Caves have sparked our imaginations and played a crucial role in human evolution, but for all their enticing history, most people still don’t know what they are and how they form. There are several ways of classifying caves. I’ll just run you through the main ones before detailing all of them:

• solutional caves are generally formed in limestone or other similar rock such as gypsum or dolomite. They form when acidic water dissolves the rock, seeping through the bedding planes.
• lava caves are also called primary caves because they form at the same time as the surrounding rock. Sometimes lava flow creates a hollow tube, which results in the cave.
• sea caves are quite self-explanatory – they’re formed by the sea, due to the constant activity of waves. They can be both over and under water.
• glacier caves are caves not in rock, but in glaciers.

There are also several more uncommon types of caves which we’ll discuss further. For now, it’s time to get our hands dirty.

Cave where the remains of Homo floresiensis, an ancient hominid, were found. Photo by Rosino.

What caves are

Before we start looking into the types of caves with more detail, let’s analyze just what caves are. Not any hole in an underground space is a cave. In order for it to be called a cave, the structure must be natural and large enough for a human to enter. This part of the definition isn’t so strict, though.

Caves can be formed by various geological processes and therefore, can vary greatly in size and structure.

Solutional caves

Limestone cave. Photo by Andrew McMillan.

These are by far the most common caves – for most people, they’re probably synonymous with caves.

Solutional caves form in carbonatic rocks, most commonly limestone, but also chalk, dolomite, marble, salt, and gypsum. Regardless of the type of rock, the process is similar. As water falls on the ground, it accumulates carbon dioxide from the atmosphere, becoming a very weak acid. But even this “weak acid” can create great things over time: as the water percolates through the rock, it dissolves more and more, creating bigger and bigger voids.

How caves form, via British Geological Survey.

It does take a lot of time, however – we’re talking geological time. This is why solutional caves often have formations like stalactites and stalagmites. Limestone caves especially have many formations, and they’re also the biggest and most impressive caves.

Lava caves

Primary or lava caves form at the same time as the rock around them, as a result of volcanic activity.

Thurston Lava Tube in Hawaii Volcanoes National Park, Hawaii. Photo by Frank Schulenburg

Lava caves are almost always lava tubes, formed by flowing lava. As the lava flows, it cools down and creates a solid crust. Liquid lava continues to flow beneath that crust, and most of it flows out, leaving behind a hollow tube. Lava tubes aren’t formed only on Earth, but also on the moon and on Venus.

In some rarer circumstances, these caves can form outside of lava tubes. Rift caves, lava mold caves, open vertical volcanic conduits, and inflationary caves can all form due to volcanic activity.

Sea caves

Sea caves are found on shores all around the world, also usually in carbonatic rocks.

Painted Cave, a large sea cave, Santa Cruz Island, California. Photo by Dave Bunnell.

The constant action of waves attacks the weaker part of the rocks and in time, starts to erode them. Sand and tiny bits of gravel can also amplify the corrosion. A special case of sea caves is called littoral caves, where the waves act on very weak areas, such as faults or bedding plane contacts. The waves “speculate” the rock’s weakness and can create a cave much faster.

Glacier caves

Until now, we’ve talked about rocky caves – but caves can also form in glaciers.

A partly submerged glacier cave on Perito Moreno Glacier. The ice facade is approximately 60 m high

They are similar to solutional caves, but it can form much faster. Melting ice and flowing water within the glacier can create surprisingly large ice caverns, which are regarded as caves. Though they are sometimes referred to as “ice caves,” that term is normally reserved for rocky caves with year-round ice formations inside.

Other types of caves

As we mentioned above, there are other types of caves too, but they’re much rarer.

• Fracture caves form when a soluble layer, such as gypsum, dissolves. After the layer disappears, the rocks around it can collapse, creating a fracture cave.
• Talus caves are formed by the openings among large boulders that have fallen down into a random heap. They should be avoided as they’re usually unstable and dangerous.
• Eolian caves are formed, like their name says, by the wind. They form only in deserts, driven by the sandblasting effect of silt or fine sand being blown against a rock face. They can be surprisingly large and impressive.
• Anchialine caves are usually coastal and contain a mixture of freshwater and saline water (usually sea water). They occur in many parts of the world and often have highly specialized, endemic fauna.