Tufa

Tufa is a variety of

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:

• Spring – Deposits form on emergence from a
  spring/seep. Morphology can vary from mineratrophic wetlands to spring aprons (see calcareous sinter
  )
• 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
  leaf litter
  , branches etc.).

Lacustrine deposits

Lacustrine tufas are generally formed at the periphery of lakes and built-up phytoherms (freshwater reefs), and on

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

Columns

Tufa columns are an unusual form of tufa typically associated with

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.

Calcite is the dominant mineral precipitate, followed by the polymorph aragonite.^{[citation needed]}

Occurrence

Tufa is common in many parts of the world, including::

• Pyramid Lake, Nevada, US – tufa formations
• Big Soda Lake, Nevada, US – tufa formations only a century old
• Mono Lake, California, US – tufa columns
• Trona Pinnacles, California, US – tufa columns
• Matlock Bath, Derbyshire, United Kingdom
• North Dock Tufa, United Kingdom
• Plitvice Lakes National Park, Croatia
• Basturs Lakes, Pallars Jussà, Catalonia – tufa mounds
• Various parts of Armenia, such as Artik
• The southwestern coastline of Western Australia
• The
  North West Province
  , South Africa
• The
  Mpumalanga Province
  , South Africa
• Various parts of southern Italy.^[10]

Some sources suggest that "tufa" was used as the primary building material for most of the châteaux of the

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]

See also

• List of types of limestone – Limestone deposits listed by location

References

External links

Wikimedia Commons has media related to:
Tufa (category)

• Mono Lake Committee: "Tufa"