The Volcanic Explosivity Index (VEI)– and some general thoughts about indices

I like a good index.

VEI 3 (Mt Cleveland, Alaska 2006 as seen from ISS)

Here’s one:

Like much else, this didn’t appear in my basic level 2 OU geology course. What I find interesting about it is the mix of quantitative and qualitative criteria. The underlying factor expressed in the index is the order of magnitude of volume of tephra emitted by the eruption, but I imagine this may be difficult to estimate, particularly for historical and pre-historical eruptions— so the scale provides a way to link this estimate to descriptive criteria obtained by retrospective methods (measuring tephra from past eruptions, historical eye witness accounts etc).

Given the infrequency of the higher VEI eruptions, presumably the scale gets less accurate at the higher end.

(This Guardian article describes a paper proposing an improved index based on the mass of ejected material.)

Short glossary/reminder for fellow non-geologists:
Tephra– the fragmental stuff that’s erupted, of any size (ash and larger).
Hawaiian eruptions- low viscosity mafic lava that dribbles or flows out of the crater.
Strombolian– ‘mildly explosive’, when tephra is thrown up out of the crater in episodic showers, caused by violent degassing in a lava that’s a bit more viscous than that of Hawaiian eruptions.
Vulcanian– when explosive release of gas from a magma chamber blows out choked debris from earlier eruptions.
Plinian– high speed ejecta thrown high into the atmosphere, giving rise to the characteristic ‘eruption column’ of big volcanoes.
Nuée ardente– a type of pyroclastic flow in which the mixture of ash, lava, blocks and volcanic gases flowing down a mountainside is hot enough to glow- hence ‘incandescent cloud’. Dangerous.

So; this tops off a few weeks of intermittent thinking about indices more generally. If an index is a summary number generated from an array of different quantities or dimensions (it’s surprisingly hard to get a concise definition from t’internet, but it seems to be something like that), than is the VEI an index at all? The number doesn’t seem to derive mathematically from the constituent criteria (although I probably should read the original papers to find out). In which case, I wonder if it would be more accurate to describe it as a scale, like another time-honoured favourite:

Beaufort scale

This one even allows for pictures.

But I’m most familiar with medical indices/scales, and use or have used all of the following examples:
Glasgow Coma Scale for assessing level of consciousness. This is a series of qualitative clinical assessments (eye, verbal and motor responses) converted into a number between 1 and 15. Notwithstanding the weird question 8, the PHQ-9 depression scale does a similar thing for mood— subjective assessments of symptoms are scored in a nine item scale.

Or there may be a a mixture of subjective and quantitative elements within the same index (like the VEI). There’s the Apgar score, a 1-5 scale for assessing the health of newborn babies (Appearance Pulse Grimace Activity Respiration). Or CRB-65 for assessing the severity of pneumonia outside of hospital (Confusion- in practice usually assessed subjectively though has its own scoring systems, Respiratory rate >30, Blood pressure <100/60, 65 yrs of age or older)

All of these medical examples have been validated to a greater or lesser extent: they have an evidence base suggesting that they can indeed predict disease severity or outcome, and this makes them useful. They may still be constructs, but the fact that studies have demonstrated their validity— and that they are regularly used in clinical practice— does suggest that they capture something of an underlying reality.

Debates around whether emerging concepts in the broader health and social sciences can be ‘indexified’ often seem to reflect uncertainty around the validity of the concept as a whole— the idea of social capital being a good example.

To me, it seems the defining characteristic of a good index/scale is utility. Regardless of the underlying theories and concepts, and whether the quality that the index measures is “real” or not, if pragmatically-oriented geologists, sailors, clinicians and public health doctors find that an index helps them do their job, than it’s a good index. Maybe that’s a somewhat trite conclusion, but it’ll have to do for now.


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