Asked by: 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.