[Geysers] Size & depth of geyser 'resevoirs'

[Davis, Brian L.]

I do tend to agree that the contact layer between the sinter and the underlying alluvium is shallow (this is visible on Geyser Hill as the simple fact that it is not a towering structure rising far above the surroundings on all sides). But there is the question of what might develop along the underlying (and deeper) alluvium/bedrock contact.

> The balance of evidence is that there are NO large, open
> reservoirs anywhere in a geyser's system. Certainly, there
> are some more-or-less open channels, but apparently most
> of a geyser's (indeed, a thermal area's) "reservoir" is permeable
> rock that can be hundreds, perhaps thousands, of feet  below 
> the surface.

I think we need to figure out in this context what the difference is between "large open reservoirs" and "more-or-less open channels". Most geysers do not appear to be very deep beneath their vent (Steamboat being one of the deepest, and notable in that regard). And given the volumes of water ejected by some (Giant, Oblong), if the immediate reservoir is not deep, it must have either a very high porosity, or have an unwieldy horizontal extent. So perhaps the question is "how open, and how spacious". Without some numbers... we're just throwing around poorly-defined words.

Note that the ejection velocities and the flux ejected (different measurements) might provide some constraints on this. The pressure at the erupting surface can be no more than hydrostatic for that depth, and the flow has to follow some form of Darcy flow (which has a *very* strong dependance on pipe size). You have to make some assumptions about the network feeding the conduit, but... huh... well, you could assume a fractal network with a certain branching ratio...

Oh, rats, you guys just gave me something else to think about. :)

We also know that macroscopic structures in the conduit systems do, indeed, vary a lot from geyser to geyser. If all geysers had a nice uniform branching structure springing from a central conduit, then the behavior of each would be very similar. Things like multi-burst geysers, or geysers with major/minor behavior, would not be expected to have the same conduit structure (or they would have the same eruptive style).

-- 
Brian Davis