Modern and fossil tufa deposits abound with wetland plants;[2]as such, many tufa deposits are characterised by their largemacrobiologicalcomponent, and are highly porous. Tufa forms either in fluvial channels or in lacustrine environments. Ford and Pedley (1996)[3]provide a review of tufa systems worldwide.
Deposits can be classified by their depositional environment (or otherwise by vegetation orpetrographically). Pedley (1990)[4]provides an extensive classification system, which includes the following classes of fluvial tufa:
Braided channel – Deposits form within a fluvial channel, dominated by oncoids (seeoncolite)
Cascade – Deposits form at waterfalls, deposition is focused here due to accelerated flow (seeGeochemistry)
Barrage – Deposits form as a series ofphytohermbarrages across a channel, which may grow up to several metres in height. Barrages often contain a significant detrital component, composed of organic material (leaf litter,branches etc.).
Rubaksatufa plug, after drying of the river, in Ethiopia
Lacustrine tufas are generally formed at the periphery of lakes and built-up phytoherms (freshwater reefs), and onstromatolites.Oncoidsare also common in these environments.
Although sometimes regarded as a distinct carbonate deposit, calcareous sinter formed from ambient temperature water can be considered a sub-type of tufa.
Calcareousspeleothemsmay be regarded as a form of calcareous sinter. They lack any significantmacrophytecomponent due to the absence of light, and for this reason they are often morphologically closer to travertine or calcareous sinter.
Tufa columns are an unusual form of tufa typically associated withsaline lakes.They are distinct from most tufa deposits in that they lack any significantmacrophytecomponent, due to the salinity excludingmesophilic organisms.[3]Some tufa columns may actually form from hot-springs, and may therefore constitute a form oftravertine.It is generally thought that such features form from CaCO3precipitated when carbonate rich source waters emerge into alkaline soda lakes. They have also been found in marine settings in the Ikkafjordof Greenland where theIkaitecolumns can reach up to 18 m (59 ft) in height.[5]
Tufa deposits form an important habitat for a diverse flora.Bryophytes(mosses, liverworts etc.) anddiatomsare well represented. The porosity of the deposits creates a wet habitat ideal for these plants.
The Pyramid and Domes tufa rock structures, Pyramid Lake, Nevada
Modern tufa is formed from alkaline waters, supersaturated with calcite. On emergence, waters degas CO2due to the loweratmosphericpCO2(seepartial pressure), resulting in an increase in pH. Since carbonate solubility decreases with increased pH,[6]precipitation is induced. Supersaturation may be enhanced by factors leading to a reduction inpCO2,for example increased air-water interactions at waterfalls may be important,[7]as may photosynthesis.[8]
Recently it has been demonstrated that microbially induced precipitation may be more important than physico-chemical precipitation. Pedley et al. (2009)[9]showed with flume experiments that precipitation does not occur unless abiofilmis present, despite supersaturation.
Tufa is occasionally shaped into a planter. Its porous consistency makes it ideal foralpine gardens.A concrete mixture calledhypertufais used for similar purposes.
Hollowed out portions of these tufa cliffs once formed back walls of rooms in a large prehistoric pueblo that stood here in Bandelier National Monument. Note outlines of masonry that were the outer portions of structure, and small holes in cliff that once supported ends of floor beams.
In the 4th century BC, tufa was used to build Roman walls up to 10m high and 3.5m thick.[13]The soft stone allows for easy sculpting. Tufa masonry was used in cemeteries, such as the one inCerveteri.[14]
^Pentecost, A. (2005).Travertine.Dordrecht, Netherlands: Kluwer Academic Publishers Group.ISBN1-4020-3523-3.
^Koban, C.G.; Schweigert, G. (1993). "Microbial origin of travertine fabrics - two examples from Southern Germany (Pleistocene Stuttgart travertines and Miocene riedöschingen Travertine)".Facies.29:251–263.doi:10.1007/BF02536931.S2CID129353316.
^Buchardt, B.; Israelson, C.; Seaman, P.; Stockmann, G. (2001). "Ikaite tufa towers in ikka fjord, southwest Greenland: their formation by mixing of seawater and alkaline spring water".Journal of Sedimentary Research.71(1): 176–189.Bibcode:2001JSedR..71..176B.doi:10.1306/042800710176.
^Forster, A.; Forster, S.C. (1996). "Troglodyte dwellings of the Loire Valley, France".Quarterly Journal of Engineering Geology and Hydrogeology.29(3): 193–197.doi:10.1144/GSL.QJEGH.1996.029.P3.01.S2CID128896993.
^"Au Turonien".Une histoire de la Touraine à travers ses roches(in French).Retrieved2010-10-01.
