Tufa is a variety of
limestone
Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms w ...
formed when
carbonate minerals precipitate 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
Travertine ( ) is a form of terrestrial limestone deposited around mineral springs, especially hot springs. It often has a fibrous or concentric appearance and exists in white, tan, cream-colored, and even rusty varieties. It is formed by a p ...
. Tufa is sometimes referred to as (meteogene) travertine.
It should not be confused with hot spring (thermogene) travertine. Tufa, which is
calcareous
Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines.
In zoology
''Calcareous'' is used as an ad ...
, should also not be confused with
tuff
Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock ...
, a porous
volcanic rock with a similar
etymology
Etymology () The New Oxford Dictionary of English (1998) – p. 633 "Etymology /ˌɛtɪˈmɒlədʒi/ the study of the class in words and the way their meanings have changed throughout time". is the study of the history of the form of words ...
that is sometimes also called "tufa".
Classification and features
Modern and fossil tufa deposits abound with wetland plants; 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)
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) 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
Geochemistry is the science that uses the tools and principles of chemistry to explain the mechanisms behind major geological systems such as the Earth's crust and its oceans. The realm of geochemistry extends beyond the Earth, encompassing th ...
)
*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
Oncolites are sedimentary structures composed of oncoids, which are layered structures formed by cyanobacterial growth. Oncolites are very similar to stromatolites, but, instead of forming columns, they form approximately spherical structures. ...
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
speleothem
A speleothem (; ) is a geological formation by mineral deposits that accumulate over time in natural caves. Speleothems most commonly form in calcareous caves due to carbonate dissolution reactions. They can take a variety of forms, dependi ...
s 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
Saline may refer to:
* Saline (medicine), a liquid with salt content to match the human body
* Saline water, non-medicinal salt water
* Saline, a historical term (especially US) for a salt works or saltern
Places
* Saline, Calvados, a commune in ...
. They are distinct from most tufa deposits in that they lack any significant
macrophyte component, due to the salinity excluding
mesophilic organisms.
Some tufa columns may actually form from hot-springs, and may therefore constitute a form of
travertine
Travertine ( ) is a form of terrestrial limestone deposited around mineral springs, especially hot springs. It often has a fibrous or concentric appearance and exists in white, tan, cream-colored, and even rusty varieties. It is formed by a p ...
. It is generally thought that such features form from CaCO
3 precipitated when carbonate rich source waters emerge into alkaline soda lakes. They have also been found in marine settings in the Ikka
fjord
In physical geography, a fjord or fiord () is a long, narrow inlet with steep sides or cliffs, created by a glacier. Fjords exist on the coasts of Alaska, Antarctica, British Columbia, Chile, Denmark, Germany, Greenland, the Faroe Islands, Icel ...
of Greenland where the
Ikaite columns can reach up to in height.
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 CO
2 due to the lower atmospheric CO
2 (see
partial pressure
In a mixture of gases, each constituent gas has a partial pressure which is the notional pressure of that constituent gas as if it alone occupied the entire volume of the original mixture at the same temperature. The total pressure of an ideal g ...
), resulting in an increase in pH. Since carbonate solubility decreases with increased pH, precipitation is induced. Supersaturation may be enhanced by factors leading to a reduction in CO
2, for example increased air-water interactions at waterfalls may be important, as may photosynthesis.
Recently it has been demonstrated that microbially induced precipitation may be more important than physico-chemical precipitation. Pedley et al. (2009) showed with flume experiments that precipitation does not occur unless a
biofilm
A biofilm comprises any syntrophic consortium of microorganisms in which cells stick to each other and often also to a surface. These adherent cells become embedded within a slimy extracellular matrix that is composed of extracellular po ...
is present, despite supersaturation.
Calcite is the dominant mineral precipitate, followed by the polymorph
aragonite.
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
The Basturs Lakes are two lakes located n the northern foothills of “Mont de Conques” and close to the villages of Basturs and Sant Romà d'Abella, belonging to the municipality of Isona and Conca Dellà, in Pallars Jussà, Lleida, Catalonia, ...
,
Pallars Jussà,
Catalonia
Catalonia (; ca, Catalunya ; Aranese Occitan: ''Catalonha'' ; es, Cataluña ) is an autonomous community of Spain, designated as a '' nationality'' by its Statute of Autonomy.
Most of the territory (except the Val d'Aran) lies on the no ...
- tufa mounds
*Various parts of Armenia, such as
Artik
Artik (Armenian: ), is a town and urban municipal community in the Shirak Province of Armenia. As of the 2011 census, the town had a population of 19,534. As per the 2016 official estimate, the population of Artik is around 18,800.
Artik is famou ...
*The southwestern coastline of
Western Australia
Western Australia (commonly abbreviated as WA) is a state of Australia occupying the western percent of the land area of Australia excluding external territories. It is bounded by the Indian Ocean to the north and west, the Southern Ocean to t ...
*Near
Groot Marico in the
North West Province, South Africa
*The
Kadishi Tufa
The Blyde River Canyon is a 26km long Canyon located in Mpumalanga, South Africa. It is the one of the larger canyons on Earth but much smaller than those of Asia, the Grand Canyon and the Fish River Canyon. Unlike the Grand and Fish River Canyon ...
,
Blyde River Canyon
The Blyde River Canyon is a 26km long Canyon located in Mpumalanga, South Africa. It is the one of the larger canyons on Earth but much smaller than those of Asia, the Grand Canyon and the Fish River Canyon. Unlike the Grand and Fish River Canyo ...
,
Mpumalanga Province
Mpumalanga () is a province of South Africa. The name means "East", or literally "The Place Where the Sun Rises" in the Swazi, Xhosa, Ndebele and Zulu languages. Mpumalanga lies in eastern South Africa, bordering Eswatini and Mozambique. I ...
, South Africa
Some sources suggest that "tufa" was used as the primary building material for most of the châteaux of the
Loire Valley, France. This results from a mis-translation of the terms "
tuffeau
Tuffeau stone — in French, simply ''tuffeau'' or ''tufeau'' — is a local limestone of the Loire Valley of France. It is characterized as a chalky or sandy, fine-grained limestone, white to yellowish-cream in appearance, and micaceous (con ...
jaune" and "tuffeau blanc", which are porous varieties of the
Late Cretaceous
The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', ...
marine limestone known as
chalk
Chalk is a soft, white, porous, sedimentary carbonate rock. It is a form of limestone composed of the mineral calcite and originally formed deep under the sea by the compression of microscopic plankton that had settled to the sea floor. C ...
.
Uses
Tufa is occasionally shaped into a planter. Its porous consistency makes it ideal for
alpine gardens. A concrete mixture called
hypertufa
Hypertufa is an anthropic rock made from various aggregates bonded together using Portland cement.
Hypertufa is intended as a manufactured substitute for natural tufa, which is a slowly precipitated limestone rock; being very porous, it is favor ...
is used for similar purposes.
See also
*
References
External links
{{commons+cat
Mono Lake Committee: "Tufa"