Steps of the VEI Scale
The VEI scale begins at 0 for eruptions that produce less than 0.0001 cubic kilometer of ejecta. Most of these eruptions are very small in size. However, some of them are "effusive" rather than being "explosive." Effusive eruptions are characterized by lava flowing from the vent instead of ejecta being blasted from the vent.
Eruptions rated at VEI 1 produce between 0.0001 and 0.001 cubic kilometers of ejecta. Above VEI 1, the scale becomes logarithmic, meaning that each step in 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 in the scale representing an explosivity increase of 10X, a VEI 5 is roughly ten times more explosive than a VEI 4. Two steps of the scale is an increase of 100X in explosivity. For example, a VEI 6 is roughly 100 times more explosive than a VEI 4. A VEI 8 is one million times more explosive than a VEI 2. All of this is based upon ejecta volume.
Because each step of the scale is a 10X increase in material ejected, there is an enormous difference in the size of an eruption on the low end of a step and an eruption on the high end of a step. For this reason, a "+" is often added to eruptions that are known to be on the upper end of their step. For example, the eruption of Katla in Southern Iceland on October 12, 1918 was rated at VEI 4+ because the eruption was a very strong VEI 4.