What are 4 of the features you will find in karst topography?

Professor Cam Nelson is on site at the Mangapohue Stream in the King Country. Cam explains how the karst landscape in this region has developed from the slow dissolution of limestone rock as a result of exposure to slightly acidic rainwater. The end result over millions of years is a karst landscape with features such as sinkholes, caves, natural bridges and fluting.

Transcript

PROFESSOR CAM NELSON
A karst landscape forms from dissolving rock, in this case, dissolving limestones. Most landscapes result from the rain, the Sun beating down on them and physically destroying them, but a karst landscape forms from the dissolution, the dissolving of rock, the dissolving of limestone in particular. And here we have an example of karst landscape that’s formed in an old cave system, we’re only about 20 kilometres to the west of the Waitomo Caves, and here is a much smaller ancestral cave system which is formed in the Mangapohue Stream, which is flowing below us here.

The cave system behind us has collapsed in its roof and so it’s now open to the skyline, but right here at the Mangapohue natural bridge, the roof of that cave system still survives, and these are called natural bridges in limestone terrains.

Rainwater is slightly acid. It actually picks up CO2 from the atmosphere, and that makes a mild carbonic acid out of the rainwater. And then when the rainwater actually enters into the soil zone, it can also pick up additional acidity from the organic acids associated with the decay of vegetation and the respiration of plants. So the water that’s flowing through the limestone can be quite acidic, and that acid will dissolve the limestone, breaking the calcium carbonate that limestone’s made up of into its various components. In particular, CO2carbon dioxide is released in that process as the water passes through the limestone. So the acidity of rainwater is certainly the active control on the production of karst features.

There’s a whole raft of terminology associated with karst topographies ranging from the large-scale cave systems to features such as natural bridges, sinkholes, fluting or lapiez weathering

And then the things that go with a karst topography that precipitate calcium carbonate. Again, once it’s dissolved, it can build up in its concentration in the water to such an extent that you can actually start to precipitate calcium carbonate again in things like stalactites and stalagmites – stalactites hanging from the ceiling, stalagmites growing from the ground. There are drip stones where dripping water rich in calcium carbonate starts to reprecipitate the dissolved calcium carbonate. There are flow stones that run down the walls and produce travertine deposits. In fact, stalactites and stalagmites, basically the calcium carbonate that’s precipitated, we wouldn’t really call it a limestone, we call it travertine – it’s still a calcium carbonate-rich rock. So there’s a myriad of these different sorts of micro-karstic features.

The characteristics of karst landscapes vary depending on factors such as: soluble bedrock type, climatic environment (more specifically precipitation and temperature), geographic position (both globally and locally), overlying soil materials, and vegetation cover (Figure 12.2.1). For example, in BC a variety of karst landscapes over a range of different climatic, geologic, and geomorphic settings such as: shoreline karst along the coast, forested karst at low and mid elevations, covered karst in the interior of BC, and alpine karst at high elevations on the coast and the Rockies. Many of these karst landscapes have their own inherent ecosystems. There are also different karst landscapes in other climatic regions, such as in tropics where there is tower karst in China, cockpit karst in Jamaica and cone karst of Cuba. Other types of karst landscapes also occur in the arctic and desert environments. The characteristics of karst landscapes may not always represent present day conditions and could have developed under different climate, geomorphic, soil cover, and vegetation conditions—processes not in evidence today.

Most small-scale karst features of a karst landscape (mm to cm in size) are associated with linear channels, furrows or grooves that form on soluble rock outcrops or rock faces, particularly limestone. These features are collectively called karren (a German term) and this term is used to describe the complex array of solutional forms and patterns generally found on limestone surfaces. There is a vast array of karren types that have been classified, based primarily on their morphological characteristics and sizes. Some examples include: rillenkarren (shallow channels with sharp ridges 2-3 cm apart), rundkarren (rounded channels separated by rounded ridges), rinnenkarren (flat bottom grooves a few cm’s deep), and spitzkarren (large groves extending down steep spires) (Figure 12.2.3).

Identifying and classifying the larger-scale surface karst features is just as confusing as the smaller-scale surface karst features. In most cases karst feature classification is based on morphological characteristics (shape and dimensions) rather than their genetic origin. However, in some cases the function (e.g., input/output of water and air) of karst features is also used as part of the classification. Examples of some of the most common surface karst features encountered are as follows:

• Sinkhole – a topographically closed depression that is circular or elliptical in shape and with steep to vertical sidewalls. (Also called a ‘doline’ in European texts),
• Swallet – a point where a stream of any size sinks underground, forming (in some cases) a cave entrance (Figure 12.2.4),
• Dry valley – a linear valley that did (or occasionally does) contain a stream,
• Karst canyon – a steep sided canyon in karst with distinctive surface erosional features (e.g., scalloping),
• Karst spring – a site where an underground stream emerges from the karst bedrock and is sometimes the site for a cave entrance,
• Polje – a large flat bottomed karst depression with water flowing at the bottom,
• Grike – a linear, narrow, and deep slot formed by dissolution along a pre-existing fracture in bedrock, and
• Solution tube – a circular or elliptical, steeply inclined tube formed by dissolution.

In many cases several karst features can occur at the same site and can be nested within each other. These are termed compound karst features. For example, a sinkhole that acts as a swallet (with water that sinks into an opening), and this opening is a cave entrance (Figure 12.2.5). Trying to define what this feature is or to classify it into a scheme is problematic. The easiest way to deal with these features is to describe them by going from the outermost enclosing feature towards the centre. In the example above we have a sinkhole in which there is a swallet (or sink point), this sink point is in turn large enough to enter and is therefore a cave entrance! Other examples of nested features could include:

• Cave entrances along the base of a karst canyon.
• Springs and sink points along the sides of polje (large flat-bottom depressions).
• Swallets and springs along the base of a dry valley.

Another useful way to classify karst features in a landscape is by using the terms exokarst, epikarst and endokarst. These three terms are common in the karst literature and are of importance to the karst system as they help explain its three-dimensional nature (Figure 12.2.6). Exokarst is used to describe all features found on the surface of the karst landscape that range from the small-scale to large-scale (e.g., karren to sinkholes to poljes). Endokarst is used to describe all components of underground karst including the smallest cavities, cave formations, erosional features, and large cave passages. Epikarst is a zone of solutionally-enlarged openings or fractures that extends for up to 10-30 m below the surface and connects the exokarst to the endokarst. Epikarst is the zone where water, air, and other materials (sediment, organic debris, and nutrients) can be transferred from the surface to the subsurface. The epikarst zone is not always obvious but is usually present in some form or another. The thickness and level of the epikarst zone depends on factors such as: climate, precipitation rates, bedrock properties, time since last glaciation, elevation and relief, groundwater circulation, vegetation type.

Epikarst is the critical linkage between the surface and subsurface karst and has some important implications for karst hydrology and the management of karst landscapes (Figure 12.2.7). In terms of hydrology, it is the zone of karst that is responsible for collecting surface water by diffuse infiltration – whereby water percolates vertically through any openings in the bedrock and gradually enlarges them by solutional processes. These openings are usually larger near the surface and gradually diminish or close off at depth in the epikarst as the water typically loses its solutional power or aggressiveness as it percolates through the bedrock. This closing off effect can make the epikarst zone a site of temporary water storage, before directing or leaking water flow to the subsurface. The epikarst zone is also a habitat site for karst biota as it contains many of the karst biospaces that exist in the three-dimensional karst landscape. In many cases, particularly in glaciated areas such as in BC, the epikarst zone can be partially, or completely, filled with sediment. This material has either been injected during glacial events or during subsequent weathering. In some cases, this might reduce the rate of water percolation, unless compaction cracks or other opening forms in the sediment. It is also likely that this sediment (especially if disturbed) will gradually moves through the epikarst into the subsurface, in effect the karst is almost analogous to a landscape vacuum cleaner!

The concept of karst ‘openness’ or the connectivity within a karst system is a key factor controlling the degree and rate of changes that can occur between various components of the system. This openness is particularly important for any human development activities on surface of karst, whereby the epikarst zone can rapidly transport the materials into the underlying karst conduits and other subsurface cavities such as: water, nutrients, soil, organic debris, and pollutants.

Karst sinkholes are naturally enclosed funnel-shaped depressions that are the prime diagnostic features of a karst landscape (Figure 12.2.8). These features can range in size from a few meters in diameter up to a kilometer or so in size. In European literature karst sinkholes are typically called ‘dolines’. This term is used to distinguish karst sinkholes from those that can develop by other natural and man-made processes (e.g., collapse of abandoned underground mine workings). In this text the North American approach is used, and the term karst sinkhole is used for any funnel-shaped karst depression. Karst sinkholes typically have steep to subvertical sidewalls that can be a few metres to 100’s of meters deep. In cases where the karst surface is covered by surficial materials, sinkholes will still form, creating steep, and in some cases, unstable sidewalls.

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