma reservoir with dissolved volatiles from an overlying zone of magma having exsolved gas bubbles, and
(2) an upper fragmentation surface that separates the middle zone of magma with exsolved gas from an upper zone of liquid to plastic pyroclasts and released gas.
The exsolution of gas from magma (or "boiling") is called vesiculation. These gas bubbles (vesicles) begin to form at the exsolution surface. Vesiculation is promoted by decompression of magma as it rises upward (where the confining pressure is less than the dissolved gas pressure). Fragmentation of the bubble walls then begins at the fragmentation surface; here, the gas bubbles grow during ascent until they become unstable and explode. This occurs when the volume of bubbles is about 75% of the total volume of the magma column.
Gas release is confined to the diameter of the magma column, and the eruption velocity is controlled mainly by the gas content. The low strength of surface rocks and the high initial exit pressure commonly results in vent erosion, so that a flared vent shape develops which enhances velocity. This marks an upward transition from subsonic to supersonic flow.
The region of hot gas and broken pyroclastic particals above the fragmentation surface is called the eruption column. It transports pyroclastic materials from the ground into the atmosphere. Common observed heights for plinian eruptive columns are between 2 and 45 km. In general the eruptive column has three parts: (1) the gas thrust region in the lower column, driven by gas expansion, (2) the convective thrust region in the upper column, driven by the constant release of thermal energy from internal ash, and (3) the umbrella region at the top of the eruption column. The umbrella region is also known as the "downwind plume".
The gas thrust region is a region of vertical movement of gas and plastic magma particles that is internally powered by gas expansion (decompression) at the base of the eruptive column. Its overall density depends on the particle-to-gas ratio. Initially, the gas thrust is denser than air because of its incorporation of pyroclastic particles. However, as these particles fall out (as ballistics), the density is reduced so that the hot gases in the column are now less than that of the atmosphere. At this point, the gas thrust gives way to convective uprise and development of the convective thrust region, which comprises about 90% of the eruptive column. The convective plume is driven by the constant release of thermal energy from internal ash. At some point, the bulk density of the column will become equal to that of the atmosphere, so that its upward mobility is only controlled by its momentum. The column thus spreads out in the umbrella region. The bottom of umbrella region is where densities of the plume and the surrounding air are equal. Continued upward mobility towards the top of the umbrella region is controlled by momentum. The umbrella region is often asymmetric due to the effect of high atmospheric winds in the stratosphere.