When scientists study volcanic eruptions, one of the challenges they face is how to compare different events, past and present, in a meaningful way. One of the tools developed to address this challenge is the Volcanic Explosivity Index (VEI), created in 1982 by Chris Hewhall and Steve Self.
According to the USGS, the VEI was originally meant to help volcanologists determine the impact of volcanic eruptions on climate. They eventually decided that the VEI was not well suited to that particular purpose, but they have continued to use it as a way of classifying the relative size of explosive eruptions.
The VEI is conceptually similar to the Richter scale, which is used to measure earthquake strength. The Richter scale starts at 1 and technically has no upper limit, but the largest earthquake ever recorded measured 9.5. Richter scale values are based on measurements of earthquake waves detected by a seismograph.
It is a logarithmic scale, which means that each step change represents an order of magnitude (or 10 times) increase over the previous step in terms of measured amplitude.
Volcanic events are difficult to measure and there is no single number that fully describes the character of an eruption. Unlike the Richter scale, the VEI, which runs from 0 to 8, is based not only on quantitative data, but also on qualitative criteria. The factors considered include:
* General description
* Volume of tephra
* Column height
* Qualitative description
* Eruption type
* CAVW maximum explosivity
* Tropospheric/Stratospheric injection
The VEI is easier to understand when viewed in chart form, and you may wish to refer to this example or this example for the following discussion. When “rating” a volcano, scientists will evaluate data in as many of the above categories as they can, and then choose an appropriate and representative VEI number.
The general description criteria of the VEI mainly refer to the size of an eruption. The categories (with associated VEI in brackets) are: non-explosive (0), small (1), moderate (2), moderate-large (3), large (4), very large (5), huge (6), humongous (7) and indescribable (8). These terms are somewhat subjective.
Volume of tephra or ejecta
Tephra is the material that is ejected from a volcano, for example lava, ash, and other pyroclasts. Its volume is measured in either cubic meters or cubic kilometers. Except at the two lowest levels of the VEI scale, each step represents a 10 fold increase in ejecta volume (but not necessarily energy).
A volcano with an ejecta volume of less than 10,000 cubic meters could be (depending on the other criteria in the chart) be given a VEI of 0. At the opposite end of the scale, a volcano with greater than 1000 cubic kilometers of ejecta would be assigned a VEI of 8.
The last eruption of this size occurred approximately 26,500 years ago. The 1980 Mount St. Helens eruption had an ejecta volume of approximately 1 cubic kilometer, giving it a VEI of 5.
When a volcano erupts, a column or cloud of ash and debris is blown into the atmosphere. The height of this column can be measured in meters or kilometers and correlates with the degree of explosivity or how powerful a volcano is.
A volcano with a column height of less than 100 meters would get a VEI of 0. At a VEI values of 5 and higher, column heights extend 25 kilometers or higher.
This category also uses subjective, descriptive terms rather than measured data. Descriptors, ranging from weakest to most powerful, include: gentle, effusive, explosive, severe, violent, terrific, cataclysmic, paroxysmal, and colossal.
There is overlap between these terms. For example, volcanoes called “gentle and effusive” may have a final VEI of 0 or 1, depending on how other criteria are assessed.
Eruptions can be divided into general types based on their behavior. The types are often named after a representative volcano or a place that typically has that type of eruption. Strombolian volcanoes, for example, are named after a type of volcanic activity common in Stromboli, Sicily.
The VEI uses several of these “type” descriptors, including (in order of explosiveness): Hawaiian, Strombolian, Vulcanian, Plinian, and Ultra-Plinian. There is some overlap in how these terms apply to the VEI. For example, a Vulcanian eruption may be assigned a VEI of 2, 3 or 4, and a Plinian may be assigned a VEI of 4 or higher.
Volcanic eruptions may last less than an hour or go on for years. Like other VEI criteria, the correspondence between duration and VEI number is not one for one, and the duration considered is only that of the “continuous blast”.
In general, shorter durations are rated lower on the VEI. An eruption lasting less than an hour rates between 0 and 2, and a volcano lasting more than 12 hours would normally be rated 4 or higher.
CAVW maximum explosivity
CAVW stands for the Catalog of Active Volcanoes of the World is a list and description of the world’s active volcanoes. The explosive characteristics of a volcano listed in the CAVW can be used to rank it on the VEI scale.
Volcanoes with lava flows, domes, or mudflows (low explosivity events) get low VEIs – 0 to 1. Phreatic volcanoes rank between 1 and 5. Volcanoes with explosions or a nuées ardentes (pyroclastic flows) will rank at 1 or higher.
Tropospheric and stratospheric injection
These criteria are another expression of column height and measure how high volcanic materials travel into the atmosphere. The troposphere is the lowest level of the atmosphere. It reaches 7 to 9 km above the earth’s surface at the poles, and up to 17 km in the tropics.
The stratosphere is the layer above the troposphere, which extends up to 50 km above the surface of the earth.
The occurrence of tropospheric injection is characterized (with VEI in brackets) as: negligible (0), minor (1), moderate (2), or substantial (3 and up). The occurrence of stratospheric injection is characterized as: none (0-2), possible (3), definite (4), and significant (5 and up).
Below are some examples of volcanic eruptions and their corresponding VEI ratings.
0 Mauna Loa, Hawaii, 1984
1 Kilauea, Hawaii, 1983 – present
2 Mount Hood, Oregon, 1865-66
3 Mount Etna, Sicily, 2002-03
4 Mount Pelée, Martinique, 1902
5 El Chichón, Mexico, 1982
6 Mount Pinatubo, Philippines, 1991
7 Mount Tambora, Indonesia, 1815
8 Yellowstone (Lava Creek eruption), 640,000 BP*
*BP means “before present” and is taken to mean the year 1950
The volcanic explosivity index or VEI is a useful means of classifying past and modern volcanic eruptions. It is similar to the Richter scale used to measure earthquakes, but it several qualitative as well as quantitative criteria are evaluated before a final VEI is assigned.
For more information on the VEI, please consult the references listed below or visit your local library.