People are often at a loss to describe the power of a volcanic eruption. When earthquakes are discussed, the classification of the Richter Scale comes readily to the public at large. News reports can even offer this information in shorthand, saying that a given tremor measured a 4.6, and no further elaboration is necessary. Volcanoes have nothing like this level of accessibility, even though a comparable system of classification does exist. It is known as the Volcanic Explosivity Index, or VEI.
The VEI is not the only system for classifying volcanic eruptions, but it is a useful one, offering a way of measuring the power of a volcanic eruption. Eruptions are measured on a scale of 0 to 8, with each point representing a level of power roughly ten times as great as the one below it.
As with the Richter Scale, the VEI is meant to be scientifically measurable, and not merely impressionistic. In the case of the VEI, the dominant characteristic is the volume of material that is expelled by the eruption, as measured in cubic meters. Any eruption that expels less than ten thousand cubic meters of material (10 to the fourth power) is left at a value of 0. A VEI of 1 ranges from ten thousand cubic meters to one million cubic meters, with each subsequent VEI number representing, at maximum, ten times the maximum of the number below it. An eruption with a VEI of 4, for example, will expel anywhere from 100 million cubic meters to a billion cubic meters of material, most of which is gas. There is no absolute upper limit, but anything more than a trillion cubic meters is a category 8 eruption, and no eruption of such a magnitude has been known to occur in recorded history.
The VEI is typically presented in a chart form, and these charts usually present additional information. The height, in kilometers, of the plume is an important element. The length of the eruption, in hours, is usually presented as well. Other factors that may be included are the speed of the eruption, measured as the mass in kilograms expelled per second, as well as descriptive information, such as characteristics of the lava involved or the kind of volcano that will create such an eruption. The numbers and descriptions associated with a given VEI number are not always completely exclusive, but at the same time, they are not independent variables, either.
The height of the plume, for example, and the duration of the eruption are both linked to the quantity of material expelled by the volcano. An eruption that expels a billion cubic meters of material into the atmosphere will go on longer than one that expels only a million cubic meters, and the plume will extend higher into the sky. All of these numbers are expressed as ranges, and there is some overlap in these ranges. An eruption with a VEI of 2 will have a plume that rises from one to five kilometers, while one with a VEI of 3 will rise from three to fifteen. There will, therefore, be some VEI 2 eruptions that rise higher than the plumes of VEI 3 eruptions, but as a whole, one will find that VEI 3 eruptions project longer plumes than VEI 2 eruptions. Beginning with a VEI of 5, these numbers are expressed only in general terms: they will extend more than 25 kilometers into the sky, and the eruption will last longer than twelve hours.
The descriptive information is also loosely applicable, but again, this information is based on real correlations. The first two categories (0 and 1) are associated with lava that will harden into basalt, while higher categories are associated with Andesite and Rhyolite. This is because of the viscosity of the lava, which governs how explosive the eruption will be. Similarly, Fissure and Shield volcanoes will only have eruptions on a scale of 0 or 1, and Scoria Cone volcanoes will remain in the vicinity of 1 or 2. Higher values are only associated with Stratovolcanoes.
For the non-specialist, many of these charts will also offer a visual impression of the kinds of eruption associated with these numbers from the Hawaiian or Icelandic types at the bottom, through the Strombolian, Vulcanian and Vesuvian types in the middle, to the Plinian and Pelean types at the high end.
The VEI has a long way to go before it is as recognizable and accessible to the public as the Richter Scale, but it remains a useful way to categorize volcanic eruptions. It lacks the precision of the Richter Scale, which registers distinctions of a tenth of a point (the difference between a 5.1 and a 5.2 level earthquake), while the VEI is expressed in whole numbers that cover a considerable range. Still, once the VEI of a given eruption is known, a fairly clear idea of the scale and character of the eruption can be inferred. The biggest caveat is that the scale of the eruption is not a clear prediction of the danger to humans posed by the eruption. The proximity and size of human settlements is even more important than the scale of the eruption. An eruption with a VEI of 6 in a remote region is less dangerous than one with a VEI of 4 near a small city.
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