The Volcanic Explosivity Index (VEI) is now universally accepted as a means of classifying the relative sizes of explosive eruptions, it was developed by Newhall and Self (1982) at the suggestion of Robert Decker, then Professor at Dartmouth College.
The higher the Volcanic Explosivity Index value, the more powerful the type of volcanic eruption.
This index is correlated with the volcanic ejecta volume (volume of volcanic products during the eruption) and several other observed physical criteria, such as the height of the eruption column and the duration of the eruption.
Furthermore, it is possible to infer some of these physical criteria based on careful field study of past eruptive deposits.
Fedotov (1985) proposed a somewhat parallel approach, devising a scale for explosive eruptions based logarithmically on the rate of lava discharge during eruptions.
Decker (1990) conducted a statistical study of the volcanic explosivity indices of eruptions to calculate the frequencies of eruptions having particular levels of explosivity.
Their research reinforced the observation that the smaller and less explosive volcanic eruptions are, the more frequently they occur.
On a log-log plot of eruption frequency versus Volcanic Explosivity Index (VEI), eruptions ranging from index 2 to 6 plot on a line with a slope of 0.5, which means that each increase in the Eruption magnitude correlates with a 5-fold decrease in frequency.
For eruptions that have an explosive index of 6 to 7, the slope of the line increases to 1 (a 10-fold decrease in frequency for each unit increase in VEI), and from 7 to 8, the slope increases to 10. .
From this analysis, he estimated that the average number of global eruptions as a function of the magnitude of the volcanic explosivity index is as follows:
| Explosion index | Frequency of eruptions |
| VEI = 2 | 15 eruptions/year; |
| VEI = 3 | 3 eruptions/year; |
| VEI = 4 | 1 rash every 2 years; |
| VEI = 5 | 1 every 10 years; |
| VEI = 6 | 1 every 50 years; |
| VEI = 7 | 1 every 450 years; |
| VEI = 8 | 1 every 300,000 years or more |
The upper limit of volcanic explosivity lies somewhere between 8 and 9 on the Volcanic Explosivity Index scale, exemplified by the Toba super-eruption.
How does it work?
The volcanic explosivity index scale starts at 0 for eruptions that produce less than 0.0001 cubic kilometers of ejecta.
Most of these eruptions are very small. However, some are “effusive” rather than “explosive.”
Eruptions classified as VEI 1 produce between 0.0001 and 0.001 cubic kilometers of ejecta.
Above VEI 1, the scale becomes logarithmic, meaning that each step on the scale represents a 10X increase in the amount of material ejected.
VEI 2 eruptions produce between 0.001 and 0.01 cubic kilometers of ejecta.
VEI 3 eruptions produce between 0.01 and 0.1 cubic kilometers of ejecta.
The progression of the scale from VEI 0 to VEI 8 is shown in the diagram on this page.
With each step on the scale representing a 10X increase in explosiveness, a VEI 5 is approximately ten times more explosive than a VEI 4.
Two steps up the scale is a 100X increase in explosiveness.
For example, a VEI 6 is approximately 100 times more explosive than a VEI 4.
A VEI 8 is a million times more explosive than a VEI 2. This is all based on ejection volume.
Because each step of the scale is a 10-fold increase in ejected material, there is a huge difference in the size of a bottom-end eruption and a top-end eruption.
For this reason, a “+” is often added to eruptions known to be at the top end of their path.
For example, the eruption of Katla in southern Iceland on October 12, 1918 was rated an explosivity index 4+ because the eruption was a very strong VEI 4.