[Nmcaver] Guatemal City "Sinkhole"

[DONALD G. DAVIS]

>      Below is a post from TAGNet with a likely explanation for the recent
>Guatemala City "sinkhole".
>
>Mark Minton
>
>
>3)The Guatemala City
>      By: Ric Finch  (Cookeville, Tennessee)
>          rfinch at tntech.edu

[INTRODUCTORY PART SNIPPED]

>With all that as background, here then, is my seat-of-the-pants, viewed
>from afar, interpretation of the Guatemala City "sinkhole":  A major
>storm water and sewer main ruptured, releasing fluid into the porous
>and easily eroded ash/pumice deposits underlying the flat ridge on
>which San Antonio lies. This downward moving water eroded a shaft
>below the ruptured drain. Shaft development likely occurred slowly at
>first, but accelerated once some through-going system was established.
>The water and the eroded sediments had to go somewhere, and that
>movement was likely towards the Rio Las Vacas, lying approx. 80 m
>below the general surface level of San Antonio, and only 500 m distant
>laterally. The water moving downward through the ash/pumice deposits
>may have reached the contact between the porous younger ash and the
>older, less permeable flow rocks supposed to underlie the ash, and
>followed this contact laterally to reach the Rio Las Vacas (which
>would serve as the local base level). Once a good conduit was
>established, one through which sediments could be moved, the
>development of the shaft may have been rapid. Residents of the San
>Antonio neighborhood report that they could hear noises underground
>for a month prior to the final breakthrough to the surface, which was
>indeed rapid and dramatic and took three unsuspecting residents down
>to a horrid death.
>
>The field datum I would like to have to confirm this interpretation
>would be the discovery of a resurgence (especially a new and very
>filthy stinky resurgence) somewhere along the west wall of the
>barranca of the Rio Las Vacas.

      Richard Finch's erosive-piping explanation of the Guatemala City
collapse pit sounds quite credible in essence.  I would postulate somewhat
different details.  Finch suggests that the shaft below the input source
was the first cavity opened.  That seems unlikely; there would be little
void space for eroded material to move into at that stage.  More likely,
diffuse flow through the permeable ash initially moved down and then
laterally to a spring at base level.  Headward sapping could then start a
tunnel extending upflow toward the source.  Once open void had reached a
point more or less directly below the source, collapse could begin to
stope a shaft rapidly upward, leading to the catastrophic surface breach. 
This is probably the stage at which residents reported hearing noises.

      However, if the ash bed is not uniformly coherent, the stoping could
have reached the surface first above some point in the conduit that is not
all the way beneath the source.  In that event, there could be further
collapses farther from the outlet gorge than the first one.  And voids
along the canyonward part of the conduit may still be migrating upward. 
In these cases, other sinkholes could appear later, to either side of the
present one.  It will be interesting to see whether that happens.  
Ultimately, the entire conduit could stope upward into an open trench.

      Since large piping caves in volcanic ash and claystone do occur in
nature (Officer's Cave, Oregon, is probably the closest reported analogy
to the Guatemala example), it is also possible that the water flow
responsible is natural.  A sewer or water-main break is not essential,
though in this urbanized context, it would be the most probable cause.

							--Donald Davis