The Volcanic Explosivity Index (VEI) was devised by Chris Newhall of the U.S. Geological Survey
and Steve Self at the University of Hawaii in 1982 to provide a relative measure of the explosiveness of volcanic eruptions.
Volume of products, eruption cloud height, and qualitative observations (using terms ranging from "gentle"
to "mega-colossal") are used to determine the explosivity value. The scale is open-ended and ranges from 0, for non-explosive
eruptions (less than 104 cubic metres of tephra* ejected), to 8, for mega-colossal explosive eruptions that can
eject 1012 cubic metres of tephra and have a cloud column height of over 25 km. Each interval on the scale represents
a ten-fold increase in observed eruption criteria. Values higher than 8 can be determined if needed.
* (Tephra) -- Air-fall material produced by a volcanic eruption regardless of composition or fragment
size.
One weakness of the VEI is that it does not take into account of the density of erupted material; ash,
volcanic bombs, and ignimbrite are all treated alike. In addition, the VEI does not take into account the magnitude of power
output of an eruption.
As you can see in Fig 1.1, the diagram of the VEI's are shown above.
