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Viscous Eruptions

Introduction

If youíve read the fluid eruptions article, then this is itís counterpart; if not, then let me say that this article intends to deal with high-viscosity magmatic eruptions- that is, explosive volcanism. This is one of the most important types of volcanic activity to monitor and study due to a number of reasons: its effects are long-reaching and can even be global, it can bring down aircraft, and it can devastate surrounding areas and lead to massive loss of life. On the other hand, none of these effects are going to be looked at in this article; if youíd like to find out about the first two, see the ash fall article; if youíd like to learn about the devastating effects on local areas, see the pyroclastic flows article.

This page will deal with the physical properties of an eruption, their controls, and the different types in relatively great detail. The understanding of these processes is integral in hazard mitigation and helping to save local communities.

Dangers

Although it was only a few moments ago that I said we would not discuss the impact of eruptions on the surrounding area, it is important to at least briefly look at what hazards exist for people around the volcano.

Pyroclastic Flows

Some scientists love terminology and giving a dozen names to roughly similar phenomena; Iím not one of those people. Here I define pyroclastic flows as a gravity driven, incandescent, high temperature, cloud of debris (aerial and ground-based) originating from the crater, slopes, or eruption column and ending at or near the base of the volcano. While I used a lot of words to describe it, this is simpler than referring to pyroclastic flows, lateral blasts, nuee ardentes, pyroclastic surges, block and ash flows, and coignimbrite pyroclastic flows separately. Victims caught by this hazard may be buried alive, suffocated by ash, crushed by debris, and/or burned to death. This hazard may be a very real danger kilometers away from the base of the volcano.

Lahars

This is one of the most deadly aspects of eruptions as itís effects can be far reaching and entirely unexpected. A lahar is essentially a flood, usually caused by flash-melting of a glacial cap during an eruption. Lahars can continue to move indefinitely through stream channels until their energy is dissipated by a lake or flood plain. There is a separate article on lahars, but dangers presented by them are similar to those created by a burst dam. Victims may be: drowned, crushed, suffocated by mud, lacerated by debris, or die in some even less pleasant ways (those of you who are morbid may want to check the lahars article).

Ashfall

The extent to which ashfall may present a hazard depends on the magnitude of the eruption; ashfall may extend for kilometers or even circle the globe. Massive eruptions can influence global climate through this mechanism (see the ashfall or volcanic hazards articles). Locally, the hazard is in silicosis or collapsing roofs on poorly constructed houses. In some areas, roof collapse can be the most deadly aspect of an eruption.

Ballistics

Ballistics are simply airborne debris, distinguished from ash only by size. Ballistics are large (few centimeters to meters) pieces of debris that are blasted from the side of a volcano during an eruption. This can present a very real hazard for nearby communities as the area becomes a virtual shooting gallery; at Sakura-jima volcano, in Japan, residents have taken precautions such as public bunkers and reinforced roofs on buildings.

Background

For information on how the magma forms and why, see the Magma Formation and Ascent article available on the main page. For information on volcano characteristics, view the basics pages at the top of the MIVO main page.

The Eruptions

Plinian Eruptions

Plinian eruptions are known for being some of the most violent, impressive, and dangerous styles of eruption seen during recorded history. There are a couple of subcategories such as subplinian or vesuvian, but these operate by the same mechanisms and differ significantly only in their magnitudes. A plinian eruption is characterized by a sustained explosion that may continue, at various levels of activity, for hours or even days. This activity produces an eruption column that can rise to heights of over 20 km, depending on the size of the eruption (height of the eruption column is actually used to determine the size or VEI of an eruptive episode). Plinian eruptions are very dangerous and commonly result in pyroclastic flows and locally heavy ashfall.

Plinian style eruptions are relatively simple compared to some others. The episode is initiated by rising magma within a vent that comes near to the surface. There is a huge density contrast between the atmosphere and the subsurface at several hundred meters depth which is important because trapped gas in the magma is the driving force for any eruption. Before the rising fluid reaches the surface, it is constantly degassing, but the high pressure of the overlying rock causes much of the gas to be retained. The triggering mechanism for an eruption can vary: common triggers are landslides, earthquakes, water-interactions, or the pressurization of a volcanic plug to the point at which it shatters. Regardless of how it happens, the magma is suddenly depressurized and the microscopic bubbles try to expand exponentially. These bubbles are unable to do this, however, due to the high silica content of the magma and the bubble walls shatter. This explosion carries the searing hot and expanding air and ash upward, creating the top of the ash column. Simultaneously, a shockwave from this huge series of microscopic explosions is directed in all directions including down. This downward propagating shockwave shatters these bubbles, accelerating the eruption downward. This combined with the progressive depressurization of lower levels of magma results in the continuation of the eruption for a variable amount of time. As you might imagine, the explosions are working their way downward into the vent and so the eruption begins to emanate from progressively lower and lower levels within the volcano- consequently, the vent is widened at the surface which can result in an increase in the magnitude of the episode. As the eruption continues, one of several events occur. A shallow magma chamber might be emptied or a dense plug may block the vent and the eruption is unable to progress beyond it; more commonly, the magma chamber has simply been degassed. Gas, having a very low density, rises to the surface of the magma chamber causing the magma at the top and in the vent to be very gas-rich compared to that at the bottom of the chamber. Once the eruption progresses to a point at which the gas content of the magma is insufficient to create an explosion of these gas bubbles, the eruption is essentially over. With time, the degassed magma will rise to the surface of the vent and create some sort of cap or lava dome that can be very dangerous to people in surrounding areas (this is dealt with in a separate article).

Vulcanian Eruptions

Vulcanian eruption events are characterized by a short burst of extremely explosive activity, followed by a longer period of repose. These bursts of activity may last a minute or more while the repose between can be on the order of minutes, hours, or days. The explosion happens quickly and is known for its volume (noise). The magma explodes upward, reaching the surface, and creating a shockwave. Behind this shockwave is a layer of compressed air which is, in turn, followed by the propagating eruptive column. The head (top) of this column is shaped much like a mushroom cloud with a thinner trail of rising ash and gas below it that extends to the crater below. There are a number of hypotheses as to why this style of eruption occurs. The first, and my favorite, is the formation of a microlite cap at the vent opening. At the top of the vent, the magma slowly degasses and cools, becoming dense and heavy, and inhibiting the further degassing of the underlying magma. The vent continues to pressurize until the breaking point is reached and the overlying cap fails. The cap failure facilitates the rapid depressurization of the vent and the explosive formation of a shockwave. Behind this shockwave follows the compressed gasses and ash, which is followed by the main bulk of the eruptive column. The second hypothesis involves the pressurization of the magma deep within the vent which may be caused by a plug obstructing the vent or a viscous strombolian-style pressurization of the magma chamber. The third hypothesis is more complex and involves the pressurization of hydrothermal fluids under the vent cap which leads to the catastrophic failure of the blockage. In reality, probably none of these hypotheses are going to be completely right. The mechanisms leading to the eruption may be different from volcano to volcano or even from eruption to eruption at a single volcano. Moreover, it may be that a combination of these events work together to create the vulcanian explosion.

Conclusion

Hopefully you now understand the different styles of eruption and their driving mechanisms better. There are a number of other articles in this website that deal with other aspects of volcanism that tie in very closely with this subject. I encourage you to keep reading and discover as much as you can about this exciting and interesting subject.