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A widely accepted measure of the size of an eruption is the volume of lava ejected as pumice and ash (tephra) during an explosive phase or the volume of lava extruded during an effusive phase. Eruption volumes are commonly expressed in cubic kilometers (km3). One km3 is roughly equivalent to 0.24 cubic mile. |
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A related measure of size is the "Volcanic Explosivity Index", or VEI. This scale is similar to the Richter scale used for indicating earthquakes size. Like the "Richter magnitude" scale, the VEI scale is logarithmic such that each unit increase in VEI (from 2 to 3, for example) corresponds to a tenfold increase in the volume of material erupted onto the surface. Descriptive phrases such as "small" or "moderate eruption" are only loosely tied to the VEI scale or the volume of explosively erupted lava.
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Relative sizes of a few well-known eruptions |
How are eruption volumes measured?
Estimates of the tephra volumes are usually obtained by mapping the distribution and thickness the tephra deposits on the ground after the eruption is over. Tephra volumes measured in this way must then be corrected for void spaces (bubbles within the pumice, empty spaces between individual chunks of pumice or ash) to get an estimate of the original volume of lava erupted. This correction can be made by comparing the bulk density of the tephra deposit with the known density of the rock-type that makes up the tephra. The result is referred to as the "dense rock equivalent" of the erupted volume.
