[Geysers] More About Nonsense

[Carlton Cross]

\Well, while we're correcting geyser knowledge (why Echinus ain't what it used to be), I can no longer resist spouting off about the thing that bugs me.  The statement, "...thus robbing it of it's ability to build pressure to erupt," is also pure nonsense.   Geysers do not build up pressure before erupting.  In fact, the eruption is started by a pressure drop.

There are two obvious processes involved in an eruption.  First, inside a geyser, the hydrostatic pressure increases with depth just as it does in a swimming pool.  Water at depth can be hotter than the boiling point at the surface because the boiling point increases as pressure increases.  (That's why things get hotter and cook faster in a pressure cooker.)  Thus, if the water at some depth gets to the boiling point and continues rising, it will start to boil just above that depth because the pressure will be just a little bit lower.  The boiling will then produce bubbles which will push off water above that point which, in turn, further reduces the pressure below.  If the water below is hot enough, it will then boil to keep the eruption going.

Second, when water passes through a constriction the velocity increases which causes the static pressure to drop.  Thus, when water that is close to boiling passes through a constriction, it will boil on the outlet side.

In both cases, the steam pushes itself and the remaining water out of the vent.  Since the volume of the steam is about 1600 times greater than the volume of the water that vaporized, it can develop considerable velocity to make room for itself.

Clearly, it does take pressure to push the water and steam upward during an eruption.  Where does the pressure come from?  A geyser such as Old Faithful that is known to have a long nearly vertical tube is probably the easiest to understand.  As the hot water rises it pushes off cooler water until the top of the column reaches the boiling point at the surface pressure (atmospheric pressure).  As soon as bubbles start forming, the pressure below will be decrease slightly.  If the water below is hot enough, it will be above its boiling point when the static pressure drops and will also boil, thus forming more bubbles.  Heat from the water causes the steam to expand, which pushes off more water.  In this way, the boiling action at the surface of the water will keep the eruption going while it drops down the tube.  Since the steam is pushing against water that has been held at pressure below the water above, it must move upward rather than downward.

Thus, the pressure of the steam can be as great as the pre-eruptive hydrostatic pressure at the depth of the boiling surface, but no greater.  Otherwise, the boiling will stop.  However, steam at the previous hydrostatic pressure can push up a rather tall column of water and steam.  If the boiling surface is 10 feet below the outlet, the steam pressure can be as high as 10 feet of water, or about 1/3 atmosphere.  That is plent of pressure to push up a good column of water.

Jeff and I have data from our model geysers showing this process very clearly.  Although natural geysers are much more complex than our models, there's little reason to think that the models are misleading.

One more point relates to the height variation during an eruption of Old Faithful.  First, it clears out the top water and pushes up to its maximum height which it holds briefly.  Then, the height gradually decreases.  Since the boiling level is dropping to depths where the pressure was greater, shouldn't the height increase because of the higher steam pressure?  I think it would except that the steam has to push through a longer length of the tube.  Since the tube is rough, there is considerable drag which decreases the exit velocity as the distance increases.  At least for me, the gradual decrease of the height is what I expect.  Because Beehive holds its height much longer, I speculate that it erupts from a relatively shallow water chamber through a shorter tube.  Thus, the height doesn't drop until the remaining water in the chamber is cooled by boiling off its steam.  An eruption from a chamber and short tube is also consistent with the higher proportion of steam in Beehive's column.  (I don't remember how much has been determined about Beehive's plumbing, but I suppose my theory could get shot down.)

At any rate, the next time you hear that a geyser is building up pressure to erupt, just close you ears.  I'll be more likely to shout "NONSENSE," but that's not such a good idea.

Carlton Cross