First 150 words of the document:
The Volcanic Eruption Index (VEI) was devised by Chris Newhall and Stephen Self in 1982, to provide a relative measure of how explosive eruptions were.
A number of factors are considered to help gauge the explosive index, these include the volume of product, the height that eruption cloud reaches and
qualitative descriptions. The scale is an open ended scale, the largest volcano in history has been given a score of 8. It is a logarithmic scale, so this means
every one point increase represents a 10 fold increase. Density and vesicularity (gas bubbling) of the volcanic products in question is not taken into account.
The power output of an eruption, isn't taken into account, this makes it hard to determine the effects of a prehistoric or unobserved eruption.
42 of the 47 eruptions that have scored an 8 on the scale have occurred in the last 36million years.