[Geysers] Hydrothermal "Thumps"

[Ron Keam]

>Question for the masses:  What causes Thumps?  Some of us were having a
>discussion about steam explosions, steam implosions, and so on.

Both steam explosions and steam implosions cause thumps, but usually 
it is more likely to be steam implosions.

Once I observed the following phenomenon in the shallows of Frying 
Pan Lake, Waimangu: About a metre off-shore was a vent, perhaps 20 cm 
below the surface, and from this was a steady discharge of steam 
causing a rapid succession of 'flashes' of scattered sunlight as 
bubbles of steam about 2cm in diameter followed each other into the 
lake.  I estimate that there might have been about five bubbles a 
second discharged.  This resulted in a succession of "popping" sounds 
as the bubbles collapsed in the cooler water (about 55degC).  The 
activity described continued without any variation or abatement 
during the whole of my time in the vicinity, perhaps about ten 
minutes.  The place where this was happening is quite a long way from 
the tourist path and, for all I know, might still be occurring.

At Whakarewarewa in the early 1950s a succession of thumps used to 
occur in the Parekohoru hot spring at the commencement of its 
approximately hourly increased outflows.  These thumps could be felt 
50 metres away in the quiet of the night.  I do not know if that 
behaviour is continuing.

I was present at Orakeikorako once (late 1950s) when there was a 
noticeable ground shock, and simultaneously Diamond Geyser gave the 
highest water shot I have ever seen it produce - perhaps 6 or 7 
metres.  This presumably was an explosion rather than an implosion 
effect.

My impression is that the thumps that one can feel in the vicinity of 
Big Blue (Kaleidoscope area) at YNP are implosion events.

Maori people were disturbed one night at Whakarewarewa by 
"explosions".  In the morning nothing untoward was immediately to be 
seen, but soon afterwards it was noticed that water from 
Roto-a-Tamaheke lake was flowing through a channel into some 
connected hot springs that, previously, had been discharging into the 
lake.  The whole area around the lake gradually cooled and remains 
so.  What seems to have happened is that water started flowing from 
the lake into the formerly dicsharging springs and started condensing 
steam pockets in the former supply to those hot springs.  This 
resulted in a runaway cooling as more and more lake water was 
swallowed.

Ron Keam
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Ron Keam
The Physics Department
The University of Auckland
Private Bag 92-019
Auckland
New Zealand
Phone +64 9 373-7599 extension 87931
FAX +64 9 373-7445
EMail r.keam at auckland.ac.nz
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