12.2 Surface and subsurface karst landforms

Karst landscapes form through the dissolution of soluble rocks, primarily limestone and dolomite, by slightly acidic groundwater. The result is a distinctive set of surface and subsurface features: sinkholes, caves, underground rivers, and sculpted rock surfaces that look unlike anything found in non-karst terrain.

These landscapes matter beyond geology. Karst aquifers supply drinking water to roughly 25% of the world's population, yet they're among the most contamination-prone groundwater systems on Earth. Understanding how surface and subsurface karst processes connect is essential for managing water resources, predicting land-surface hazards, and protecting fragile karst ecosystems.

Surface Karst Landforms

Sinkholes and Depressions

Sinkholes are closed depressions that form when surface material collapses into underground voids or when bedrock slowly dissolves away. They're the most recognizable karst feature, and they come in three main types:

• Solution sinkholes develop from slow, steady dissolution of exposed bedrock at the surface. These tend to be bowl-shaped and form over long timescales.
• Collapse sinkholes form suddenly when the roof of an underground cavern gives way. These can appear without warning and pose serious hazards to infrastructure.
• Subsidence sinkholes occur gradually as unconsolidated sediment (soil, sand, clay) settles or is washed downward into voids in the underlying bedrock. They're common where a thick soil mantle overlies cavernous limestone.

Poljes are much larger features: broad, flat-floored depressions with steep bounding walls and internal drainage (meaning water doesn't flow out via a surface stream). They can stretch for kilometers. Three subtypes exist based on their geologic setting:

• Border poljes form along the contact between soluble karst rock and insoluble non-karst rock
• Structural poljes develop along faults or between tectonic ridges
• Baselevel poljes sit near the regional water table in lowland areas and frequently flood

Karst windows are spots where an underground stream or cave passage is briefly exposed at the surface, giving you a direct view into the subsurface drainage system. Mammoth Cave in Kentucky has well-known examples.

Exposed Rock Features

Karren refers to small-scale dissolution sculptures on exposed limestone surfaces, ranging from millimeter-wide grooves to meter-scale channels. Different types reflect different formation conditions:

• Rillenkarren: parallel, straight grooves on steep rock faces, carved by thin sheets of rainwater flowing downslope
• Rinnenkarren: larger, often meandering channels that collect more water and cut deeper than rillenkarren
• Kamenitzas: shallow, pan-shaped depressions on flat or gently sloping surfaces that hold standing water, which accelerates further dissolution

At a much larger scale, karst dissolution can produce dramatic hill-and-depression topography:

• Tower karst (fenglin) consists of isolated, steep-sided limestone towers rising abruptly from flat alluvial plains. The iconic landscape around Guilin, China, is the classic example. Towers form in advanced stages of karst evolution where most of the surrounding rock has been dissolved or eroded away.
• Cone karst (fengcong) features closely spaced conical hills with rounded summits and star-shaped depressions between them. The Chocolate Hills of the Philippines illustrate this morphology. Cone karst represents an earlier evolutionary stage than tower karst, where hills still share a connected limestone base.

Subsurface Karst Features

Cave Formation and Types

A cave is a natural underground void large enough for a person to enter. In karst, caves form primarily through rock dissolution by chemically aggressive groundwater. The process of cave formation, called speleogenesis, depends on the interaction between rock chemistry (mineralogy, solubility), hydrology (water flow paths, pressure), and geological structure (bedding planes, joints, faults).

Two fundamental cave types correspond to their position relative to the water table:

• Phreatic caves develop below the water table, fully submerged in the saturated zone. Water moves slowly under hydrostatic pressure, dissolving rock in all directions. This produces rounded, tubular cross-sections and complex, anastomosing (braided) passage networks. Carlsbad Caverns in New Mexico formed largely under phreatic conditions.
• Vadose caves develop above the water table, in the unsaturated zone where air fills the upper part of passages. Water flows downward under gravity, cutting narrow, canyon-like passages and vertical shafts. Mammoth Cave in Kentucky has extensive vadose-zone passages.

Many real cave systems contain both phreatic and vadose elements, reflecting changes in water table position over geologic time.

Cave Features and Hydrology

Caverns are large underground chambers, sometimes interconnected by narrower passages. Conduits are the underground channels that carry water through karst systems, ranging from tight fractures only millimeters wide to passages large enough to kayak through.

Speleothems are secondary mineral deposits that form inside caves after the cave void already exists. They develop when water carrying dissolved calcium carbonate enters a cave, loses $CO_2$ to the cave atmosphere (degassing), and precipitates calcite. The key reaction is the reverse of the dissolution equation:

$Ca^{2+} + 2HCO_3^- \rightarrow CaCO_3 + H_2O + CO_2$

Common speleothem types include:

• Stalactites: grow downward from cave ceilings as water drips and deposits calcite ring by ring
• Stalagmites: build upward from cave floors where drip water lands and spreads
• Flowstones: sheet-like calcite deposits on walls and floors formed by thin films of flowing water
• Helictites: twisted, seemingly gravity-defying formations that grow in multiple directions, likely driven by capillary forces and crystal growth pressures rather than dripping water

Groundwater's Role in Karst

Dissolution Processes

Groundwater is the primary agent shaping karst. The process starts when rainwater absorbs $CO_2$ from the atmosphere and, more significantly, from soil (where biological respiration produces $CO_2$ concentrations 10 to 100 times higher than in the atmosphere). This creates a weak carbonic acid solution that attacks carbonate minerals.

The fundamental dissolution reaction for limestone is:

$CaCO_3 + H_2O + CO_2 \rightleftharpoons Ca^{2+} + 2HCO_3^-$

This reaction is reversible, which is why speleothems can form when conditions shift to favor precipitation.

Several factors control the rate of dissolution:

• Temperature: colder water holds more dissolved $CO_2$, increasing its aggressiveness, but warmer temperatures speed up reaction kinetics. The net effect depends on the specific setting.
• Pressure: higher $CO_2$ partial pressure (as found in soil zones) keeps more carbonic acid in solution.
• Other acids: in some systems, sulfuric acid ($H_2SO_4$) from oxidation of sulfide minerals drives dissolution. Carlsbad Caverns and Lechuguilla Cave formed partly through this mechanism.

The epikarst is the highly weathered, fractured zone at the very top of the bedrock surface. It acts as a sponge, temporarily storing infiltrating water and funneling it into deeper fractures and conduits. The epikarst is a critical zone for both recharge and contaminant transport.

Karst Hydrology

Groundwater flow in karst aquifers is fundamentally different from flow in most other aquifer types. Instead of moving uniformly through pore spaces, water travels along two very different pathways simultaneously:

• Diffuse flow through the rock matrix and small fractures (slow, like a typical aquifer)
• Conduit flow through enlarged fractures, solution channels, and cave passages (fast, sometimes reaching velocities of kilometers per day)

This dual-flow nature means karst aquifers behave unpredictably. Spring discharge can spike rapidly after storms, and contaminants can travel long distances with minimal filtration.

Water table fluctuations over time create multi-level cave systems, where older, higher passages were abandoned as the water table dropped (often in response to river incision or tectonic uplift), and new passages formed at lower levels.

Karst springs show distinctive behavior:

• Discharge can be highly variable, surging after rainfall
• Estavelles are features that function as springs during high water and as sinks (swallowing surface water) during low water, reversing flow direction seasonally

Because water moves so quickly through conduits with little natural filtration, karst aquifers are extremely vulnerable to contamination. A pollutant entering a sinkhole can reach a drinking-water spring kilometers away within hours or days.

Surface vs Subsurface Karst Impacts

Landscape Evolution

Surface and subsurface karst features don't develop independently. They form interconnected three-dimensional drainage networks where what happens at the surface directly affects the subsurface, and vice versa. For example, a sinkhole captures surface runoff and funnels it underground, accelerating conduit enlargement below; that enlarged conduit then draws down the water table, promoting further sinkhole development above.

This feedback loop means karst landscapes evolve as coupled systems. Over time, climate, tectonics, and base level changes (the elevation of the lowest drainage outlet) steer the style of karst that develops:

• Tropical karst tends toward tower and cone karst morphologies, driven by intense rainfall and high biological $CO_2$ production in soils (Southeast Asia, Caribbean)
• Temperate karst more commonly develops extensive horizontal cave systems with well-defined sinkhole plains (Appalachian Mountains, the Dinaric Alps)

Hydrological and Environmental Impacts

Karst systems reshape surface water hydrology in ways that can seem counterintuitive:

• Sinking streams disappear entirely underground at discrete points, called swallow holes (Lost River, Indiana, vanishes and re-emerges 8 km away)
• Losing rivers gradually diminish in flow as water seeps into the streambed along their course
• Underground piracy occurs when subsurface conduits capture flow that previously belonged to a surface drainage basin, rerouting water across topographic divides

Subsurface void collapse continually creates new surface features like sinkholes and collapse dolines, meaning the karst landscape is always actively reshaping itself.

The high spatial variability of karst terrain presents real challenges:

• Land use planning must account for sinkhole hazards, uneven bedrock depth, and unpredictable subsurface drainage
• Water resource management requires specialized techniques like dye tracing to map underground flow paths
• Groundwater protection demands strict controls on surface activities near sinkholes and recharge zones, since the natural filtration capacity of karst is minimal