What Is A Sinkhole And Why Do Sinkholes Form?
    By News Staff | May 12th 2010 03:12 PM | 8 comments | Print | E-mail | Track Comments
    A sinkhole in Saint-Jude, Quebec has just collapsed a house, killing a family of four inside, leading people to ask what they are, how they occur and obviously who is at risk.

    Sinkholes are depressions that occur when soil or bedrock has been removed and only air remains - commonly when the rock below the land surface is limestone,  carbonate rock or salt beds that have been naturally dissolved by ground water circulating through them and then the water dries up.

    As the rock is dissolved by water(dissolution), spaces are created (suffosion) and typically sinkholes happen suddenly because the surface stays intact until the weight becomes too much. 

    Sinkholes occur more often in places like Florida and Alabama because they have more underground rivers than most other states.   When sinkholes form it is commonly because the ground has simply collapsed over underground rivers that were once filled with water.

    So if you want to look for signs of a major drought in the making, disgruntled skiers at Lake Tahoe are not going to be as accurate as sinkholes.  

    This map from the USGS shows where underground cavities have more frequently occurred and resulted in sinkholes.   As they note, these susceptible types of rock are called evaporites, such as salt, gypsum, and anhydrite, and carbonates, such as limestone and dolomite.  Evaporite rocks are in 35-40% of the US, but at various depths, which is why not all areas are equally affected.

    sinkhole frequency map by US region

    Can sinkholes be caused by man?

    Land-use practices have also been implicated in sinkholes.   When natural water drainage patterns are changed or water is otherwise diverted, the upward pressure of water is no longer keeping the surface stable.   Likewise, heavy industrial building or artificial lakes can cause weight issues that result in sinkholes.

    Obviously if underground mining has occurred plain ol' gravity can cause sinkholes.

    3 types of sinkholes

    Dissolution sinkholes - where water first contacts the rock surface and also in joints and fractures.

    Cover-subsidence sinkholes -  usually when the covering is permeable and contain sand. 

    Cover-collapse sinkholes - usually when the covering contains a large amount of clay and surface erosion and drainage cause the 'bowl' people commonly see.


    Only this one is not a sinkhole. See Dave Petley's reporting here:

    Hey Ron, good to see you again.   So Dave's blog didn't say (other than they were not unheard of) but is the landscape there prone to quick clay slides?   

    One thing we never considered was 'is it a sinkhole at all?' because we assumed when everyone called it one, that is what it was.    It gets even messier when it comes to fruit flies in biology.
    When I was a small child, my father pointed out to me that using drains for surface water instead of soakaways leads to drying out of soil and eventual subsidence.

    I took the photo below last year in Chatham, Kent.  It shows the cracks in a building as the extension tilts away from the main building.

    Listed as an ancient monument, the Brook Pumping Station was opened in 1929.  It was part of Chatham’s main drainage system until 1980.  It is now a museum.  When built it was in mostly open land.  That land was formerly a river bed.  Today the building is surrounded on all sides by roads and buildings.  These have modern drains which take the surface water away from the area.

    Fortunately the local geology isn't quite right for sinkhole formation.  But subsidence in a drying out river bed makes a good second best, don't you think?

    Also used for dumping the bodies of political opponents, as in the Foibe massacres.  I found out about these during a synchrotron trip to Elettra last year.
    Robert H. Olley / Quondam Physics Department / University of Reading / England
    Tropical storm Agatha has caused a 200 ft deep sink hole in Guatemala.


    Here in Indiana, in the southern portion of the state where the limestone bedrock is exposed at the surface, sinkholes are quite pervasive and a serious problem for local residents.

    When the carbon dioxide in the atmosphere dissolve into water vapor, forming carbonic acid, and when the sulfur dioxide emissions from the burning of coal oxidizes forming sulfur trioxide which in turn forms sulfuric upon contact with water vapor, the two, when precipitated out of the atmosphere in the form of rain, slowly react with the alkaline calcium carbonate in the limestone, in time carving out large subterranean caverns as the acids percolate down into the bedrock. When the stresses on top of the roof-rock of these caverns exceeds the tensile strength of the roof-rock itself, catastrophic failure occurs, and the roof-rock collapses into the underlying cavern forming sometimes incredibly large sinkholes. This occurs when there is a drought and the water table, which helps support the roof rocks, is lowered and is replaced by gases.

    For a sedimentary petrologist it's a petrological paradise. But for local residents it is an extremely hazardous threat to life, limb and property.
    More sinkhole news: 

    Sinkhole appears in Guatemala City

    But especially given that it opened up under a factory, happily there are no reported fatalities.
    Robert H. Olley / Quondam Physics Department / University of Reading / England

    Moses the camel rescued from a sinkhole

    Handsome chap, isn't he?

    Robert H. Olley / Quondam Physics Department / University of Reading / England