[Nmcaver] Guatamala "sinkhole" info, revised

[J. LaRue Bills]

Folks,
I just received the following from Ric:

>Here's a submission I just sent to TAG-Net, and I hope >Mark [Minton] will 
>pick this one up for circulation >among  the Texas cavers as well.
>
>    Ric
>
> Cavers--
>
>    Guatemalan emergency authorities have released some information that 
> substantially changes the picture of the Guatemala City "sinkhole" and the 
> cause of its formation.
>
>    It is now reported to have a diameter of 25 - 30 m and a depth of 
> approximately 50 meters-- just half the original depth estimate!  (Not 
> really a big surprise that the first reports were exaggerated....just like 
> the length and depth of most real caves when estimated rather than 
> surveyed!)
>
>    Most startling to me is to learn that there is a major storm water 
> and/or sewer collector pipe buried at a depth of 60 m in this zone!   I 
> would never have imagined a pipe at this depth.
>
>    It now seems likely that a portion of this major drain collapsed and 
> was  blocked.  Effluent then escaped from the drain and eroded the soft 
> ash  layers, creating a shaft that grew upward by a process called piping. 
> Collapse of overlying ash beds ruptured more pipes and drains, 
> contributing  to the process.   The ashy and pumiceous sediments may have 
> simply washed on  down and out the downstream end of the ruptured major 
> drain.
>
>    This new scenario eliminates the need for the creation of a conduit 
> along the ash/lava flow contact or the involvement of any possible 
> pre-existing karst conduit at depth, ideas proposed earlier based on the 
> reported depth of the shaft and the known geology of the area.   This new 
> scenario is simpler and more likely correct than the earlier proposed 
> concept.
>
>    This new scenario would also allow the shaft to have developed very 
> rapidly, not requiring months or years as earlier speculated for the more 
> geologically complex scenario.
>
>    If you want to take a look at the official Guatemalan disaster 
> bulletins and maps (in Spanish), you can find them here:
>
> http://www.conred.org/principal.php
>
>
>    Ric Finch, NSS 5560 RL,
>    and geologist led astray by relying on unconfirmed facts!