How karst features are formed

The development of karst (karstification) takes place best in Ireland on limestone rocks that are hard and almost impermeable to water. The rock must have cracks and fissures into which rain water can seep, and there must be a plentiful supply of rain.

Rain water is slightly acidic and this acid (carbonic acid) readily dissolves the rock, carrying it away as an invisible solution. Rain water which first passes through soil before reaching the limestone becomes much more acidic and is capable of dissolving a greater quantity of rock.

As the acidified rain water trickles down the cracks in the limestone, it progressively enlarges them which allows a greater quantity of water to enter and hence enlarge the cracks even more. In time, the fissures are sufficiently enlarged to engulf all rainwater within moments of its falling. In some areas, rivers which rise on non-limestone rocks, flow on to the limestone and sink underground in swallow holes – again formed by the corrosive action of the river water on the soluble rock.

Underground, the waters from fissures unite to form small streams and in turn these join and excavate correspondingly large conduits. Conduits accessible to humans are called caves. At some point the underground waters return to the surface as springs, except where local geological conditions may cause the waters to emerge from the sea bed some distance off-shore, as on the Burren coast of Co. Clare. Thus caves and cave streams are the equivalent of valleys and rivers in a non-karstic area. Caves and karst fissures are common at shallow depths beneath the ground surface but they are also known to exist at great depths. The mechanism by which deep karst features are formed is not wholly understood.


Pinnacles and towers of limestone remaining when the surrounding rocks have been dissolved away (David Drew)

 
Small solutional hollows (types of karren) pit the limestone surface on the Atlantic coast of Co. Clare (David Drew)

In Ireland, some hundreds of caves are known, some containing rivers (Marble Arch Cave in Co. Fermanagh, for example), some long abandoned by the streams that formed them (e.g. the Mitchelstown Caves in Co. Tipperary). Unlike the mountains of the world, all of which are known, new caves are continually being discovered or new passages discovered in known caves – even in Ireland. Thus no one knows where the longest and deepest caves in the world really are.

Ireland’s longest cave is Pollnagollum in the Burren, Co. Clare where over 15 km of passageways have been explored thus far. The deepest cave in Ireland is the Reyfad system near Boho, Co. Fermanagh. This cave has been explored to a depth of over 180 m below the entrance and its exploration involves descending deep vertical shafts. Recent water well drilling near Kinvara in Co. Galway discovered caves at 48 m below the present day sea level.

The longest explored cave in the world (by far) is the Mammoth Cave in Kentucky, with some 530 km of passages mapped. The longest cave developed in gypsum rock is Optimisticheskaja in Ukraine. The 165 km of passages in this system form a maze contained in an area of less than one square kilometre in extent. The world’s deepest known cave at - 1535 m is the System Jean Bernard in the French Alps. The largest underground caverns yet discovered include a chamber in Lubang Nasib Bagus in Sarawak 700 m long, 300 m wide and 120 m high, whilst the main chamber of Majlis Al Jinn cave (now renamed) in Oman has a volume of 4 million cubic metres.

When cave passages are abandoned or partly abandoned by the streams that formed them, drips of water entering the cave from above may deposit crystals of pure calcium carbonate (calcite) which gradually form calcite deposits, e.g stalactites or, as in the photograph of Dunmore Cave, (page 19), stalagmites. The white calcite may be coloured by impurities such as iron dissolved from the rocks above the cave.

Thus a mature karst landscape is devoid of surface water. The surface may be pitted with deep hollows, conical or saucer shaped, and sometimes hundreds of metres deep and several kilometres in diameter. These dolines (small to medium sized enclosed depressions) act as funnels, collecting rainwater and leading it underground into cave systems.


A river emerging from a large cave near the Mediterranean coast of Turkey (David Drew)


In the uppermost layers of limestone, solutional enlargements of joints and bedding planes is greatest. This layer is called the Epikarst (David Drew)


A desolate karst plateau in Mallorca. There is almost no level ground, doline depressions are abundant and soils are thin or non-existent (David Drew)

Unless deposits of loose material blanket the limestone, e.g. glacially derived materials in the case of much of the Irish midlands, the soils which develop are typically very thin and patchy and are liable to erosion. Thick soils suitable for cultivation are confined to the hollows or dolines.

The solution of limestone rock takes place at the surface as well as underground. Many limestone surfaces are pitted with small hollows or runnels, collectively termed karren, which range from a few centimetres to a few metres in depth and length. The dissolution of the limestone is again due to acidified water standing in pools or running over the surface and sculpting the rock.