To understand caves as a geologic feature, one must begin to understand how they are formed. Caver are formed by water, usually rain water, from the surface penetrate into the ground, trickling down till it reaches an impermeable surface. While water is moving through the tops soil it picks up carbon dioxide, making carbonic acid. This carbonic acid reacts with the limestone bedrock to cause chemical weathering. This weathering process is sped up by over 25% due to the addition of carbon dioxide gas to the water. This weathering generally only happens where limestone is present. Limestone is a sedimentary rock that is composed of calcium carbonate (CaCO3). Limestone is soluble in water in and other weak acids, meaning that in can be weathered down easy by the addition of water. Generally in areas that have limestone as the underlying bedrock you see very little lakes and ponds because the water can penetrate through weathered away cracks in the bedrock, and drain downward. While the water travels through these cracks it takes some of the calcium carbonate away with it, making the holes bigger and bigger. The holes begin to get really large until it opens up on the surface making a cave. This is how the caves in Borneo have been formed.
There are other ways that caves can be formed. Depending on what type of cave it is it could be formed by water erosion, chemical processes like mention above, or even by molten rock from volcanoes. Sea caves are generally hollowed out by the pressure that the sea exerts on the rocks. The constant beating of the waves can cause the rock there to be weathered away forming sea caves around coastal areas. Caves can also be formed by volcanic lava tubes. Lava tubes are areas where lava flows underground to the central source or volcano. Once the volcano has become dormant or the lava no longer flows from that area, there is a cave left behind. These caves are generally perfectly cylindrical due to the fast moving lava weathering away any felsic or intermediate material it comes in contact with. The ideal lava for these types to tubes is pahoehoe. This type of lava is smooth in appearance and is not as viscous as many other types of lavas. This low viscosity allows for fast flow and high temperature, allowing the lava to destroy any unlike rocks or matter in its path. This is also how these types of caves become uniformly cylindrical. The lava flows evenly through the elongated tubes at a constant rate smoothing out the edges making it almost a perfect cylinder. Less viscous lava causes the caves to have uneven ceilings and floors.
There are also many exciting geologic features that unique to caves. Stalactites and stalagmites are two that come to mind right off the bat. Stalactite comes from the Greek word “stalasso”, which means “that which drips.” Stalactites are formed from the deposition of calcium carbonate that was picked up from the chemical weathering in the formation of the cave. Each stalactite is formed from a single drop of water that contains a whole hosp of minerals. These minerals, once in contact with air, begins to precipitate and become solid. Some water drops off and forms another feature that will be talked about shortly. These precipitated minerals begin to build up after a while creating a pointed object coming down from the ceiling, stalactites. The water that drops off from these stalactites forms another geologic feature in caves, stalagmites. These are the counterparts of stalactites. The water dripping off the stalactites forms in puddles on the ground. Once the mineral content of the these puddles increases, the minerals begin to precipitate out of the solution and harden. The mineral rich water from the stalactite above continues to fall a gather on that hardened rock below. These stalagmites and stalactite grow almost towards each other, making some of the most beautiful sight in the world.