Geology expert explains Cayman’s sinkholes

| 23/01/2010

Cayman Islands News, Grand Cayman local news, Cayman earthquake, sinkholes, Professor Brian Jones(CNS): Brian Jones, the  Professor of Geology at the University of Alberta who has been doing research on the geology of the Cayman Islands since 1981, has told CNS that there are numerous  caves and caverns on the Cayman Islands, both above and below sea level, that can account for the sinkhole that appeared in the wake of the recent earthquake. Although Cayman is in an area susceptible to earth tremors, Professor Jones also says that in general the seismic events affecting the islands are usually very deep and, coupled with our strong infrastructure the islands, are less likely to suffer major damage.

The professor explained that over the last 30 million years or so the Cayman Islands have evolved as sea level has risen and fallen.  When sea level rises, the island is submerged and deposition of sediments take place; when sea level falls, the island is exposed to the atmosphere and weathering takes place.  The rocks on the Cayman Islands are limestone, formed of calcite (CaCO3) and dolomite (CaMg(CO3)2).  “When exposed to rainwater, these minerals will slowly dissolve – both at the surface and below the surface.  Surface dissolution produces the rugged terrain seen over much of the islands, whereas dissolution below the surface produces caves and widens fractures,” Prof Jones added.

He explained that there are numerous caves here and plenty of evidence of them, not only from the caves above sea level that we can access on Grand Cayman and Cayman Brac, but when drilling the bit commonly drops through large caves, some as much as 350 feet below sea level.  “There is considerable evidence that there are numerous caves throughout the rocks of the islands.  Similarly, there are numerous fractures throughout the islands that have been widened by dissolution caused by the action of rain water,” Prof Jones said.

When an earthquake hits, the vibrations cause any weak parts of the rocks to move and even collapse, which means the rocks above a cave or cavity may collapse down and the surface manifestation of that collapse will be a sinkhole.

However, Jones said the area in South Sound, where the large sinkhole was well documented and smaller holes were reportedly sighted, is, from  a geological perspective, poorly known – largely because it is built up and there is little rock exposed at the surface.  “It is difficult to gain first hand knowledge of the rocks in that area,” he noted. “From what I have been able to see, it appears that the surface deposits are either peat and swamp deposits, formed in the mangroves, or limestone that belong to the Ironshore Formation.”

Most of the rocks seen at the surface of this formation will probably be about 125,000 years old, the professor said.  “These rocks overlay dolostones that belong to the Pedro Castle Formation and/or the CaymanFormation.  The former is 2-5 million years old, whereas the latter is about 12 million years old in its upper part.  Caves, if present, will primarily be in these formations, and if some of those caves collapsed, then sinkholes would be produced.”

Asked about the reports of the smaller holes that appeared to give off a sulphur smell and be bubbling along the South Sound seashore, he said that without seeing them he could not be absolutely sure but suspected that the tremors had caused the loose sediments to move in such a way that gases trapped in the subsurface were released.  “If you shake water-saturated sands, they become, in effect, quick-sands.  As the gas bubbles escape, they will create small holes,” he explained.

Despite the ups and downs of Cayman’s geology, however, Prof Jones said residents in Cayman need not be alarmed about the ‘holey’ ground on which they walk as most of the earthquakes that hit the Cayman Islands have a fairly deep point of origin. He acknowledged that, given what had happened in Haiti, it was not surprising, but the circumstances were very different because of the severity and shallowness of that earthquake. 

Category: Science and Nature

Comments (21)

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  1. Richard Wadd says:

     Nowhere in the Bible does it say that the Earth is only 7,000 years old.

    That "7,000 year old" figure is actually borne out of a Scientific approximation of the Known Historical Record of Human Civilization.

    If one reads the bible, it clearly explains to us the insignificance of ‘Time’ when trying to understand the many wonders of our Creator.

    "… a day is as a thousand years, and a thousand years, as a day".

  2. Anonymous says:

    I somehow doubt it.

    Did he look at the pictures?

    The first noticble thing is the drain pipes. One from the block with 2 units which crosses the hole (you can see the pipe) and one from the adjacent block. I believe the first pipe is a 4" (the hole is just below the tree in the right near the veranda) and the second a 3" (which can clearly be seen)joining via a "T" and going to the left of the picture and onward to the cistern.

    Now we had some rain last year (lots of it from memory). These drains were rainwater drains from the roofs of the two buildings. If there had been a major leak at the join (which was coincidently located at the centre of the hole) the ground, which is pretty well sand with about 2ft of compacted marl on top, would have liquified over time and the sand would have slowly disapeared leaving a void below the compacted marl.

    Based on this the hole would have appeared in the future sooner or later it just needed a trigger, fortunately it was a quake and not a heavy lawn mower or we may have had an injury.

     

    Now based on the good Professors statement abouut haiti being shallow and ours deep I think he should have done some homework.

    This is the Haiti 7.0  at 8.1 miles deep.

    http://earthquake.usgs.gov/earthquakes/recenteqsww/Quakes/us2010rja6.php

    This is the Cayman quake  at 6.2 miles deep.

    http://earthquake.usgs.gov/earthquakes/recenteqsww/Quakes/us2010rrbc.html

     

    Did he even look at the photos and reports from USGS?

    • anonymous says:

      Haiti quake was caused by two sides of a fault SLIDING by each other. this creates huge lateral effects…Cayman’s quake is at a techtonic boundary where one plate is going under the other. The quakes here are usually deeper, in the trench.

       

      The key difference however was the magnitude…a 7.0 quake is many times stronger that what we had. A 6 is 10 times stronger than a 5 and a 7 is 10 times stronger that a 6.  The equation is an exponential one not linear.  Wiki it, too technical to explain here.

       

      • Anonymous says:

        The quake was the same as Haiti.

        I hope it is not plates going under one another. That is the sort that is most likely to produce Tsunamis as in the Asian Tsunami in 2004.

        However it was expected that our fault was indeedthat way case during  the Eocene epoch, (34-56 million yearsago).

        Today the tectonic boundary is a 100-250 km wide seismogenic zone of generally left-lateral strike-slip deformation which covers about 2000 km in length.

        This means we are slowly moving in a westerly direction(with a bit north as well)

        Actually there is much more to it than magnitude. We say there was a 5.9 in Cayman, well it was 32 miles off Cayman. Take into effect the medium through which the surface waves travel as well as the construction standards and duration of the shaking.

        A high 5 in Haiti may of had a simlar effect if the duration of shaking was more than a minute.

    • Mozzie Fodder says:

      Wow, your explanation makes much more sense that that of a Professor who has studied this islands geology for over 20 years. Do you have any relevant qualifications to make these assumptions other than the fact that you "looked at the pictures"?

       

      • Anonymous says:

        Only a Structual Engineering Degree the UK.

        And I actually went down to the hole and looked closely at it. 

  3. Anonymous says:

    I know this man was making sense… but like everything here, everyone wants to poke holes in someone else option….

    Well look who has gotten the last laugh… apologize to the man!!!

  4. Historical perspective says:

    While this explanation seems to be meritorious in many senses, I note carefully that he refers to the fact that it is hard to definitively state what was the cause without examining the hole directly.

    Well, if I can shed some historical perspective on the matter, maybe we can arrive at an explanation quicker.

    This area was originally all part of a large pond- Tarpon lake or something like that.  Pirates Lair, Anne Bonnie and Mary Read Crescent almost down to Cayman Crossing was all dredged out.

    Remember Burns mentioned this on the radio on Thursday?  It’s an inconvenient truth for many living in the areaa nd for those who directly benefited from the development.  In fact some of the developers’ family members are ironically now ardent environmentalists- iron of irony.

    So it’s not hard for me to imagine a sink hole occurring over an area where there would not have been much rock to begin with and an area that was eventually filled artificially. 

    Just a thought…

    • Anonymous says:

      Except the large pond was a bout half a mile away from the site of Pirates Lair

      Just a thought, I wouldn’t believe everything you hear on the radio

      • anonymous says:

        Ask the Old South Sounders where that lake was…right under Berna Cummings house across the road from the sink hole…!!!

         

        However, I do not think it was the pond. That  would have caused a much bigger area to collapse through "liquefaction".  This was a cave and a big one…I understand that over 30 loads of material disappeared down it already.

         

        Professor is correct about cause. The cave may go under the other buildings or just go straight down.  Ground penetrating geotechnical radar should be brought in to verify the safety of the surrounding buildings and road. Even if it does go under the building it does not however mean they will collapse. The sink hole was likely the thinnest area of rock in the caves roof.

  5. Anonymous says:

    see, some people gave me thumbs down and accused me of smoking "glue" when i said on the sinkhole first article here on cns that it is a gigantic cave underneath that whole side of Pirate’s Lair and this was a small part of that cave roof that has collapsed, now what you going to tell me, i think you owe me an apology.

    see what he said, that he’s been studying Cayman since 1981 and that area is built up so he doesn’t know the underneath of that area where the sink hole is.

    see, i told you that area was filled in in the early 70’s along with Tarpon Pond that was on the other side of the road.  I was told this from old time Caymanians.

    You think old time Caymanians don’t know their land???

    if he knew the truth, he wouldalso warn that side of Pirate’s Lair to watch out, they are in danger, cause that cave it’s sitting on has been breached and weakened and could have further collapses causing the buildings to collapse which in my opinion is exactly what’s going to happen, but of course i’ll probably be accused of smoking glue for saying that too, … until…the inevetable … until… it collapses…  better to be safe than sorry when it’s too late and lives are lost.

  6. Anonymous says:

    Thank you CNS for publishing this very informative article. It is good to have such a great resource such as Professor Jones to give such an educated explanation.

  7. Joe Grinder says:

    P.S. " ‘Holey’  ground they walk on".Got to love it.

  8. Joe Grinder says:

    Oh.Oh.125,000 years old. Heritic.Kick him off the island,everyone knows the earth is only 7,000 years old .Oh yeah, just like the Devil, he starts at 125,000,then goes to 2 to 5 million years at Pedro Castle,dosn’t this guy read the Bible.We all know this is impossible.How can CNS publish such rubbish.

    • fuzzy says:

      Nice try Joe.Trying to stir um up again eh?

    • The Scientist says:

      The Earth Is Not 7,000 years old, Ha,ha,ha!!! LOL!!!. You really made me laugh, i think you need to get your facts right before you post on how old the Earth is. Roughly the Earth is about 4.4 to 4.6 Billion Years Old. We are now in the 21st century, 2010 years. And before that time it was BCE. (Before Common Era). CE (Common Era). A.D. (After Death). And then before that time B.C (Before Christ). With all that said, 7,000 years is very short for the Earth. 4.6 billion years old with approxomately 6.8 billion People, 3.4 billion Men and 3.4 billion Women songs correct. The reason I state that, is that we have a minority of people that says the World have more Women than Men, Humanly impossible! Because every time a person dies another one born and even so as the population increases over the Centuries, Women and Men remain equal.

    • Anonymous says:

      FYI: The Earth is approximately 4.6 billion years old. This data has been calculated using rocks and radioactivie dating.

    • Anonymous says:

      Why try and stir things up? Dn’t you have anything better do do with your life?

    • Anonymous says:

      As always Mr. Joe quotes the Bible as being the only truth on this planet we live on, clearly SCIENCE is a myth….