Karst topography

Karst topography is a landform produced by the dissolving action of surface water and groundwater on the underlying bedrock of a region. The landforms produced are unique because they represent internal or underground drainage, forming such features as sinking streams, sinkholes, caves, natural bridges, and springs.

Dissolving Bedrock

Karst topography is a unique landscape produced by the dissolving of the bedrock of a region, with the consequent development of underground drainage. Most common bedrock materials, such as granite, sandstone, and shale, are resistant to dissolving (also known as dissolution), and landscapes are carved into these bedrock materials by the mechanical action of water, wind, and ice. Some bedrock materials, such as limestone, dolomite, gypsum, and rock salt, dissolve relatively easily. Gypsum and rock salt are soluble in plain water and, in most landscapes, are chemically destroyed very quickly. Karst landscapes on these two rock types persist only in dryer climates, such as the American Southwest. Gypsum and rock salt are also mechanically weak rocks and are destroyed rapidly by mechanical erosive activities. Limestone and its close cousin dolomite are mechanically strong rocks; therefore, they resist normal erosive activity. Yet calcite, the mineral of which limestone is composed, is especially vulnerable to dissolving by water that is slightly acidic.

Under normal conditions, the source of such acidity is carbonic acid, a natural combination of carbon dioxide and water. Rainwater absorbs carbon dioxide from the atmosphere and is naturally slightly acidic (modern pollution has accentuated this natural tendency to produce acid rain). In the soil, organic activity can increase the amount of carbon dioxide to levels well above those found in the atmosphere, and water moving through such soils can become very acidic. When rainwater or soil water charged with carbonic acid meets limestone bedrock, it will slowly dissolve the limestone. Limestone is mechanically resistant and supports the development of karst topography well. Dolomite reacts more slowly to acid waters, and generally, the karst features found on dolomites are subdued and take longer to form than those found on limestone. Discussion of karst topography is, therefore, a discussion of the dissolving of limestone in most situations.

The rate at which limestone dissolves depends on the amount of water in the environment and the amount of carbon dioxide available. Atmospheric carbon dioxide levels are similar worldwide, but the amount of carbon dioxide in the soil varies greatly. Organic activity controls the amount of carbon dioxide available. For that reason, the most acidic groundwater tends to be found in the warm, wet zones of the tropics, where organic activity is high. Dry, cold climates have erosion rates of as little as a few millimeters per one thousand years, whereas warm, wet climates can have rates up to 150 millimeters per one thousand years. These erosion rates are averages of the rate of surface lowering for a region. Throughout hundreds of thousands of years, significant landforms can be developed.

The dissolving action of carbon dioxide-charged water produces several etching patterns on exposed limestone bedrock, from microscopic features to large trough structures more than 1 meter deep and many meters long. Karst topography develops when the water penetrates down pores, cracks, and openings into the limestone. Once inside the rock, the water is capable, over long periods, of dissolving voids and passageways in the rock. These openings in the rock become integrated into underground flow networks similar to stream patterns on the Earth's surface. The result is a series of underground passageways called caves, which collect water from sinkholes and sinking streams on the Earth's surface and transmit the water back to the Earth's surface at springs. While the flow pattern of caves is often similar to the pattern of surface streams, caves are tubes in bedrock and may migrate up and down, as well as from side to side, in a manner that surface streams cannot. As the cave system grows and matures, it can capture larger volumes of surface water and enlarge. Eventually, it will not only carry material in solution but also mechanically transport sediment underground. On the land surface, this underground or internal drainage produces sinking streams and sinkholes. In mature systems, the sinkholes may be kilometers across and hundreds of meters deep, containing many sinking streams.

Depending on the nature of the limestone, the climate, and the presence of mountains, a wide variety of karst landscapes may appear. The landscape may be as simple as rolling hills or a flat plain with mostly normal surface drainage and only a few scattered sinkholes, sinking streams, and caves. However, other landscapes may have no significant surface drainage and sinkholes covering the land surface, producing a sinkhole plain. One of the best examples of a sinkhole plain is in the Mammoth Cave area of Kentucky. Under extreme conditions, usually associated with the tropics, the landscape is so altered by dissolution that the sinkholes deepen faster than they widen, producing a landscape of tall limestone towers and pinnacles standing above a flat plain. This landform is called tower karst and is best known from southern China, where thousands of towers several hundred meters high dot the landscape.

The downward erosion by groundwater can produce cave systems at lower elevations, causing earlier, higher cave systems to become abandoned. These abandoned caves may persist for hundreds of thousands of years, developing complex mineral displays of stalactites, stalagmites, and other deposits. Animals and people may use the cave for shelter, leaving important fossils and cultural material behind. The continuing land surface erosion will eventually breach into underlying cave systems to produce new entrances and truncated cave fragments called natural bridges. Sinkholes become natural traps, collecting unwary animals, plant remains, soil, and pollen. Transporting material into the subsurface allows the material to be preserved from destruction in the earth's surface environment.

Like all landscapes, karst topography can go through a series of developmental stages. Karst development begins when the soluble rock is first exposed at the Earth's surface. In some cases, groundwater reaches the soluble rock before it is exposed, and cave development can begin before the rock is on the surface. In either case, the progressive development of a karst landscape by chemical and mechanical erosion will remove the layer of soluble rock responsible for the karst processes. In time, karst processes become less critical in the landscape as the soluble bedrock disappears, and the landscape will revert to a more typical, mechanically produced form.

In the United States, limestones form important areas of karst topography in the Virginias, Pennsylvania, Indiana, Kentucky, Tennessee, Alabama, Florida, Missouri, Arkansas, and Texas. Many other states, such as New York, Minnesota, Iowa, Colorado, and New Mexico, have minor amounts of limestone karst. Some states have major cave systems in minor karst areas, such as Carlsbad Caverns in New Mexico and the Jewel and Wind Caves in South Dakota. Gypsum karst is locally vital in Kansas, Oklahoma, and New Mexico, while dolomite karst is found in Missouri. Marble—limestone altered by heat and pressure—forms small karst areas in California, New England, and Oregon. Rock salt karst is rare in the United States but can be found in Spain and the Middle East. Almost every state and every country have some form of karst development.

Other landscapes can mimic the unique features of karst topography in special cases. Such cases are called pseudokarst because they imitate karst topography but are produced by other phenomena. For example, volcanic areas produce eruptive craters and lava tubes that look like sinkholes and caves from karst areas. In deserts, streams can dry up and appear to be sinking, and winds can produce excavated hollows in the sand. Glaciers can produce depressions in the ground that look like sinkholes. In fine-grained clay deposits—seen in the Badlands of South Dakota—caves, bridges, and sinkholes are produced by mechanical flushing of the clay particles, a process called suffusion. Wave activity on rocky coastlines can carve sea caves in various rock materials.

Karst topography is unique because of the chemical way the bedrock is destroyed and the internal or underground drainage that results. Most karst is developed in limestone because of this rock's abundance, slow chemical dissolution rates, and great mechanical strength. In certain areas of the world, karst topography is the dominant landscape for thousands of square kilometers. Cave systems over 500 kilometers of surveyed passage exist, and caves have been followed as deep as 1,700 meters beneath the surface.

Research into karst topography continues in the twenty-first century as scientists continue to make discoveries about the Earth’s past, informing them about its future potential. In 2023, stalagmites from the Cave of the Mounds in Wisconsin revealed important findings about a warmer period at the end of the most recent ice age that may have altered the Greenland ice sheet. One specific area of new research is karst aquifers and their ability to resupply groundwater and resist pollution. The US Geological Survey (USGS) continues to study karst aquifer systems across the United States, including the Arbuckle-Simpson Aquifer and Edwards Balcones Fault Zone Aquifer, to understand karst aquifers and the threats they face from global climate change. The USGS Karst Interest Group (KIG) holds annual workshops to link scientists studying karst areas to discuss their vast research projects.

Principal Terms

carbonic acid: a weak acid formed by mixing water and carbon dioxide; it is important in the dissolving of the most common karst rock, limestone

cave: an opening or hole in the ground enterable by people; in karst topography, caves have been dissolved out and act (or once acted) as underground conduit flow routes for water

limestone: a common sedimentary rock containing the mineral calcite; the calcite originated from fossil shells of marine plants and animals

natural bridge: a bridge over an abandoned or active watercourse; in karst topography, it may be a short cave or a remnant of an old, long cave

sinkhole: a hole or depression in the landscape produced by dissolving bedrock; sinkholes can range in size from a few meters across and deep to kilometers wide and hundreds of meters deep

sinking stream: a stream or river that loses part or all of its water to pathways dissolved underground in the bedrock

spring: a place where groundwater reappears on the Earth's surface; in karst topography, a spring represents the discharge point of a cave

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