Stalactites can grow very quickly. Some have been observed to grow more
than half an inch per year. The largest stalactites and flowstones could
have formed in a few thousand years.
The fast-growing stalactites form via processes very different from
calcium carbonate stalactites found in limestone caves. Limestone is
not soluble in water. When carbon dioxide (from decaying plants in the
soil above the cave) mixes with water, it forms a very weak carbonic
acid. This turns the calcium carbonate into calcium bicarbonate, which
dissolves. When drips are exposed to air in the cave, a little carbon
dioxide escapes from them into the atmosphere, which reverses the
process and precipitates a small amount of calcium carbonate. The
upper average rate for limestone stalactite growth is ten centimeters
per thousand years, with lower growth rates outside of tropical areas.
Fast-growing stalactites, on the other hand, either grow from gypsum
through an evaporative process, or they form from concrete or mortar.
When water is added to concrete, one product is calcium hydroxide,
which is about 100 times more soluble than calcite. The calcium
hydroxide absorbs carbon dioxide from the atmosphere to reconstitute
calcium carbonate.
The time for stalactite growth also has to allow for time for the cave
to dissolve in the first place, which is a very slow process, sometimes
on the order of tens of millions of years. Then the geological
conditions have to change so that the cave is no longer under water.
Only then can stalactite growth begin.
Direct measurement via radiometric dating gives stalactite ages over
190,000 years (Ford and Hill 1999). Other deposits in caves have
been dated to several million years old. For example, argon-argon
dating of alunite (an aluminum sulfate mineral) gives an age of 11.3
million years for a cave near Carlsbad Caverns (Polyak et al. 1998).
Oxygen isotope measurements in stalactites give an indication of
outside temperatures. They are consistent with the coming and going of
ice ages back at least 160,000 years (Dorale et al. 1998; Wang et
al. 2001; Zhang et al. 2004).
Dorale, J. A. Dorale, R. L. Edwards, E. Ito and Luis A.
González, 1998. Climate and vegetation history of the
midcontinent from 75 to 25 ka: A speleothem record from Crevice Cave,
Missouri, USA. Science 282: 1871-1874.
Polyak, V. J., W. C. McIntosh, N. Güven and P. Provencio, 1998.
Age and origin of Carlsbad Cavern and related caves from
40Ar/39Ar of alunite. Science 279: 1919-1922.
See also Sasowsky, I. D., 1998. Determining the age of what is not
there. Science 279: 1874.
Wang, Y. J. et al., 2001. A high-resolution absolute-dated Late
Pleistocene monsoon record from Hulu Cave, China.
Science 294: 2345-2348.
Zhang, M., D. Yuan, Y Lin, H. Cheng, J. Qin and H Zhang, 2004. The
record of paleoclimatic change from stalagmites and the determination
of termination II in the south of Guizhou Province, China. Science in
China Ser. D 47(1): 1-12.
http://www.karst.edu.cn/publication/Zhang%20Ml200401.pdf
