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From wikipedia:

With a margin of error of 10 minutes, Old Faithful will erupt 65 minutes after an eruption lasting less than 2.5 minutes or 91 minutes after an eruption lasting more than 2.5 minutes. The reliability of Old Faithful can be attributed to the fact that it is not connected to any other thermal features of the Upper Geyser Basin

Relatively recent discoveries led scientists to believe that there is a substantial underground chamber where the water and steam build up:

A previously unknown underground cavity might help trigger the timely eruptions of the famous Yellowstone geyser Old Faithful, a new study shows. The researchers who uncovered new evidence of a chamber suspect that it stores the pressurized near-boiling water, steam, and other gases that propel Old Faithful’s eruptions.

Are the 65 minute and 91 minute values given for the eruption times simply a byproduct of needing more time for the steam to "recharge" after a larger eruption, or is there a deeper interplay between the underground cavity and the main channel that causes some sort of a vortex that has a (somewhat) regular periodicity?

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  • $\begingroup$ I'm a rank amateur in this area, so if I have abused terminology, please let me know! $\endgroup$ – jonsca Aug 21 '14 at 6:17
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    $\begingroup$ Welcome to the site! This is one of the best questions I have seen on here. Very interesting. My first guess would be that since the more-time-to-recharge hypothesis could basically give you a linear model, the distribution it would produce would be more or less normal, not bimodal. There's likely something more interesting going on here. The process makes me think of the bimodal patterns that water flow forms make. Could be a similar process, but with 2 unequal sized side cavities. But this is speculation on my part, I have no experience in the area :) $\endgroup$ – naught101 Aug 21 '14 at 14:25
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First I would like to say this is not my area of expertise, so this should perhaps be a comment but I am not allowed yet to comment on this site.

In a simplistic view a geyser has heated water chamber with a column of water above it. The water is heated above the normal boiling temperature but is kept liquid by the pressure of the water column above it. When the water eventually starts to vaporize this reduces the pressure on the water in the chamber resulting in more rapid vaporization of the water in the chamber resulting in the eruption.

For Old Faithful assume two chambers, one chamber is lower than the other. If the temperatures in both chambers are high enough the vaporization in the upper chamber would also reduce the pressure in the lower chamber allowing it to vaporize resulting in both chambers emptying, resulting in a longer eruption. Assuming the infill and heating rates are essentially constant then the time before the next eruption would take longer because both chambers are "empty". On the other hand if the upper chamber vaporizes and but the lower one does not, either because it is not hot enough or the pressure hasn't dropped enough or back pressure from the upper chamber keeps the lower chamber liquid, then only the upper chamber will empty resulting in less time to infill and reheat.

Again not my area.

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  • $\begingroup$ I am accepting this because it seems reasonable to me, so thanks for the answer! $\endgroup$ – jonsca Aug 30 '14 at 0:23

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