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Stunning Limestone caverns | Debating how they were ever formed

Have you ever visited the Son Doong Cave in the forests of Vietnam. It is among the largest and most spectacular limestone caverns in the world, measuring 5.5 miles in length, with some of its largest  caverns high enough to house a 40 storey building. Inside the cave is a pristine jungle 600 feet below the surface with an underground river flowing through it.

Mammoth National Park in Kentucky possesses an area of 80 square miles of a maze of caves. The cave has not yet been fully explored, with new chambers being discovered constantly. The largest cave chamber in the world in terms of total surface area though, is located in the Mulu cave system in the island of Borneo, Indonesia – the Sarawak chamber – with an area of 1.66 million square feet. The deepest cave in the world is the Krubera cave, with a depth of up to 2,708 feet below the Earth (N. Geiling, 2014).

Son doong limestone caverns

Fig: The Son Doong Cave in Vietnam
Source: abstravel.asia

A cavern or a cave may be defined as a naturally formed running subterranean void. Limestone caverns can be simple, accentuating their basic structural features such as joints and bedding planes, or they can be complex and intricate. Limestone caverns can occur in many forms, extending horizontally or vertically, with a single level or many levels, some having streams running through them, etc.

Cavern Structures

Depositional features in limestone caverns can exhibit a wide variety of formations, out of which the most popularly known features are aggregations of calcium carbonate protruding from the ceilings, or from walls and floors.

The many forms of deposits of such carbonate rocks can be described with the inclusive term cave travertine, although the term speleothems is also sometimes used. Limestone caverns often have water laced with calcium carbonate and other minerals dripping from the ceilings of these limestone caverns, which lead to certain formations in limestone caverns for which the pioneering American Geologist W.H. Davis (1930) proposed the term dripstone to describe them collectively.

Those aggregations extending downwards are called stalactites and those growing upwards from surfaces are called stalagmites. These aggregations can also occur in the form of columns and pillars inside limestone caverns. These can occur in a multitude of shapes and sizes. Sometimes, these forms can appear in irregular outgrowths, growing obliquely,  horizontally, with curvatures as well as downward for example. These are called helictites.

Types of limestone caverns

Fig: Examples of (left) Stalactites, and (right) Stalagmites
Sources: (left) Courtney Hedrick, flickr, and (right) Marko Erman, flickr

Among other features, the routes going through limestone caverns are seldom uniform in terms of size throughout the cavern system. Their size can characteristically expand or contract as one passes from one end of the cavern to another.

Limestone caverns are seldom stable in their dimensions, and numerous limestone caverns are evidenced with littered debris of rock infalls. Sometimes large rooms inside limestone caverns might be connected or interconnected by passages or conduits varying in dimensions, their floors composed of clay or silt.

The locations with smooth surfaces in limestone caverns might be evidence of a body of still water or streams present there in previous time periods. Cavern floors can also exhibit mudbanks and bars, and pebbles and rocks may also be present, providing evidence for the flow of water in limestone caverns.

The erosion caused by flowing water in limestone caverns can be evidenced by grooves and flutings (traces and sets of narrow channels and depressions) at the floors, sides and also the ceilings of limestone caverns (W.D. Thornbury, 1990). The action of water in limestone caverns thus makes a significant impact on the formation of structures inside limestone caverns. The role of water in shaping limestone caverns is however, not limited only to structures inside limestone caverns, and is a significant constituent also of the debates that arose concerning the formation of limestone caverns.

The Debates Over Limestone Caverns Formation

Prior to 1930 the prevailing views on the formation of limestone caverns were vague, with non-conclusive theories attributing their formation to action by groundwater. In this period Matson (1909) provided the analytical explanation that indicated that the development of these caverns chiefly occurred above the water table, and the formation of limestone caverns was attributed to the action of surface waters that were diverted to these locations.

Local base levels of surface water were said to be the determining factor in the subterranean development of caverns. Subterranean caverns were said to be formed during the process of downcutting of valleys on the surface whereby cave travertines were said to be formed by the abandonment of a cavern level, sometimes extending multiple times, forming multiple levels. This dominant view was challenged by W.M. Davis in his paper ‘Origin of Limestone Caverns’, published in 1930.

Davis instead proposed his two-cycle theory as the alternative, in which limestone caverns are formed by the action of water below the water table instead of above. This takes place by the action of water in two stages or cycles. In the first cycle, phreatic water forms a solution for the dissolving processes that form a major part of the development of subterranean caverns. Phreatic water is water from the phreatic zone i.e. the area below the water table in an aquifer where all the pores and fractures in the soil are saturated with water.

Davis instead proposed his two-cycle theory as the alternative, in which limestone caverns are formed by the action of water below the water table instead of above. This takes place by the action of water in two stages or cycles. In the first cycle, phreatic water forms a solution for the dissolving processes that form a major part of the development of subterranean caverns. Phreatic water is water from the phreatic zone i.e. the area below the water table in an aquifer where all the pores and fractures in the soil are saturated with water.

Davis proposed that this process in karst topographies is followed by the second cycle in which a lowering of the level of the water table that leads to the draining of these areas, leaving behind vacant spaces that form subterranean caverns. These vacant spaces are often occupied by air and vadose water i.e. water at atmospheric pressure above the water table that is moving downward depending on gravitational forces (Encyclopaedia Britannica, 2017).

The two-cycle theory provided by Davis that challenged the view that surface water alone was responsible for subterranean cavern formation invigorated a debate that till contemporary times leads to two opposing camps in the debates regarding the formation of limestone caverns.

Much of the disagreement can be traced to an ambiguity over whether unlike other topographies, karst topographies contain groundwater as a continuous mass or not. There is a debate over whether the water table in karst topographies forms continuous or discontinuous boundaries with the soil above.

Discrediting or arguing against geomorphological theories provided by Davis is difficult, as it is based on the prevailing patterns of geomorphological thought. The theories of landform development provided by Davis based on cycles of erosion are among the most popular theories of the development of landforms.

Davis with his theory of geographical cycles of erosion (1889) provided a systematic genealogical system of landform classification that undertakes sequential studies of landform development. With his work Davis sought to study landforms in the same manner that biologists study evolution, which represents the dominant trend in geomorphological thought.

Davis supported his theory with numerous examples such as those of limestone caverns in Cuba and Florida and also with the help of various geomorphological theoretical models such as caverns exhibiting non-branching patterns having instead network patterns of passages, blind galleries, a lack of graded longitudinal profiles and numerous other features that streams do not normally provide.

Bretz (1953) supported the two-cycle theory after a detailed study of caverns at the Ozark region and attributed cavern formation to events below the water table, specifically due to waters circulating beneath the land surface that are under the requisite hydrostatic head (or levels of water required for it to seep through). The limestone caverns Bretz studied were the result of old topographies where the water table had receded further down after the formation of the caverns. The ubiquitous presence of clay deposits in limestone caverns studied by Bretz led him to infer that these were part of the silt brought in by the action of water that had since disappeared below the surface and very little usually remained as vadose water.

Swinnerton (1929, 1932) added a new dimension by attributing cavern formation to water flowing laterally at water levels equal to the water table, called the water table theory. Malott (1937) introduced the invasion theory after his study of limestone caverns in Kentucky and Indiana (W.D. Thornbury, 1990). Malott’s theory, giving the examples of these caverns, held that they were formed due to the action of subterranean streams and appear intermittently throughout the karst topographies in the regions he studied.

limestone caverns Development

Fig: Some theories concerning the formation of Limestone Caverns

Source: http://yunus.hacettepe.edu.tr

The contemporary theoretical model attempts to find a general underlying framework common to all basins located in karst topographies. White (1988) delineated some variable factors and grouped karst processes into chemical, physical and hydrogeological processes. The chemical processes involve the result of carbonate dissolution, whose reactions are greatly influenced by climate and carbon di oxide from flora.

The physical processes involve the maintenance of a hydrostatic head of water, as described in Bretz’s analysis, and gravitational forces that work to dissolve the carbonate rocks. The hydrogeological processes can encompass a variety of settings and can include the structural environment determining the characteristic features of regional geomorphology, the thickness of the strata of soluble rock, and the statigraphic (nature of soil stratification) and lithologic (or physical) properties such as porosity, permeability, etc of the region.

These variables were brought together in a new model called the Ford-Ewers Model (1968-1982) that seeks to offer a comprehensive geomorphological explanation for the formation of limestone caverns. Bringing together earlier studies, this is a genealogical model in which the most potent triggers for cavern formation are the frequency of fractures (fissures) in soluble rocks and the potentiality for chemically aggressive waters to attack these fissures (C. Werner, 2017). The model thus somewhat validates the two-cycle theory of Davis, although it cannot be considered the only concomitant model, and a proper geological study of limestone caverns individually might be able to provide the optimum  site-specific answers.

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