How Are Caves Formed

Millions of years in the making, caves are wondrous and diverse natural phenomena, which have held humans in both awe and dread for thousands of years.

Solutional caves, as found across the Yucatan peninsula, are the most commonly occurring Earth cavities found across the globe. They are formed when a soluble rock such as limestone or marble is dissolved slowly by natural acid in the resident groundwater that seeps through the planes, faults and joints which, over epochs, slowly become cracks, then gulleys and finally caves. This dissolving process produces a distinctive landform known as ‘karst’, which is characterised by subterranean drainage, sinkholes and extensive interlinked cave networks.

The other most notable feature of solutional caves are the striking calcium carbonate formations that are produced by the slow precipitation of acid-laced groundwater. These formations include: stalactites – from the Greek “that which drips”, a type of secondary mineral that hangs from the ceiling of caves; stalagmites – from the Greek “drop”, a secondary mineral material which drops from the ceiling to the floor where it forms a calcium carbonate deposit; and soda straws, which are thin mineral tubes that grow out of cracks and carry water in their interior.

While solutional caves are by far the most common caves found worldwide, other varieties also exist and can be formed in numerous different ways.

Primary caves for example are formed at the same time as the surrounding rock, instead of afterwards like the solutional varieties that we’ve mentioned before. These caves are mostly formed by lava flowing downward and cooling and solidifying on top, while continuing to progress at the base, creating a lava tube once dissipated.

Another variety of cave formed in a similar manner to primary caves are glacial caves. Here, caves and tunnels are formed when embedded ice melts under glaciers and – as with the lava – flow downwards before eventually freezing again on top and solidifying once more. Finally, littoral caves (commonly referred to as sea caves) are formed when coastal rock is eroded away by the tidal action of the ocean waves, eating away at soluble rockalong weakened points such as fault lines.

Caves facts

Soluble rock – This, a good cross-section diagram of a formation of solutional-formed caves, demonstrates how soft soluble rock is eroded overtime by acidic groundwater. Water becomes acidic through a combination of climate effects (pollution) and by being absorbed by/ passed over organic hydrocarbons.

Calcium carbonate – After the acidic groundwater penetrates the rock’s planes, faults and joints, it can drip down from the ceiling of a preformed cave leaving mineral deposits either at its base, or on its roof, which then hardens into calcium carbonate formations such as stalactites and stalagmites.

Karst formation – As the soluble rock is eroded, a series of extensive tunnels, channels and holes is formed underground, culminating with many cave mouths often leading into valleys or rivers. In addition, if the erosion is severe then sinkholes may be formed (as can be seen here) when the roof of hollowed-out areas collapse in on itself. These are the characteristic features of karst topography.

Subterranean drainage – Once formed, any drainage channels will continue to erode if the area is subjected to heavy rainfall, with the gulleys being carved out by underground streams and rivers.