Cave

Caves and caverns are natural spaces found under the surface of the Earth, and scientists have even found similar features on other rocky worlds like Mars. These amazing openings in the ground form mostly through the weathering of rock and can stretch far beneath our feet. Some smaller caves, called exogene caves, don't go very deep, like rock shelters, while others that go deeper than their opening are wide are known as endogene caves.

The study of caves and everything around them is called speleology. Many people enjoy visiting caves as a fun activity, and this is sometimes called caving, potholing, or spelunking. Exploring these hidden spaces lets us learn about Earth's history and the unique life that can live inside caves.

Formation types

The formation and development of caves is known as speleogenesis; it can occur over the course of millions of years. Caves can range widely in size, and are formed by various geological processes. These may involve a combination of chemical processes, erosion by water, tectonic forces, microorganisms, pressure, and atmospheric influences. Isotopic dating techniques can be applied to cave sediments, to determine the timescale of the geological events which formed and shaped present-day caves.

It is estimated that a cave cannot be more than 3,000 metres (9,800 ft) vertically beneath the surface due to the pressure of overlying rocks. Most caves are formed in limestone by dissolution.

Solutional

Main article: Solutional cave

Solutional caves or karst caves are the most frequently occurring caves. Such caves form in rock that is soluble; most occur in limestone, but they can also form in other rocks including chalk, dolomite, marble, salt, and gypsum. Except for salt caves, solutional caves result when rock is dissolved by natural acid in groundwater that seeps through bedding planes, faults, joints, and comparable features. Over time cracks enlarge to become caves and cave systems.

The largest and most abundant solutional caves are located in limestone. Limestone dissolves under the action of rainwater and groundwater charged with H₂CO₃ (carbonic acid) and naturally occurring organic acids. Limestone caves are often adorned with calcium carbonate formations produced through slow precipitation. These include flowstones, stalactites, stalagmites, helictites, soda straws and columns.

Primary

Caves formed at the same time as the surrounding rock are called primary caves.

Lava tubes

Lava tubes are formed through volcanic activity and are the most common primary caves. As molten lava flows downhill, its surface cools and forms a solid outer crust. This crust insulates the interior from the cool air outside, which allows hot liquid lava to continue to flow underneath. If most of the lava flows out, a hollow tube remains. Such caves can be found in the Canary Islands, Jeju-do, the basaltic plains of Eastern Idaho, and in other places. Kazumura Cave near Hilo, Hawaii is a remarkably long and deep lava tube; it is 65.6 km long (40.8 mi).

Lava caves include but are not limited to lava tubes. Other caves formed through volcanic activity include rifts, lava molds, open vertical conduits, and inflationary blisters, among others.

Sea or littoral

Main article: Sea cave

Sea caves are found along coasts around the world. A special case is littoral caves, which are formed by wave action in zones of weakness in sea cliffs. Often these weaknesses are faults, but they may also be dykes or bedding-plane contacts. Some wave-cut caves are now above sea level because of later uplift. Elsewhere, in places such as Thailand's Phang Nga Bay, solutional caves have been flooded by the sea and are now subject to littoral erosion. Sea caves are generally around 5 to 50 metres (16 to 164 ft) in length, but may exceed 300 metres (980 ft).

Erosional

Erosional caves are those that form entirely by erosion by flowing streams carrying rocks and other sediments. These can form in any type of rock, including hard rocks such as granite. Generally there must be some zone of weakness to guide the water, such as a fault or joint.

Glacier

Main article: Glacier cave

Glacier caves are formed by melting ice and flowing water within and under glaciers. The cavities are influenced by the very slow flow of the ice, which tends to collapse the caves again.

Fracture

Fracture caves are formed when layers of more soluble minerals, such as gypsum, dissolve out from between layers of less soluble rock. These rocks fracture and collapse in blocks of stone.

Talus

Main article: Talus cave

Talus caves are formed by the openings among large boulders that have fallen down into a random heap, often at the bases of cliffs. These unstable deposits are called talus or scree, and may be subject to frequent rockfalls and landslides.

Anchialine

Main article: Anchialine cave

Anchialine caves are caves, usually coastal, containing a mixture of freshwater and saline water (usually sea water). They occur in many parts of the world, and often contain highly specialized and endemic fauna.

Physical patterns

Caves can form in different patterns depending on how water and rock interact. Branchwork caves are the most common type. They look like tiny rivers on the surface, with passages joining together like streams. These caves usually form near places where water goes into the ground.

Other types of caves include angular network caves, which have narrow, straight passages formed by cracks in the rock. Anastomotic caves look like braided streams, with passages that split and then join again. Spongework caves have a random, sponge-like pattern of cavities. Ramiform caves feature large, irregular rooms created by rising water. Finally, pit caves are vertical shafts instead of horizontal passages.

Geographic distribution

Caves can be found all around the world, but we know more about them in places where people have spent a lot of time exploring them. Countries like France, Italy, Australia, the UK, and the United States have many well-known caves. These caves are mostly in Europe, Asia, North America, and Oceania. However, there are fewer known caves in South America, Africa, and Antarctica.

Even within these general areas, the number of known caves changes. Some big parts of North America and Asia don’t have many recorded caves, while places like the Madagascar dry deciduous forests and parts of Brazil have lots of them. As more people explore different rocky areas around the world, we will probably find many more caves in places we haven’t looked closely at yet. For example, China has a lot of rock that could form caves, but we still don’t know about many of them.

Records and superlatives

Some caves are known for being very long, deep, or large. The longest cave system ever explored is Mammoth Cave in Kentucky, US, stretching for over 675 kilometers. Another amazing cave is Sistema Ox Bel Ha in Yucatán, Mexico, which is the longest underwater cave known.

The deepest cave ever found is Veryovkina Cave in the Republic of Abkhazia, diving more than 2,200 meters below its entrance. The biggest room inside a cave is in Son Doong Cave in Vietnam, which has a passage that is nearly 5 kilometers long!

Ecology

Main article: Biospeleology

Caves are special places where unique animals live. Some animals can only live in caves, called troglobites, and they often look different from other animals. They might be pale or white because they don’t need colour, and they may not have eyes because there’s no light underground. These animals can sense vibrations very well to find food and each other.

Some cave animals, like bats, visit caves but live outside too. Others, like certain insects and small shrimp, spend part of their lives in caves. Because caves are very delicate environments, many of these animals are rare and need protection.

Archaeological and cultural importance

See also: Underground living

People have used caves for a very long time. Some of the oldest human remains found on Earth come from caves in South Africa. These remains show us that early humans lived or were brought into caves by animals that might have hunted them.

Caves have also been important places for early humans all over the world. For example, in China, Indonesia, Europe, and South Africa, early humans used caves for shelter and sometimes as special places for art. One of the oldest known art sites is in a cave in South Africa, where humans began using sea caves about 180,000 years ago.

We have found some of the world’s oldest cave paintings in many places. These paintings show that humans have been creating art in caves for tens of thousands of years.

Caves and acoustics

Caves have been important for sound for a very long time. Ancient people in Spain and France made paintings in cave areas that had special sounds. Some of these paintings showed animals, and the sounds in these spots could even copy the animals' calls. People might have also used their voices to find their way in dark parts of the caves where lights didn’t help.

Today, caves are still used for music. Cumberland Caverns is a great example, where musicians enjoy the special sounds the cave makes. The unusual shapes of the cave walls help spread out sounds evenly, making it feel like a music studio. In the past, famous musicians performed in caves like Luray Caverns in Virginia, where a huge instrument called the "Great Stalacpipe Organ" uses mallets to hit stalactites and create different musical notes.