[Geysers] Blog post about geyser mechanisms

[Carlton Cross]

A quote from the below link,

"There, after an eruption, more and more steam can accumulate between 
the surface of the water and the roof of the cavity, gradually 
building up pressure. When the pressure grows too high, the steam and 
water escape through the geyser's vertical shaft."

and

"They found that pressure builds up in a bubble trap there between 
geyser eruptions, just as in the Russian study."

I haven't had time to locate and read the referenced sources,  but I 
think it's important to note that pressure build-up is not what 
causes an eruption.  Also, the Cross driveway experiments have 
produced geyser models that demonstrate eruptions from an entirely 
vertical system with no places for trapping steam.  Pressure gages 
along the water column show clearly that the pressure everywhere 
decreases continuously once the eruption has started.  The 
temperature also drops because the steam carries heat out of the 
system.  Eruptions in a vertical system were not noticeably different 
from those of a horizontal system.

The static pressure within a fluid system is determined by the depth 
below the surface.  When you dive into water, you feel greater 
pressure as you go deeper.  It doesn't matter whether you're in a 
chamber with vapor or not.  In a horizontal chamber, the static 
pressure will be determined by the pressure at the chamber exit to 
the surface, and the pressure at the exit will be determined by the 
depth below the surface.  As a geyser system fills after an eruption, 
the depth of the water increases until the start of overflow.  After 
that, the temperature will increase, but not the static pressure.

Steam within a horizontal chamber will displace water from the 
chamber.  That water must exit through whatever passage leads to the 
surface where overflow will occur.  Hence, the effective depth of the 
water above the chamber will not change and the static pressure will 
NOT increase.

Once a geyser system has reached overflow, it can and does continue 
to heat, and, at some point, a small section of upward-moving water 
will rise until it reaches a place where the static pressure is low 
enough for the water to boil and produce steam.  The expansion of the 
steam will displace water from that region, and, simultaneously, the 
steam bubbles will begin to rise.  As the bubbles rise in the water 
column, the static pressure at all points below the bubbles will 
decrease because water with bubbles weighs less than water without 
bubbles.  Finally, when the pressure drops, the boiling point drops 
and more water will boil which produces more bubbles which allows 
more water to boil, etc.  The system has gone unstable and the 
expanding steam will begin to rush toward the surface exit - an eruption.

So far, I have talked only about the static pressure which is 
determined by the depth within the system.  There are, of course, 
dynamic pressure changes related to water movement.  Once steam has 
accumulated within a chamber or the water column, the whole column 
can bounce up and down because the steam below is compressible.  When 
the column rises, the steam expands and the pressure drops eventually 
to the point where the upward motion will decrease, possibly until it 
stops and then begins to fall.  The downward motion will compress the 
steam below and the pressure will rise, possibly causing the steam to 
condense into water.  When the pressure is finally great enough to 
stop the downward motion, expansion can begin again, pushing the water upward.

Our driveway experiments clearly produced two forms, rapid and slow, 
of a bouncing water column as the system neared an eruption.  In the 
rapid form, there was only slight movement of the water at about one 
cycle per second with no overflow.  The slow form was more like a 
series of overflow surges separated by many seconds.

Carlton Cross
cross at bmi.net



At 07:38 PM 9/7/2013, you wrote:
>Thinking this might interest some gazers who do not read geological 
>magazines or journals, I'll send along the URL to a post I just put 
>up about some interesting new studies on geysers:
>
><http://www.yellowstonetreasures.com/author-blog/>http://www.yellowstonetreasures.com/author-blog/
>
>Happy geyser gazing to those of you who get to enjoy the late season!
>
>Janet Chapple
>
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>Geysers at lists.wallawalla.edu
>