Caves and karst landscapes

In some places, the ground has hidden holes and tunnels.

Rivers can vanish into the ground. Water can pop out again as a spring. Under the ground there may be a cave.

In some places, streams suddenly disappear into the ground, round hollows dot the fields, and water bubbles out of springs at the bottom of hills. Underneath, there may be caves. This kind of landscape is called karst.

The U.S. National Park Service (NPS) explains that karst forms where water has dissolved the rock under the ground. It happens in rocks that dissolve, mostly limestone, as well as marble and gypsum. Rain soaks into cracks, travels underground, sometimes for long distances, and comes back out at springs, many of which are cave entrances.

Karst is a landscape in which dissolution of the bedrock has produced sinkholes, sinking streams, caves, springs and related features (NPS). It develops on soluble rocks such as limestone, marble and gypsum; the British Geological Survey (BGS) adds chalk and salt. In a typical karst, much of the water falling on the surface enters the subsurface through cracks and dissolved openings, flows underground, sometimes for long distances, and discharges from springs, many of them cave entrances.

Karst matters well beyond caves. The NPS notes that about 20 percent of the United States is underlain by karst and that 40 percent of the groundwater used for drinking there comes from karst aquifers. Karst landscapes appear in parks from Mammoth Cave and Carlsbad Caverns to the Everglades and Grand Canyon.

Rainwater soaks into the soil.

It picks up a gas that makes it a tiny bit sour, like fizzy water. This water slowly dissolves the rock. Cracks get wider and wider. At last, they become a cave.

Solution caves are the most common kind of cave. The NPS explains how they start. Rain and melting snow soak into the soil, where they pick up carbon dioxide gas. This makes a very weak acid called carbonic acid, the same acid that is in fizzy drinks.

The weak acid trickles down into cracks in the limestone and slowly dissolves the rock. As a crack gets wider, more water can flow through it, so it grows faster. Underground streams carve passages and carry sand that scrapes the rock. Even at their fastest, cave passages grow only about 1 millimeter a year, so many caves are hundreds of thousands or even millions of years old.

Solution caves form by dissolution along joints, faults and bedding planes, through both chemical corrosion and physical erosion (NPS). The usual agent is carbonic acid: carbon dioxide from the atmosphere and especially the soil dissolves in infiltrating rain and snowmelt. Though weak, this acid dissolves limestone over long periods. Dolomite behaves similarly; marble, metamorphosed limestone, dissolves the same way; gypsum and halite dissolve far more easily and so survive mainly in dry climates.

Growth accelerates once a crack is wide enough for water to flow rather than cling to its walls. Active streams then enlarge passages, and their sediment abrades and scours the rock. The NPS gives a maximum growth rate of about 1 mm per year for passages with active streams, about a meter in a thousand years, and notes that many caves are hundreds of thousands or millions of years old. The stream eventually returns to the surface as a spring, where many caves end.

A sinkhole is a round dip in the ground.

Rainwater drains down through it into the rock below. Some streams flow into a hole and disappear underground!

A sinkhole is a hollow in the ground made when the rock underneath dissolves or a cave roof below gives way. The NPS says sinkholes can be anything from a few feet to a few miles across. Rain that falls into a sinkhole drains out through the bottom into caves below.

Sometimes a whole stream flows into a hole and vanishes underground. This is called a sinking stream, or swallow hole. The water keeps flowing through the caves and comes back out somewhere else as a spring. Most sinkholes form slowly, but BGS notes that leaking pipes, heavy rain or building work can trigger one to open suddenly.

Sinkholes are depressions formed when the land surface sinks because of bedrock dissolution or cave collapse, from a few feet to a few miles across (NPS). BGS classifies them by process: solution sinkholes, where local chemical weathering enlarges a hollow around a fissure; collapse sinkholes, where a cave passage gradually fails upward, sometimes through rock that is not itself soluble; and suffosion sinkholes, where soil subsides into a dissolved depression in the bedrock, called shake holes in Yorkshire, from about 1 to 15 m across on Malham Moor.

Although sinkholes are natural, BGS lists human triggers too: leaking drains and water mains, altered drainage, building loads, changes in the water table from drought or pumping, and mining. Streams that enter a sinkhole and disappear are swallow holes, or sinking streams; where many sinkholes feed a cave, passages can join like the branches of a tree.

Water drips from the cave roof.

Each drip leaves a tiny bit of rock behind. Slowly, stone icicles grow down. Stone towers grow up from the floor. When they meet, they make a column.

Once a cave fills with air instead of water, it can start to grow decorations. Cave scientists call them speleothems. They form from water dripping through the ceiling, carrying dissolved rock.

Stalactites hang from the ceiling like icicles. Each one starts as a thin, hollow tube called a soda straw: every drop leaves a tiny ring of mineral behind, and drop after drop builds the tube longer. Stalagmites grow up from the floor where the drops land. Because drops splash, stalagmites are wider and rounder. When a stalactite and stalagmite meet, they make a column. The NPS says columns can be over 20 meters (65 feet) tall.

Speleothems are mineral deposits defined by their form; calcite is the most common cave mineral, and Cave Minerals of the World (Hill, 1997) describes 38 types (NPS). They form only once a cave lies above the water table, in the air-filled zone. Stalactites begin as hollow soda straws: as a drop loses carbon dioxide, a thin carbonate film forms on it; when the drop falls, a ring is left on the ceiling, and successive drops extend it into a tube that may later thicken into a massive pendant tens of feet long.

Stalagmites build up beneath drips; splashing spreads them, so they lack central tubes and are usually wider, with rounded tops. A stalagmite that joins its feeder stalactite forms a column, sometimes over 20 m tall and wide, typically aligned along ceiling joints where most water enters. Flowstone, deposited in layers from flowing water, is one of the most common speleothems; helictites twist in any direction, fed through tiny central channels.

Some caves are very, very long.

Mammoth Cave in Kentucky is the longest cave on Earth. Its tunnels go on and on, more than 400 miles.

The NPS calls Mammoth Cave in Kentucky the longest cave on Earth, with more than 400 miles (about 640 kilometers) of passages. It lies under four counties on several levels, one above another, and was carved by dozens of rivers and streams.

Some caves are steep, with deep vertical pits and underground waterfalls. Others are flat and completely full of water, and can only be explored by trained cave divers. The NPS says the second-longest cave in the world, in Quintana Roo, Mexico, is almost entirely filled with water and has been mapped by divers over many decades.

Cave form depends heavily on gradient, the vertical drop from where water enters to the spring where it leaves (NPS). Steep caves are chains of vertical pits and waterfalls, explored with ropes; low-gradient caves near river valleys and coasts are often entirely flooded and explored only by cave diving. Some form mazes of junctions and parallel passages.

Mammoth Cave in Kentucky, which the NPS calls the longest cave on Earth at more than 400 miles, lies beneath four counties on multiple levels and was carved by dozens of rivers and streams. The second longest, in Quintana Roo, Mexico, is almost entirely water-filled and has been mapped by divers over decades through many entrances.

Karst holds lots of clean water underground.

Many people drink it. Dirty water can flow into it fast. So we keep karst clean.

Karst is full of holes, like a Swiss cheese, and it stores a lot of water. The NPS says about 40 percent of the groundwater used for drinking in the United States comes from karst.

But because water rushes quickly through the holes, it is not filtered much on the way. Anything harmful spilled on the ground above can quickly reach the water people drink. That is why protecting karst landscapes matters for people, plants and animals.

Karst aquifers store and transmit large volumes of water, and the NPS reports that about 20 percent of the United States is underlain by karst and that 40 percent of the groundwater used for drinking comes from karst aquifers. Their porous, Swiss-cheese structure means water flows quickly and receives little filtration, so contaminants entering at the surface are transported rapidly, creating water-quality problems. The NPS concludes that protecting karst landscapes is imperative for our health and safety.

Caves themselves are slow to make and slow to decorate: passages grow at most about 1 mm a year, and a stalactite only lengthens as long as water keeps dripping through it (NPS). BGS adds that human activity, from leaking pipes to building work and pumping groundwater, can trigger sinkholes, so how land above karst is used matters too.