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Mysterious Life of Caves

How Caves Form


Caves homepage

Cave-Making Agent: Rainwater

Limestone caves, which are formed primarily by rainwater and snowmelt, are by far the most numerous of all cave types.

Limestone formations were created millions of years ago, often in shallow seas, largely from the accumulated remains of marine animals such as corals. This type of sedimentary rock forms large, solid, rectangular blocks.

illustration: rain falls onto landscape and seeps into ground

Rainwater and snowmelt seeping into the ground absorb large amounts of carbon dioxide, which is a product of decaying organic matter.

The water and carbon dioxide together form carbonic acid.

illustration: cross-sectional view of limestone formation; show water moved into ground and indication of CO2 entering water

This carbonic acid continues to seep into the soil and through the limestone until it reaches the water table, which is the upper limit at which ground is saturated with water.

The acid moves fast enough through the limestone that it does not create channels as it makes its way to the water table.

illustration: water seeps to water table and through cracks in the limestone

The carbonic acid eats away at the limestone at a level directly below the water table, eventually forming channels. The larger a channel becomes, the more water it holds and the faster the limestone dissolves.

These channels may form along the water table or they may extend down below the table along fissures within the limestone.

illustration: cave forming along the water table

The cave typically forms over a period of a few million years, slowly increasing in size.

Later, if the water table lowers, the enlarging of the cave stops. It is at this point that dagger-like stalactites and stalagmites sometimes begin to form as mineral-laden water drips through the cave's ceiling.

illustration: cave is wider and water table is lower

If it's moving fast enough within the cave, water can also carve a larger, deeper path through the rock by corrasion (erosion by abrasion), just as a river carves through rock to create a canyon.

Moving water can also form caves within glaciers and sandstone caves at the base of cliffs.

Choose another way that caves can form.

illustration: new cave is present

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