Science Puzzle
How Are Limestone Caves Made?
Beneath the Mendips, the Yorkshire Dales and many other limestone areas lie caves big enough to hold a cathedral, linked by passages that run for kilometres. Mammoth Cave in Kentucky has more than 680 kilometres of mapped passages. Nobody dug them, and often there's no river nearby big enough to have carved them out.
How are limestone caves made?
The Answer
Rainwater dissolves them. Rain picks up carbon dioxide, which makes it a very weak acid, and limestone slowly dissolves in that acid, so over hundreds of thousands of years cracks in the rock are widened into passages and caves.
Carbon dioxide dissolves in water to make carbonic acid, the same weak acid that gives fizzy water its slight tang. Rain picks up a little from the air and much more as it soaks through soil, where roots and rotting plants release plenty of the gas. Limestone is calcium carbonate, which reacts with the acid and dissolves.
The water follows cracks and the joints between layers of rock, eating away at their sides. Narrow cracks become pipes, pipes become passages, and where water collects, chambers open up. When the water table drops, the caves drain and are left full of air.
The dissolved limestone isn't gone for good, though. Stalactites are the same chemistry in reverse. When a drip of water reaches the open air of a cave, some of its carbon dioxide escapes, just as a fizzy drink goes flat. The water can no longer hold all its dissolved limestone, so it leaves a thin ring of it behind. Drip by drip, often less than a millimetre a year, stalactites grow down from the roof and stalagmites build up from the floor.
The principle: Chemical weathering. Rainwater made slightly acidic by carbon dioxide dissolves limestone, slowly widening cracks into caves; when the gas escapes again, the rock is laid back down as stalactites.