by Teresa » Apr 7, 2007 8:56 pm
Wildwolf--
You are spot on. All it takes for the calcite to come out of solution is the normal rough and tumble of water running downhill, though the steeper the drop, the more rock gets bounced out of solution faster. Algae pulling CO2 out of the water can make calcite deposit faster. We're talking FAST. I've seen living moss being coated by tufa--weeks to months, not years to centuries of a stalactite dripping.
Evan--
Wikipedia isn't quite right when it tries to make it sound like tufa forms primarily in lakes, and travertine only in streams. It's more a quality of the stone--just like in a cave you can have very hard, compact and finely crystalline flowstone, (travertine) and you can have punky flowstone incorporating mud, airbubbles, sticks, and so forth. It literally looks very close to those 'punk' sticks which come with 4th of July fireworks (though harder, of course.)
Tufa is more like the punky flowstone, or the calcite that deposits in bathtubs, sinks, and hot water heaters in hard water regions. There are also regional names, like marl, calcareous limestone, and a few more.
A couple of good references (if you are really into this) and by NSS people to boot are:
Ford, T.D., and Pedley, H.M. 1996. A review of tufa and travertine deposits of the world. Elsevier, Earth-Science Reviews, Vol. 41. p. 117-175.
Herman, J. S., and D. A. Hubbard, eds. Travertine-Marl: Stream Deposits in Virginia. 1990. Va. Div. Mineral Resources Pub 101, Dept. of Mines, Minerals and Energy, Div. of Mineral Resources, Charlottesville, Va. 184 p.
I prefer to use travertine for hard, pretty, highly crystalline banded water-laid calcium carbonate like show cave trinkets are made out of, tufa for the gray-green stuff like Wildwolf's photo, (and which I studied for about 5 years) and marl for what forms around my faucets.
Hope this helps.