Introduction The word ‘Karst’ comes from the Krs Plateau in Slovenia, where karst environments were first studied closely. The Slav people use ‘Krs’ to refere to ‘bare stony ground’. More specifically geomorphology refers to karst as a type of terrain characterised mainly by caves, sinkholes and complex drainage networks. The majority of karst formations occur in soluble rocks such as limestone and dolomite (although recent discoveries have revealed karst formations in more resistant rocks such as quartz, but are rare) and the reason for this is connected to the underlying processes.
The two chief processes responsible for the weathering of rocks are physical weathering and chemical weathering, but it chemical weathering that is particularly present in soluble rocks (Christopherson 2012). Around 25% of the earth’s surface is composed of limestone and almost 15% of the earth’s surface contains karst features. Considering the potential hazards of karst features together with their significant proportions it is no surprise that karst environments are highly studied. These hazards include sinkholes, sinkhole flooding and the contamination of groundwater.
This essay will also discuss the impact humans have had on karst landscapes. Figure 1: Karst landscapes and limestone regions (Christopherson 2012) 2. Karst Processes 2. 1 Brief Description The hydrological cycle is very important in the process of karst formations. The formation of karst starts with precipitation that often reacts to carbon dioxide to form carbonic acid. When the water reaches the earth it percolates through very permeable limestone. The water will erode the limestone along cracks and over long periods of time these cracks increase in size to form caves and other karst features.
In many instances tectonic processes are responsible for the exposure of limestone. As uplift takes place and as the limestone starts bulging cracks start to appear making the rock more vulnerable to erosion along these cracks. As mentioned before the principle process of weathering in soluble rocks is the dissolving of minerals by water. The process of dissolving is a rather simple concept, however several conditions are needed for the formation of karst topography (Blair 1986). Figure 2: Hydrological Cycle in Karst Regions (American Geological Institute) 2.
2 Lithology The solubility of rocks is determined by the amount of calcium they contain. The higher the concentration of calcium becomes the higher the chances of karst forming. Karst topography can form in rocks that constitute less than 80% carbonates but the development of karst favours rocks with higher concentrations of carbonates. Consequently the majority of karst landforms appear in limestone and dolomite(Jennings 1985). Any rock that contains concentrations of calcium carbonate (CaCO3) greater than 50% is considered to be limestone.
Pure limestone contains at least 90% and in places such as Jamaica and Slovenia the limestone is at least 95% pure. Dolomite rocks contain at least 50% calcium-magnesium carbonate, also known as dolomite ((CaMg)(CO3)2), while pure dolomitic rocks (also called dolostones) contain at least 90% dolomite (Huggett 2011). Additionally the thickness of the bed rock as well as the nature of the bed influence the degree of karsification. For instance in the Dinaric Mountains the soluble rock is quite shallow before it meets insoluble rock, resulting in broad flat floors in the caves.
Figure 3: Limestones close to Dover (Lutgens 2011) 2. 3 Structure The limestone, or other soluble rock, needs to have complex patterns of joints in order for water to enter the rock. Water will slowly erode the rock to form subterranean drainage channels that could later on increase in size and form caves. Vertical karst features, such as pinnacle karst, are formed where the limestone is exposed at the surface. Tectonic processes also have an important impact since tectonic forces can uplift and deform plateaus of carbonate rock increasing the fracture density.
Such an elevated plateau would also likely receive greater amounts of precipitation that would increase erosion. Figure 4: Pinnacle karst in Shilin National Park, Yunnan, China (Huggett 2011) 2. 4 Relief The vadose zone is the layer above the water table. A shallow vadose zone means the water table is relatively high, this high water table supports the earth above it. A reduction in the water table could leave the ground above unsupported and a depression can be formed and in extreme cases collapse can occur. 2. 5 Hydrology 2. 5. 1 Carbonates(Limestone and Dolomite)
As mentioned before the chief process of erosion in karst systems is chemical weathering. Limestone is moderately soluble in pure water, but in water that contains carbonic acid the solubility of limestone increases greatly (Huggett 2011). There are three components present in chemical weathering of limestone. This process usually starts in the atmosphere(or in the soil for that matter) where water molecules mix with carbon dioxide and forms carbonic acid. CO2 + H20 H2CO3 As this solution of water and carbonic acid infiltrates carbonate rocks, especially limestone, the carbonic acid dissolves the calcite.
CaCO3 + H20 + C02 Ca++ + 2HCO3- Limestone Dissolved Limestone Once this reaction reaches equilibrium it will remain in that state until pressure decreases or temperature increases at which point the dissolved limestone will return to limestone that will be deposited again, similar reactions are responsible for the formations of stalagmite and stalactites in caves. Dolomite reacts the same way as limestone when it comes into contact with carbonic acid, but other rocks, such as evaporates, do not react with carbonic acid. The rate of erosion of limestone is not constant, erosion fluctuates along with storm water.
2. 5. 2 Silicate Rocks Even though the majority of karst formations occur in carbonate rocks karst formations can occur in other rock types such as silicate rocks. Silicate rocks are much more resistant to chemical weathering, rocks such as quartzite have a very low solubility. However, even though they are rare, there are instances of chemical weathering of silicate rocks. Since it is very difficult to dissolve the quartzite grains themselves a different idea has been proposed. Instead of dissolving the entire quartzite grain only the cementing material needs to be dissolved.
Once individual grains have been separated by hydrolysis they are susceptible to physical weathering (Huggett 2011). 2. 6 Climate There has been much debate around which climates create the most karst landforms. Some geomorphologists argue that cold climates are the most erosive climate while others advocate the tropics as the most erosive climate of karst landscapes. Those in favour of cold climate being more erosive point out that colder water can hold greater amounts of carbon dioxide and therefore making the water more acidic.
On the other hand those in favour of warmer climates discard the former argument and show that at cold climates rates reactions are slower. So despite water carrying greater amounts of carbon dioxide in colder areas the rates of reaction are higher in warm areas. Instead of the amount of carbon dioxide held in water, these geomorphologists emphasise the amounts of precipitation. Hence tropical areas are more erosive with greater amounts of rain with more vegetation. It is for this reason that there is a distinction made between classic karst and tropical karst formations (Huggett 2011).
2. 7 Vegetation Vegetation is the most important source of carbon dioxide for acidic solutions. This carbon dioxide is released by the roots of plants and by the decomposition of dead plant matter by bacteria. Areas with sparse vegetation will therefore erode more slowly than areas with dense vegetation that release much greater amounts of biogenic carbon dioxide(Jennings 1985). The existence of dense vegetation also supports the argument that karst areas are more active in the tropics. 2. 8 Hydrothermal Processes In areas where hot springs are present hydrothermal processes can occur.
Hot water can dissolve much more compounds and can become much more acidic than carbonic acid. Hot water that has dissolved pyrites contains a high concentration of sulphuric acid and other minerals that is much more corrosive than carbonic acid. Figure 5: A hot spring at Yosemite Park, USA 2. 9 Pseudokarst – Other Processes In regions such as in China, chemical weathering is almost the only form of erosion, however most of the time chemical weathering works with other processes of weathering. These karst formations , where chemical weathering is not the principle form of erosion, are called pseudokarst.
Such a process of karst formation is the evacuation of molten rock (Huggett 2011). As molten rock flows beneath the surface it forms channels of drainage. However once the lava flow has stopped the channels remain behind as seen in Kazumura Cave in Hawaii. Figure 6: Kazumura Cave in Hawaii (Vulcanospeleology. org 2009) 3. Karst Hazards Karst features like caves are often spectacular landscapes but they do not come without their fair share of potential hazards. However before karst hazards are explored it is necessary to identify some of the most important karst features.
Karst forms are categorised between to those features that appear on the surface of the earth and those that appear beneath the surface of the earth. 3. 1 Karst Features 3. 1. 1Surface Features Amongst other surface karst features some of them are: Karren – describes the small scale solutional features(Huggett 2011); Limestone Pavements – these are large karrens covering a large area that occur in flattish areas as the result of rain; Pinnacle Karst – large pinnacles (up to 40m high) that commonly occur in Asia. 3. 1. 2 Subterranean Features
Erosion does not stop at the surface the water starts to percolate through the soil and starts to erode limestone. Consequently complex drainage networks are formed below the surface in areas of soluble rocks. Under the right conditions fissures and conduits can grow in size as dissolution and scouring slowly erode the walls and eventually forming caves. As will be seen in the next section caves are sometimes responsible for sinkhole subsidence. Deposition also takes place in caves, as temperature or pressure varies various minerals are deposited.
This process of deposition is very slow but over time speleothems (cave formations such as stalagmites, stalactites and columns) can form. Figure 7: Stalactites, stalagmites, and columns in New Mexico’s Carlsbad Caverns National Park (Lutgens 2011) 3. 2 Dolines – A potential hazard and the management thereof Dolines, also called sinkholes, are circular depressions that occur in limestone areas. Sinkholes can be formed by several processes and are also categorised according the underlying process. Solution dolines are formed slowly as erosive water gets trapped in a conduit.
This water erodes at the bedrock and over time a depression is formed at the surface. This process has a positive feedback, as the size of the depression increases more water can be trapped and erosion increases. Because this type of doline forms slowly it is not a threat to human life. 3. 2. 1 Sinkhole Subsidence Collapse dolines are caused by the collapse of the roof in a cave. These types of sinkholes happen suddenly and are a possible hazard for humans. One of the most detrimental and recent sinkhole incidents took place in Gautemala. On the 30 May 2010 a sinkhole swallowed an entire three storey building and killed at least one person.
The sinkhole was 30 storeys deep and about 18m wide (National Geographic 2010). Another sinkhole in Florida took the life of Jeff Bush in April of 2013. These cases were only two of many sinkholes that occur every year. Sinkholes do kill people but they do so on a very small scale with usually only one or two casualties. More significant damage is done to the built environment. This raises the question of what can be done to prevent such damage to building and the loss of life. Figure 8: Sinkhole in Gautemala on the 30 May 2010 (Society, N. 2010) Management: Cases such as the sinkhole in Florida can be reduced by government policies.
In South Africa such a policy is the Development Facilitation Act 67 of 1995 which forces developers to do an investigation on the stability of the site before developing starts. Such an investigation would include the assessment of possible sinkholes, especially in limestone areas. Human developments also have an effect on the probability of sinkhole subsidence. Urban development increases runoff to the extent where existing natural drainage networks cannot cope with such volumes of water. As mentioned earlier, groundwater pumping can remove the buoyancy of the land by lowering the water table.
This is what happened in Winter Park, Florida in May of 1981. The obvious solution to such problems would be to closely monitor water tables and reduce the amount of water being pumped out of the ground. Figure 9: Winter Park, Florida, May 1981 3. 2. 2 Sinkhole Flooding Once sinkholes have formed they can collect water from rain. However, just like a river, sinkholes have a maximum capacity and they also have outlet capacities. A severe storm can fill a sinkhole with water faster than the sinkhole can drain the water causing sinkhole flooding. Management: Sinkhole floodplains are often overlooked by developers.
One possible solution of such flooding is to fill sinkholes with material. Another solution is better planning and building buildings above the historical sinkhole floodplain 3. 2. 3 Groundwater Contamination Unlike other terrain, karst terrain is very fractured. Usually water will be filtered through a number of layers before it reaches the water level and in this process pollutants are removed from the water. However in karst areas the water does not always filter through soil before it reaches the water level. For instance a sinkhole allows direct access to groundwater making the groundwater more vulnerable to pollution.
Management: In such areas extra care needs to be taken with waste disposal, storm drains, rubbish dumps, septic tanks, fertilizers, pesticides and any other chemical that can contaminate groundwater. 4. Conclusion To conclude karsification is the chemical and physical weathering of soluble rock, mostly on limestone and dolomite, however it is also possible for karst features to appear in less soluble rock. Almost a quarter of rocks on earth are limestone and more than half of these contain karst formations. This is a considerable proportion considering the large amount of humans that live on such potentially hazardous terrain.
It is for this reason that it is essential that policies, such as the Development Facilitation Act 67 are implemented properly to prevent the loss of life and damage to property. 5. References Belo, B. (2013) Natural Hazard Mitigation Planning For Karst Terrains Available at: http://scholar. lib. vt. edu/theses/available/etd-05222003-230312/unrestricted/etd. pdf [Accessed: 16 April 2013]. Blair, R. (1986) Geomorphology from Space. California: National Aeronautics and Space Administration, p. 407-430. Christopherson, R. (2012) Geosystems – An Introduction to Physical Geography.
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com/news/2010/06/photogalleries/100601-sinkhole-in-guatemala-2010-pictures-world/ [Accessed: 16 Apr 2013]. Veni, G. et al. (2013) Living with Karst. [e-book] Available through: American Geological Institute www. agiweb. org/environment/publications/karst. pdf [Accessed: 16 April 2013]. Vulcanospeleology. org (2009) Kazumura Cave, Hawaii:Commission on Volcanic Caves Photo Gallery. [online] Available at: http://www. vulcanospeleology. org/pgkazu/index. html [Accessed: 16 Apr 2013]. Ziervogel, P. (2007) Managing karst landscapes. Cape Town Environment & Urbanization, 19 (1).
