Author: Michał Gradziński
Institute of Geological Sciences, Jagiellonian University, Kraków
Dr. Michał Gradziński, an assistant professor at the Department of Sedimentology and Paleoenvironmental Analysis (Institue of Geological Sciences, Jagiellonian Univeristy), studies karst geology, carbonate sedimentology, and microbial sediments.
The progressive development of karst caves, which are created as a consequence of the successive dissolution of rock by water, has been a subject of research for more than 100 years now. Initially involving just first-hand observation, the methods used have also come to embrace experimental testing and more recently even numerical modeling. It is now clear that a typical karst cave passes through several distinct stages in its “lifecycle”: an initial stage, followed by a stage of rapid expansion (when it becomes human-accessible), then a mature stage, a period of old age (stagnation), and finally destruction.
According to the encyclopedia definition, a cave is a naturally-formed opening in rock that is human-accessible, in other words one that has attained a certain sufficient size. The duration of the initial, formation stage can roughly be inferred by calculating the rate of the rock dissolution processes involved. However, pinpointing the age of a specific cave involves various objective difficulties. A certain maximum possible age of any rock cavity is set by the age of the rocks themselves, which is a routine task for geologists to identify. A minimum age, in turn, can often be established by a certain indirect method: it is assumed that every such rock cavity must be older than the oldest sediments to be found inside it (called cave-filling sediments).
The age of cave-filling sediments can be studied in various ways, analogous to the methods used to study the age of many other kinds of sediments. These include various paleontological methods, which seek to identify a “relative” age by scrutinizing the remains of fossilized flora and fauna. One inconvenience is that cave sediments are frequently fossil-free, not containing any organic remains at all. Another shortcoming is that the flora and fauna remains that are found in caves are frequently not of great stratigraphic value, not making it possible to precisely identify their age. For example, the cave bear (Ursus spelaeus), whose numerous bone remains are found in many caves of Europe, went extinct upwards of 20,000 years ago. When such bones are found, therefore, they provide only a certain suggestion about the age of the sediments in which they occur. The bones of small vertebrate animals, especially rodents, are more useful in this regard. We should also mention plant pollen studies, which in recent years have been performed on material from cave sediments as well.
The past 30 years have seen very dynamic development in the technique of studying the age of speleothems (cave formations such as stalactites and stalagmites) isotopically, mainly based on the uranium series isotopes. This can yield very reliable dates for cave formations less than around 600,000 years old. Various efforts are now being made to extend this dating technique to older speleothems as well. Recently, paleomagnetic methods have also been successfully used in identifying the age of cave sediments. All told, a combination of various methods for dating sediments can be used to narrow down the age of a given cave, and frequently to identify how far it has progressed through the successive stages of development.
Such experimentation and calculations indicate that the initial stage of a cave’s lifespan usually (under favorable conditions) lasts several thousand years. Of course the rate at which the rocks are dissolved depends on many factors, such as the water supply, its aggressiveness with respect to the foundation rocks, and the properties of those rocks, such as their porosity and the presence of previously-existing cracks. Under unfavorable conditions, however, the duration of this initial stage might be prolonged significantly, even to more than a million years.
To growth and maturity…
Once the dissolution processes widen the initial rock cavity to above 5–15 mm, a stage of rapid expansion begins, because this width permits the passage of a turbulent flow, which is conducive to significantly faster dissolution. At this stage, as in the previous one, the cavity is still fully filled with water, that is to say it is still at the “phreatic” stage. The duration of this stage also hinges on many factors; in the case of cavities developing in carbonate rocks it usually ranges from several thousand to ten thousand years, although it can sometimes last more than 100,000 years. Then the cavity, or cave, attains a size that enables it to be penetrated by humans (with a diameter from tens of centimeters up to several meters or more) and it enters the mature stage.
The duration of a given cave’s stages of rapid expansion and maturity depends, among other factors, on the geomorphological development of the area in which it occurs. If situated in a rapidly deepening valley, a cave will relatively quickly end up above the water table, in other words in what is called the “vadose” zone, and become partly or fully filled with air. That means that its continued expansion will be either completely halted or significantly curbed. Of course there are some exceptions, including caves that are fed by concentrated flood waters or from melting glaciers. Caves of this latter category are called “proglacial”: many of the caves in the Tatra Mountains and Alpine ranges, for instance, belong to this category.
Waters flowing through caves deposit various types of clastic sediments in them, from pebbles to fine-grained sediments made of very small mineral particles. Such sediments slowly fill the cave, although they also can be subject to erosion. As a consequence, many caves become repeatedly filled up with sediments and then cleared out again, although generally they slowly become filled with clastic sediments. In the vadose zone of caves, various kinds of speleothems crystallize.
To old age…
When a cave becomes completely dried up, deprived of flowing streams, its expansion essentially halts and it enters what we might call “old age.” In the deeper sections of caves of this sort, all processes proceed very slowly. The accumulation of clastic sediments is very restricted, but speleothems continue to grow, and instability of the ceiling leads to rockfalls and the accumulation of angular clasts on the cave floor. It is quite paradoxical that many caves open to visitors, which give the impression of being vibrantly alive in view of the impressive, continually growing speleothem formations so admired by tourists, have already reached this “elderly” stage of their development.
Some caves become completely filled up with sediments, and under favorable circumstances they may survive in this form, together with the surrounding rocks, for millions, tens of millions, or even hundreds of millions of years. Such caves are known as paleokarst formations.
But most caves and the sediments they are filled with end up being worn away by surface erosion, together with the surrounding rocks. Complete destruction is preceded by a stage of fragmentation, when only a small, unconnected cave portions remain. Examples of such relicts of former large caves, created under hydrological and climactic conditions that were distinctly different from those we know today, can be encountered in Poland, for instance, in the Krakow-Wieluń Upland region. The final destruction of a cave usually occurs a million or more years after its formation. However, that depends on the rate of denudation within the region, which in turn is conditioned by a diverse set of factors.
In exceptional situations, the development of caves in carbonate rocks might proceed much more quickly. For example, caves may develop very quickly in areas where salt and fresh waters mix, or in young, highly porous carbonate rocks.
In closing, we should point out that the principles sketched out above apply mainly to karst caves, and the estimated speed of these processes applies exclusively to such caves formed in carbonate rocks. Karst caves that are formed in sulfate rocks (gypsum and anhydrite) or in rock salt develop much more quickly. Sulfate rocks dissolve at least two orders of magnitude faster than carbonate rocks, and salt dissolves around eight orders of magnitude faster. That means that caves in sulfate or salt rocks are also subject to destruction more quickly than those formed in carbonate rocks. Other types of rapidly-developing caves include those that form in lava flows, which are estimated to develop in less than a month. Of course, they are initially inaccessible to humans given the very high temperature and anaerobic atmosphere prevailing inside. Such caves also, for different reasons, undergo destruction much more quickly, and so their “lifecycle” is shorter than that of karst caves in carbonate rocks.
Further reading:Bosák P. (2008). Karst processes and time. Geologos, 14: 19–36.
Ford D.C., Williams P. W. (2007). Karst Hydrogeology and Geomorphology. Wiley, Chichester (pp. 562).
White W.B. (1988). Geomorphology and Hydrology of Karst Terrains. Oxford University Press, New York (pp. 464).
© Academia 3 (43) 2014