Sinkholes Tellus Border Earthquake - Geological Survey of Ireland

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Geology Matters: Summer 2013 Sinkholes in Ireland Dr. Caoimhe Hickey, Hydrogeologist, Groundwater GSI What is a sinkhole or doline? A sinkhole (or doline as it is more commonly called outside of North America), is a natural enclosed depression found in karst landscapes. Karst landscapes are formed in rocks that can be dissolved by acidic water. This is usually on carbonate rocks, such as limestones but karst can develop on evaporitic rocks and siliceous rocks such as quartzite. Approximately half of the Republic of Ireland is underlain by Carboniferous Limestone and consequently karst is an important aspect of the Irish landscape. Dolines are the most common landform in karst areas. They are described as small to medium sized closed depressions, ranging from metres to hundreds of metres in both diameter and depth. Once created dolines function as funnels, allowing the direct transmission of surface water into the underlying karstic bedrock aquifer. They may occur as isolated features or in clusters causing a pock-marked land surface How do dolines form? Dolines are formed by two main methods: the slow solutional removal of rock from the surface downward (solution doline), or by the collapse of overlying rock or overlying material into an underground cave or chamber (collapse doline). Most dolines are considered polygenetic in origin and are usually formed from a combination of solution and collapse, however one of these processes usually dominates their appearance, whether it is catastrophic or gradual. Solution dolines form in places such as vertical or near vertical joint intersections. Surface runoff will focus at these areas of weakness, leading to the solution of the bedrock. Water and solutes will then move downwards through the bedrock openings (such as at joints and bedding planes). The result is a funnel-shaped depression on the surface. Solution dolines are considered to be formed by a gradual process of sagging or settling of the overlying deposits into the hollow left by an area of dissolving rock. Solution dolines are usually characterised by gentle slopes with no obvious rupturing of the soil or surface. Collapse dolines usually occur very suddenly where the bedrock or subsoil material collapse into an underlying void. Cover collapse dolines (sometimes known as dropout dolines), which are very common in Ireland, occur in karst areas covered by unconsolidated material, such as glacial till. They form by the sudden downward movement of the overburden and usually form in areas where the overburden is somewhat cohesive. They occur in a process called 'piping', where a soil or subsoil arch, which has formed due to removal of material at the bottom of a layer of overburden, suddenly gives way (White, 1988). Although there must be a highly efficient pathway established for sediment transportation in order for the soil arch to form and grow, a large bedrock hollow is not necessary for their development. Cover collapse dolines are characterised by vertical or steep-sided collapses, with a very sharp break in slope and often have stepped sides, where soil is exposed. Over time, however, their slopes may degrade and infilling sediment may build up giving these dolines the morphology of solution dolines (Ford and Williams, 2007). If you are worried about sinkholes in Ireland, or have additional information about sinkholes or potential karst collapse features please contact The Groundwater Section at The Geological Survey of Ireland groundwaterinfo@gsi.ie page 18
Geology Matters: Summer 2013 Solution Doline Formation (Jennings 1985) Solution Dolines in Co. Roscommon The Formation of a Cover Collapse Doline. A) Solutional openings in the bedrock wash material downward, B) A small arch forms in the subsoil where the material is being washed away, C) The void grows in size as more material is being downwashed until it reaches a critical point and starts to rupture, D) The arch suddenly collapses as it can no longer support its own weight, E) Overtime the vertical sides will degrade and the hole will become less deep. page 19
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Sinkholes Tellus Border Earthquake - Geological Survey of Ireland

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