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karst

Of Karst! – short episodes about karst

Of Karst! – short episodes about karst

Episode 2: Dissolving rock? (or, how karst evolves).

Post by Andreas Hartmann, Lecturer in Hydrology at the University of Freiburg (Universität Freiburg), in Germany. You can follow Andreas on twitter at @sub_heterogenty.

Didn’t get to read Episode 1? Click this link here to do so!

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In the previous episode, I introduced karst by showing how it looks in different regions in the world. This episode will now deal with the processes that create such amazing surface and subsurface landforms. The widely used term “karstification” refers to the chemical weathering of easily soluble rock composed of carbonate rock or gypsum. Most typical is karstification of limestone (consisting of the mineral calcite, CaCO3) or dolostone (consisting of the mineral dolomite, CaMg(CO3)2). If exposed to CO2 rich water these rocks are dissolved to form aqueous calcium (Ca2+) or magnesium (Mg2+) and bicarbonate (HCO3 ) ions. For calcite, karstification is described by the following chemical equilibrium:

The dissolution of carbonate rock depends on various factors. Imagine a solid block of salt, which you pour water on. If completely solid, the water will flow down the salt surface slowly dissolving the block. If fractured, water will eventually enlarge the fractures in the salt block and dissolution will occur much faster. Now imagine smashing the salt block before pouring water on it. In such circumstances the salt will dissolve even faster as the surface area exposed to the water is much larger.

Karst and its evolution (educational video provided by Jennifer Calva on Youtube).

The same is true for karstification. If the carbonate rock is heavily fractured, it will dissolve faster than unfractured carbonate rock. Another factor is the availability of CO2, that depends on the relative amount of CO2 in the air, air temperature and soil microbiotic processes. Other factors are the purity of the carbonate rock, the availability of water, and the supply of CO2 from the surface. As soon as karstification takes place, more water will be able to pass the dissolution enlarged fractures providing more and more CO2, and creating a positive feedback between rock dissolution and water flow:

Positive feedback between carbonate rock dissolution and water flow (Hartmann et al., 2014, modified).

The hydrochemical processes described in this episode of the Of Karst! Series not only create beautiful karst landscapes but they also have a strong and particular impact on water flow paths in the subsurface, which will the topic of episode 4 that can be expected in early 2018. Before, I will present a special feature about karst in the movies as topic of episode 3 in autumn 2017.

Further reading

Hartmann, A., Goldscheider, N., Wagener, T., Lange, J. & Weiler, M. 2014. Karst water resources in a changing world: Review of hydrological modeling approaches. Reviews of Geophysics, 52, 218–242, doi: 10.1002/2013rg000443.

Ford, D.C. & Williams, P.W. 2013. Karst Hydrogeology and Geomorphology. John Wiley & Sons, 576 pages.

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Andreas Hartmann is a lecturer in Hydrology at the University of Freiburg. His primary field of interest is karst hydrology and hydrological modelling. Find out more at his personal webpage www.subsurface-heterogeneity.com.

 

Of Karst! – short episodes about karst

Of Karst! – short episodes about karst

Episode 1 – A different introduction to karst

by Andreas Hartmann Lecturer in Hydrology at the University of Freiburg

Usually, textbooks or lectures start with the theoretical background and basic knowledge of the topic they try to cover. Writing my first contribution to the Water Underground blog I want to take advantage of this less formal environment. I will introduce karst as I and many others around the world see it. As the most beautiful environment to explore and study.

Some of you may not be familiar with the term karst, its geomorphology or hydrological consequences. But I am almost certain that most of you have seen the landforms in the four pictures below.

Tower karst (1st photo) is typical of tropical regions. The picture below is taken close to Guilin, Southwest China, and I am sure many of you remember James Bond “The Man with the Golden Gun” and the beautiful tower karst islands at which parts of movie takes place (episode 3 will be a special feature about karst in the movies). Tower karst reaches heights up to 300m and often referred to by its Chinese name Fenglin or Fengcong karst, when occurring in a large number.

The 2nd photo shows the opposite landform: a huge hole in the forest ground. This is not a crater but a very big collapse sinkhole at Vermillion Creek, Northwest territories, Canada. It has an ellipse shape (60m x 120m) and 40 m below the surface, it has a lake whose depth has not yet been determined. You may not have previously heard the term sinkhole. But on the news one day you will hear stories of holes suddenly swallowing cars or entire houses in Florida or Mexico. If not due to mining, those were most probably collapses that occurred due to karstification.

Figure 1: (1) amazing tower karst Li River, Gulin, China (duskyswondersite.com), (2) collapse sinkhole , Vermillion Creek, Northwest territories, Canada (pinterest.com), (3) Kalisuci Cave at Jogjakarta, Indonesia (ourtheholiday.blogspot.com), (4) spring of the Loue River, France (wikiwand.com)

The most popular features of karst are caves, some of them as large as entire buildings. The 3rd photo shows how it may look inside a karstic cave (Kalisuci Cave at Jogjakarta, Indonesia). Note that there are plenty of stalactites and that there is a lot of water that will eventually find its way back to the surface discharging a karstic spring.

The 4th photo shows the spring of the Loue River, France, which is one of the largest springs in Europe. The volumes of water coming out easily compare to the discharge of medium size rivers. If you ever saw a spring that big it must have been a karst spring!

In the Of Karst! series, I will take you on a journey through more of these amazing characteristics of karst. I will show how its evolution over time can produce the landforms shown here. I will show how karstification affects the resulting movement of water on the surface, in caves systems and in karstic rock. And I will explain why karst is so relevant for our societies. In episode 2 (late June 2017) I will speak of how karst evolves. Episode 3 (early October 2017) will a special feature about karst in James Bond other famous movies.

Andreas Hartmann is a lecturer in Hydrology at the University of Freiburg. His primary field of interest is karst hydrology and hydrological modelling. Find out more at his personal webpage www.subsurface-heterogeneity.com

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