[Geysers] Exactly what is, and is not, a geyser?

[Bill Johnson]

No, this isn't an attempt to be a smart aleck, and it is definitely not a
newbie question either.  Rather, it's a thing that arose from a very
interesting conversation as I was researching an article I'm writing for
the Sput, and a question for which I had no fully satisfying answer.

The article I'm writing is on the subject of geyser-like fountaining at
volcanic crater lakes -- specifically, a "geyser" that I had the
opportunity to observe a few years ago as a member of a research party
working on a particular volcano that I won't name here.  (This is called a
narrative hook; read the article when it comes out.)  To help with
assembling a reference list, I had a conversation with a world-class expert
on hydrothermal phenomena at active volcanoes, an expert who happens to be
my son Peter, a Ph.D. student in volcanology whose thesis topic -- I
oversimplify -- is the movement of ground water in response to magmatic
incursions at awakening volcanoes.  He was a member of that research party
(nice to do father/son things like that!) and is well acquainted with the
event, and posed the subject question; more specifically, he posed the
question "and the thing we saw differs from what we call a 'geyser' in what
way?"  And I realized I had no good answer for him.

There are certain things we usually exclude from the definition of "geyser"
even though they throw water and other stuff into the air: man-made wells
and other artificial structures, cold-water springs where the water is
propelled by carbon dioxide or other gases, and so on.  However, things get
ticklish when talking about two-phase ejection of water from a geological
system as a result of interactions of ground water with a magma body with
unknown mechanisms not necessarily relating to water flashing to steam
beneath a constriction.  That's the semantic space we're in with this
article, and there are several practical reasons for exploring it.

Any thoughts on this from this august body of gazers assembled?  All
opinions appreciated.

-- Bill Johnson
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