Written by Holly
Caves are something that many of us are familiar with. Maybe you’ve been in one, maybe you’ve been in a hundred, or maybe the only cave you’ve encountered was in a video game or movie. Regardless of your experience or lack thereof, we all have an idea of what a cave is: a hole in the ground. But as scientists, we have to be a little more specific.
Scientifically, a cave is a cave when it meets three requirements:
- Made of rock
- Made by natural forces
- Large enough to fit a human
Sometimes, a fourth trait is required:
4. Large enough to extend beyond the reach of light
However, this trait is not always included since some caves, such as sea caves, cannot meet it due to the nature of their formation. Therefore, we only use the main three.
Caves are found all over the world, on every continent. In the United States, they are found in all states
except Rhode Island and Louisiana. There are also many different types of cave around the world, and each type is defined by how or where it forms. The formation of a cave is known as speleogenesis.
Solution Caves are the most common type in the world. These caves form in rock that can be dissolved; usually it’s limestone, but rocks like marble, gypsum, and chalk are also capable of hosting solution caves. These caves begin as a crack in the rock known as a fissure or joint.
As rain falls and snow melts, water slowly seeps into the ground and combines with carbon dioxide (CO2) and becomes carbonic acid, which is a weak acid capable of dissolving rock over time. Carbonic acid may sound scary, but it is in fact the main ingredient in all carbonated drinks like soda pop and sparkling water.
Eventually, this weak acid slips into the crack in the rock and gets stuck. While sitting for long periods, it slowly eats away at the stone and creates a larger opening. By opening up these pockets of air in the rock, the acid creates room for cave formations, or speleothems, to build up.
Sometimes, a stronger acid known as sulphuric acid can get involved. This will form when the weaker carbonic acid comes in contact with certain minerals, such as pyrite, which is made up of iron and sulphur. Inside solution caves, it is usually fairly obvious when sulphuric acid did the dissolving: the rock walls will be much smoother and, usually, steeper. This is because sulphuric acid is much stronger than the rock and can dissolve it quickly, often creating domes. Domes are features in the ceiling where the ceiling rises very high and sharp, like a church steeple.
Sea Caves, also known as Littoral Caves, are a type of cave seen around large bodies of water like oceans, lakes, and rivers. These caves form as water beats against exposed rocks and slowly wears them away over time in a process known as mechanical weathering. Over time, holes begin to appear in the rock face and can even create archways that you can kayak through. Sea caves are seen anywhere with an amount of water large enough to have waves or a substantial flow (so not streams or ponds).
Lava Tubes are caves that form in areas with volcanic activity such as Hawaii, Iceland, and even the Moon. Lava tubes will form when lava is flowing on the surface. The top layer of lava will cool down faster because it is exposed to the cooler air surrounding it, creating a hard shell of rock. Since only the outside of the lava is cooled, the insides keep flowing until all that’s left is a tube in the shape of the lava flow. You can think of it in terms of roasting marshmallows, where the marshmallow is the lava. Stick the marshmallow in the campfire, and the outside becomes charred and burnt. You can then pull this shell off, leaving behind the gooey center of the marshmallow. The gooey center is the lava, and the shell is the lava tube.
Glacier Caves form in glaciers, of course. Glaciers are huge masses of ice that exist on land, cover at least 0.1 kilometers (~25 acres), and stick around year after year. They exist on every continent except Australia and contain over 2% of Earth’s water, so glacier caves can be found almost everywhere. Glacier caves form when glaciers start to melt and crack, creating large openings with hypnotic wavy walls and ceilings. These caves usually stick around but are often the most susceptible to disappearing. The Paradise Ice Caves at Mount Rainier were one of the most famous sets of glacier caves in the world and reached a maximum length of 13.25 kilometers (~8 miles) before they disappeared entirely due to glacial recession.
Ice Caves sound like they are very similar to glacier caves, but they are in fact very different. Ice caves are not caves made out of ice, but rather decorated by ice. They are a hole in the rock that has icicles as stalactites and piles of ice as flowstone and stalagmites, and they have this ice for at least half of the year. Among the most famous ice caves are Northern Wisconsin’s Mainland Ice Caves at the Apostle Islands.
Aeolian Caves are found in desert biomes. Named for the Greek god of the winds, these caves form when wind picks up sediments and rubs them against rock faces, slowly wearing the rock away. They are very rarely significant in size, often not reaching more than a few meters into the cliffside. These caves, however, do act as a natural shelter and were used as sites for homes by the Ancestral Pueblo peoples in places like Mesa Verde National Park, Colorado.
Talus Caves are usually the final type of cave that is acknowledged. These are probably the simplest and quickest-forming caves: they are the gaps between huge boulders and rocks that have piled and stacked up over time at the bases of much larger rock structures like mountains and bluffs. These big pieces of rock that break off of the main rock structure and collect themselves into piles are known as talus. Talus Caves can form anywhere with exposed rock, making them incredibly common. In Wisconsin, these can be found all around the shoreline of Devil’s Lake in Baraboo along with many, many other locations.