What is karst?
Karst is a landscape or terrain that results from the chemical weathering (corrosion) of bedrock types that are soluble in water. These bedrock types are primarily carbonate rocks (limestone and rarely dolomite), salts and gypsum (least elastic and flexible). Carbonate rocks were made through sedimentation during the Mesozoic age including the Trias, the Jurassic and the Cretaceous periods (230-66 million years ago) and a part of the Tertiary period. Because of the calcium carbonate (CaCO3), which is soluble in water, carbonate rocks dissolve in water and consequently let it through. The dissolving does not immediately take place but over a long period of time and permeability is considerable because of chemically induced corrosion causing cracks, which enable the water to run through the rock.
In Croatia karst covers its Dinaric area. It could be said that the town of Karlovac is the so called “boundary rock” dividing karst areas of Croatia from non-karst ones. Southern suburbs of Karlovac are situated on the undulating area and these hills are made of carbonate rocks and belong to the tectonic unit of the Dinarides. Therefore, almost the entire part of Croatia south of Karlovac is karst area and the other part north of this town is not, although there are isolated karst areas in the Pannonian Croatia( on Slavonian mountains Medvednica and Žumberk).
Chemical processes play the main role in shaping the karst landscape. Let’s not forget tectonics and mechanical processes, but without chemical processes we wouldn’t talk about karst landscape. Karst is a result of chemical processes that have an impact on soluble sedimentary rocks. Karst landscape is also a result of climate and pedologic and vegetation characteristics. Calcium carbonate ( CaCO3) in contact with water (H2O) and carbon dioxide (CO2) dissolves into ions of calcium (Ca2+) and hydrogen carbonate (HCO32-).The power of corrosion, the dissolution of compact limestone, that is calcium carbonate ( CaCO3), depends on the amount of dissolved carbon dioxide (CO2) in water, in which process calcium hydrogen carbonate is made.
Chemical equation for limestone corrosion is as following:
CaCO3 + H2O + CO2 ↔ Ca2+ + 2HCO32-
There are many cavities in karst because of corrosion. Porosity is one of the dominant characteristics of karst rocks. That is how numerous caves and pits are formed. However, the reaction of the karst process is reversible, that is, the reverse course is also possible: in specific conditions calcium carbonate will be extracted again.
There is a differentiation between surface and underground karst formations.
Surface karst formations include:
Škrape (cracks) – the smallest but most numerous karst formations on the surface. Most common forms are grooves and small winding or meandering channels.
Ponikva (swallow hole, karst valleys) – are oval depressions caused by corrosion of water disappearing underground. Their width and depth most often range from a few to hundred meters. According to their shape it is possible to distinguish among oval, flat-bottomed or funnel-shaped ones. The area abundant in ponikve is called boginjavi krš.
Polja (fields) in karst and plateau – the biggest karst formations. Polja were formed through the fusion of rifts and side corrosive action of water streaming in from impermeable or permeable areas and disappearing in swallow holes. Such waters are called ponornice (lost rivers). Karst plateaux are typical for tropical regions. The quantity of substances of biogenic origin in soil is favourable to fast side corrosion and when more resistant parts lag behind in the form of conical hills the so called kupasti krš (Conical karst) is formed.
Underground karst formations:
Jame (pits) - the passage connects them to the surface and they stretch vertically into the underground (more than 45º). There are 44 known pits deeper than 250 m in Croatia. The largest number of pits may be found on the Velebit and Biokovo mountains. Lukina jama with the depth of 1392m is the deepest pit in Croatia and it is situated in the north-eastern part of the Hajdučki kukovi in the area of the North Velebit National Park.
Spilje (caves) - the passage connects them to the surface but they stretch horizontally into the underground (up to 45º). The best known caves in Croatia are Cerovačke spilje ( Cerovačke Caves) 1 km south of Gračac. They consist of two caves, Gornja (1295m) and Donja Cerovačka spilja (2685m) interconnected at numerous points.
Kaverna (cavern) – not connected to the surface and may be huge (Tounj Cave, which used to ba a cavern, with the length of 8487m is third in Croatia and it was discovered during the works in quarry).
According to their origin karst formations may be divided into three groups. In the early phase the channels are filled with water. In the transitive phase they are partially filled with water and calcium carbonate (dripstones) starts to settle. In the last phase there is no water and dripstones are formed. Dripstones are formed (under the change of circumstances) when calcium carbonate resettles. They are divided into stalactites (formed by dripping water and hanging from a cave roof), stalagmites (formed by upward growth from a cave floor) and stalagnates (cave pillars).