Reading: Chapter 5, The Blue Planet
Explosive eruptions of volcanoes are controlled by a small number of processes, that may be predominant in particular types of eruptions, but most explosive eruptions have some of each component.
Look at photographic material on the internet:
Interpret what you see, using the following ideas: É
Magma contains gas that is dissolved under pressure (like CO2 in a soda bottle). As the magma rises, the confining pressure decreases. Gas exsolves and makes bubbles. Exsolving gas makes the magma lighter and allows it to rise further, leading to a signficant pressure build-up in surface magma reservoirs.
Near the surface, magma may come in contact with water (groundwater, rivers, lakes, ice, shallow seas) that turns into steam from the heat of the volcano. The steam begins to fragment the magma, allowing more water to be heated to boiling, leading very large explosions. Similar explosions occur in industry (dust explosions; fuel-coolant interactions). Such processes are called hydromagmatic (or phreato - magmatic when the water is strictly groundwater), and they are one of the most important (and dangerous) type of volcanic explosions. É
Pressure buildup eventually leads to a catastrophic release by blasting a vent to the surface, or pressure may be released continuously through an existing vent. Catastrophic explosions may lead to a variety of phenomena, including the formation of craters, calderas, or lateral blasts, like at Mt. St. Helens, for example. É
Explosive energy, derived from outgassing or evaporation of external water propels magma into an eruptive column, that behaves very much like a water fountain with a fluid with the viscosity of honey (that may actually be frothy and fragmented). Such a fountain may be a few meters high and feed a lava flow, or many hundred meters in the case of major explosive eruptions. However, while gas thrust can propel blocks and bombs of up to house-size, the height is quite limited. Other processes are needed to make eruption columns reach several km height. É
Eruptive systems are typically convective and particles bounce off each other, very much like atoms in a gas. These bouncing particles and the gas between them gives a system a bulk density that determines the fate of this eruption system. Very dense systems will not rise very high and collapse immediately in a solid/liquid - dominated pyroclastic flow, while less dense systems will produce nue ardentes, or ignimbrites that can move at very high speeds. É
As eruption systems convect, they tend to entrain air from the atmosphere that is immediately heated when it comes in contact with the magmatic gases and solids. This reduces the density of an eruption system and causes critical increase in bouyancy, that may make an eruption cloud lighter than air. Air entrainment may occur in vertical eruption clouds or in ground-based pyroclstic flows. In large eruptive systems, this buoyancy can propel eruption clouds into the stratosphere, where volcanic products may remain for very long time, circling the earth many times. É
As an eruption system leaves the vent of a volcano, it begins to loose energy, and the biggest fragments drop out first. As the system develops, more and more solid materials drops out, decreasing the density of an eruption cloud and allowing it to rise or stay boyoant for longer time.
The direction and the speed of local wind has a major influence on the distribution of volcanic fallout. In particularly eruption systems that are dominated by buyoant forces (and minor inertia) are deflected by winds. Thus, most of the volcanic products are deposited down-wind.