How long does it take stalagmites and stalactites to form?

Monday 23rd September 2013
Submitted by Luis Villazon
Naomi Chambers, Chelmsford

Stalactites form when water containing dissolved calcium bicarbonate from the limestone rock drips from the ceiling of a cave. As the water comes into contact with the air, some of the calcium bicarbonate precipitates back into limestone to form a tiny ring, which gradually elongates to form a stalactite.

Stalagmites grow upwards from the drips that fall to the floor. They spread outwards more, so they have a wider, flatter shape than stalactites, but they gain mass at roughly the same rate. Limestone stalactites form extremely slowly – usually less than 10cm every thousand years – and radiometric dating has shown that some are over 190,000 years old.

Stalactites can also form by a different chemical process when water drips through concrete, and this is much faster. Stalactites under concrete bridges can grow as fast as a centimetre per year.

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