these eruptions has been described for decades by a semi-quantitative “Volcanic

Explosivity Index (VEI)” based on a number of interdependent factors (eruption

duration, volume of ejected material, eruptive column height, tropospheric and

stratospheric injection, and many qualitative descriptors), most of which could only

be assessed after eruption had ceased. The utility of this semi-quantitative index

has been questioned since then, yet its common usage persists. We suggest that if

VEI is to be used in the future, it may as well be an index that can be determined in

real time during an eruption, and one that strictly reflects eruption intensity. We find

that the same value for the “traditional” Volcanic Explosivity Index of past eruptions

can be estimated in real time during an eruption, based solely on eruption column

height. This approach is not intended to replace the traditional VEI scale or supplant

more detailed mechanistic approaches to mass eruption rate (MER); rather it can be

used to provide a more immediate way of calculating the VEI and linking to MER, in

real time while an eruption is taking place. As such, if the easily observed height

can be measured, both MER and VEI can be estimated. This may facilitate

emergency response by aviation, hazard management, and others to mitigate

societal impact of hazardous eruptions. This scheme can be inverted to use

volume-based VEI of ancient eruptions to roughly retrodict unobserved column

height for key historic and pre-historic eruptions. Further, ash particle morphology

reflects explosivity in terms of decompression rate at the vent of unobserved

eruptions. With sufficiently rapid decompression, this could result in an observable

bi-modal bubble size distribution. The morphology of ash particles reveals a

threshold between VEI 3 and 4, above which there is sufficiently rapid

decompression to trigger a second nucleation event near the vent.