2 Karst Conduit Flow Develops by Geochemical and Flow Processes
Karst is formed in soluble rocks especially limestone and to a lesser extent in dolomite but can develop in gypsum or salt. Karst is the most dramatic evidence of the ability of groundwater to alter the form of the earth’s surface and subsurface as illustrated on the cover of this book and in Figure 1. More than one episode of karst development can occur as geological conditions change, which adds complexity to the karst system. Formation of substantial-sized openings in rocks requires chemical dissolution operating over geologic time scales. For example, Mammoth cave in the state of Kentucky in the USA includes multiple bedrock formations and several levels of conduits that extend over an estimated 200 km2 (Figure 3). It is believed that groundwater began interacting with the uppermost cave-forming limestone formation—about 10 million years ago and that the upper levels of the cave system were fully formed by 3.2 million years ago, based on radiometric dating of quartz pebbles (Quinlin and Ewers, 1989; USGS website, 2021). Mammoth cave is the longest known cave system in the world with mapped passages extending more than 600 km. Personnel of Mammoth Cave National Park estimate there may be as much as 1000 km of unmapped caves (Toomey et al., 2017).
Figure 3 – Systems of cave networks that constitute karst terrane can be extensive and can require decades of mapping as evidenced from this map of the known passages in the Mammoth Cave system and the adjacent caves from those known in 1908 and those explored as of 2016. Modified from Toomey and others (2017).
Karst openings result from the combination of water flow and mineral dissolution that removes rock mass to create and/or enlarge openings. However, much of the beauty observed in karst results from the creation of mineral mass by chemical precipitation that forms striking cave structures (Figure 4). Most karst aquifers are formed as a consequence of the infiltration of precipitation, subsurface diversion of stormwater runoff and surface stream flow, and the subsequent movement of groundwater. These are epigene karst aquifers and are active components of the meteoric water system and the critical zone, and are the topic of this book. The critical zone is Earth’s permeable near-surface boundary layer where rock, soil, water, air, and living organisms interact. Another type of karst, hypogene karst, develops by deep-sourced corrosive waters or gases upwelling through fault zones and fractures and/or following the geologic dip of permeable and soluble bedrock strata. Hypogene karst is generally located in, or near, regions of tectonic, volcanic, or high-temperature geothermal activity past or present, such as Carlsbad Caverns, New Mexico, USA. Depending on the depth and hydrogeological isolation, geological strata altered by hypogene karst may or may not interact with shallow meteoric waters to store and transmit fresh groundwater. Thus, they may or may not be identified as aquifers, and may or may not play a role in the occurrence of near-surface karst geomorphic features.
Figure 4 – Underground cave photograph showing common structures formed by precipitation of minerals from groundwater (Dave Bunnell, 2006. “Photo by Dave Bunnell showing the most common speleothems” by Dave Bunnell is licensed under CC BY-SA 2.5).