Tufa is a variety of
limestone formed when
carbonate mineralsprecipitate out of water in
unheated rivers or lakes.
Geothermally heated hot springs sometimes produce similar (but less porous) carbonate deposits, which are known as
travertine. Tufa is sometimes referred to as (meteogene) travertine.[1] It should not be confused with hot spring (thermogene) travertine. Tufa, which is
calcareous, should also not be confused with
tuff, a porous
volcanic rock with a similar
etymology that is sometimes also called "tufa".
Classification and features
Modern and fossil tufa deposits abound with wetland plants;[2] as such, many tufa deposits are characterised by their large
macrobiological component, 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.
Fluvial deposits
Deposits can be classified by their depositional environment (or otherwise by vegetation or
petrographically). 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 (see
oncolite)
Cascade – Deposits form at waterfalls, deposition is focused here due to accelerated flow (see
Geochemistry)
Barrage – Deposits form as a series of
phytoherm barrages 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.).
Lacustrine deposits
Lacustrine tufas are generally formed at the periphery of lakes and built-up phytoherms (freshwater reefs), and on
stromatolites.
Oncoids are also common in these environments.
Calcareous sinter
Although sometimes regarded as a distinct carbonate deposit, calcareous sinter formed from ambient temperature water can be considered a sub-type of tufa.
Speleothems
Calcareous
speleothems may be regarded as a form of calcareous sinter. They lack any significant
macrophyte component due to the absence of light, and for this reason they are often morphologically closer to travertine or calcareous sinter.
Columns
Tufa columns are an unusual form of tufa typically associated with
saline lakes. They are distinct from most tufa deposits in that they lack any significant
macrophyte component, due to the salinity excluding
mesophilic organisms.[3] Some tufa columns may actually form from hot-springs, and may therefore constitute a form of
travertine. It is generally thought that such features form from CaCO3 precipitated when carbonate rich source waters emerge into alkaline soda lakes. They have also been found in marine settings in the Ikka
fjord of Greenland where the
Ikaite columns can reach up to 18 m (59 ft) in height.[5]
Biology
Tufa deposits form an important habitat for a diverse flora.
Bryophytes (mosses, liverworts etc.) and
diatoms are well represented. The porosity of the deposits creates a wet habitat ideal for these plants.
Geochemistry
Modern tufa is formed from alkaline waters, supersaturated with calcite. On emergence, waters degas CO2 due to the lower atmospheric pCO2 (see
partial 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 in pCO2, 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 a
biofilm is present, despite supersaturation.
Tufa is occasionally shaped into a planter. Its porous consistency makes it ideal for
alpine gardens. A concrete mixture called
hypertufa is used for similar purposes.
Tufa is used to build Roman walls in the 4th century BC, for up to 10m high and 3.5m thick.[13] They are soft, allowing easy sculpting. Tufa masonry was used in cemeteries, such as the one in
Cerveteri.[14]
^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). Retrieved 2010-10-01.