The topics covered in this chapter can be summarized as follows:
|12.1 Karst Landscapes and Systems
|Like everything else in geology, karst is part of the Earth system, and it has some unique attributes, such as the link between the atmosphere and the hydrosphere that contributes to the dissolution of limestone, and the nature of caves as ecosystems. Karst is present on every continent, and almost in every country.
|12.2 Karst Landforms and Features
|Limestone karst regions are characterized by a wide range of surface features on scales ranging from centimetre-sized karren to hills and ravines with vertical relief of hundreds of metres. Karst regions feature sinkholes and swallets where streams disappear underground, but there is also significant diffuse flow from surface to depth through epikarst.
|12.3 Karst Hydrogeology
|Limestone karst has unique hydrogeological features, including discrete and diffuse surface input of water, subsurface flow along passages and through fissures and cracks, and springs that may have continuous or intermittent flow. Water is stored within caves within passages, within sediments, and within fissures and fractures. The path from a sink to a spring can complex, and it can be difficult to determine where the water in a spring is coming from.
|12.4 Cave Features, Cave Contents, and Subterranean Life
|Although limestone caves are the most common, there are many other types, and they can form in several types of bedrock or in surficial materials. The morphology of limestone caves is controlled by bedding and fractures, and limestone caves have a wide range of solution-related features: speleogens and speleothems. All caves also have a variety of cave sediments. Cave ecosystems include organisms that only live in caves (troglobites), those that can also inhabit similar dark and damp environments (troglophiles), those that only spend part of their time underground (trogloxenes) and those that end up in caves by accident.
|12.5 Origin and Genesis of Caves
|Genesis of limestone caves has long been debated, but there is now some consensus that the original enlargement of passages likely takes place in the epiphreatic zone, close to the boundary between the aerated vadose zone and the saturated phreatic zone. The evolution of a cave system is significantly affected by changes in the level of the water table, and those might be related to crustal movements, climate change or sea level changes.
|12.6 Human Interactions with Karst and Caves
|Humans have always used caves as sites of refuge, as places to live or as places to store things, and so caves are hugely significant sites for archaeological studies. More recently, they have become tourist attractions and recreational opportunities. Caves are also important for geological and climate-change research because past conditions can be determined by sampling speleothems and cave sediments.
Answers for the review questions can be found in Appendix 1.
- Describe the role of carbon dioxide in the process of the dissolution of limestone.
- Explain why soil is important to this process.
- Explain how exokarst differs from epikarst.
- What feature of a karst aquifer defines the position of the epiphreatic zone?
- How would you expect the chemical composition of water from a karst aquifer to differ from that from a sandstone aquifer?
- While limestone is the main host rock of karst, there are others. List some of the types of rock, or situations, where caves can exist.
- Explain the difference between a speleogen and a speleothem.
- Describe the typical process for the deposition of calcium carbonate to form speleothems.
- Explain the difference between allogenic (allochthonous) and authigenic (autochthonous) cave sediments?
- What is the term for an organism that might live in a cave, but also could live in a non-cave dark and damp environment?
- According to the prevailing theory of limestone karst genesis, does the original development of cave conduits take place in the vadose zone, near to the boundary between the phreatic and vadose zones, or deeper in the phreatic zone?
- What sequence of events might lead to a multi-level cave system?