all 3 comments

[–]MightyMorphinFaggot 3 insightful - 1 fun3 insightful - 0 fun4 insightful - 1 fun -  (0 children)

Anyone with a tap in a hard water area could tell you that.

[–]chadwickofwv 2 insightful - 1 fun2 insightful - 0 fun3 insightful - 1 fun -  (0 children)

It's an electrical phenomenon.

[–]weavilsatemyface 1 insightful - 1 fun1 insightful - 0 fun2 insightful - 1 fun -  (0 children)

Despite what some 19th century geologists thought, we can work out the age of stalagmites and stalactites quite well, and have learned a lot about how they form.

It is well known that some kinds of stalagmites and stalactites ("dripstones") can form rapidly. Those made from ice (or icicles in common language) can form in a matter of hours or days. Those formed from lava can form in days or weeks. Those which form from water dripping through concrete can form in years or decades. Other chemical and physical processes can occur over varying time frames, e.g. evaporation in gypsum, and the rate can be affected by temperature and rainfall. (Well duh.)

There are fast and slow growing dripstones, and anyone in an area with hard water knows that leaky showerheads and faucets can become coated with calcite in a matter of months. This is the same process as the famous "Coke bottle stalagmites".

But under other conditions, the flowstones and dripstones that form in limestone caves grow very slowly, about a millimetre per decade. We can directly measure the age of these with radioactive dating, or count the rings that are caused by seasonal changes in rainfall.

Another factor is the width of the dripstone. Wide low stalagmites like the Breast Of Venus occur from relatively rapid drips, while tall narrow stalagmites require a very slow rate of dripping. So when you see a tall, narrow stalagmite, you know that it can't have formed rapidly, or it would be wide and short.