Does the altitude of the site of volcanic activity accelerate/impede
its effects on the environment? The other question is: how do you compute
VEI?
Shaleen Jain
Hi Shaleen,
That is an interesting question. Certainly the altitude of a vent on a
volcano will be important regarding the effects on the local environment.
For example many volcanoes are snow-covered so that an eruption up at the
summit stands a good chance of melting a lot of snow and ice, and even if
it is only a small eruption, the resulting floods and lahars could be
deadly. On the other hand, a lower-elevation eruption down on the flanks
might not be such a danger.
With regard to really big eruptions and their effects on the atmosphere
and around the world, the altitude of the vent probably doesn't make much
difference. The tallest active volcanoes reach almost 6 km high. The
highest eruption columns are 35-45 km high, so that 6 km more or less
won't matter too much--the ash and gas can still get into the stratosphere
where it will be carried far and wide.
As for VEI, it is calculated in a number of ways. There is a section on
VEI in VolcanoWorld (type VEI in the VolcanoWorld searcher). The most
straightforward means of calculating a VEI value is to determine the
volume of material that was erupted. This is often very tedious work,
especially if the eruption was long ago and much of the erupted material
has been eroded away. However, if the eruption was long ago there were
probably no accounts of it so the other VEI characteristics can't be
applied. For eruptions that were/are wittnessed, the height of the
eruption column, the duration of single blasts, and any qualitative
descriptions can also be added in to help determine the VEI. There is a
good table explaining this on page 23 of Volcanoes of the World by Tom
Simkin and Lee Seibert.
Sincerely,
Scott Rowland
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