Tufa
Encyclopedia
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
, formed by the precipitation
Precipitation (chemistry)
Precipitation is the formation of a solid in a solution or inside anothersolid during a chemical reaction or by diffusion in a solid. When the reaction occurs in a liquid, the solid formed is called the precipitate, or when compacted by a centrifuge, a pellet. The liquid remaining above the solid...
of carbonate minerals
Carbonate minerals
Carbonate minerals are those minerals containing the carbonate ion: CO32-.-Anhydrous carbonates:*Calcite group: Trigonal**Calcite CaCO3**Gaspeite CO3**Magnesite MgCO3**Otavite CdCO3**Rhodochrosite MnCO3**Siderite FeCO3**Smithsonite ZnCO3...
from ambient temperature water bodies. Geothermally heated hot-springs
Hot spring
A hot spring is a spring that is produced by the emergence of geothermally heated groundwater from the Earth's crust. There are geothermal hot springs in many locations all over the crust of the earth.-Definitions:...
sometimes produce similar (but less porous) carbonate deposits known as travertine
Travertine
Travertine is a form of limestone deposited by mineral springs, especially hot springs. Travertine often has a fibrous or concentric appearance and exists in white, tan, and cream-colored varieties. It is formed by a process of rapid precipitation of calcium carbonate, often at the mouth of a hot...
. Tufa is sometimes referred to as (meteogene) travertine; care must be taken when searching through literature to prevent confusion with hot spring (thermogene) travertine. Calcareous
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:...
tufa should not be confused with tuff
Tuff
Tuff is a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption. Tuff is sometimes called tufa, particularly when used as construction material, although tufa also refers to a quite different rock. Rock that contains greater than 50% tuff is considered...
, a porous volcanic rock
Volcanic rock
Volcanic rock is a rock formed from magma erupted from a volcano. In other words, it is an igneous rock of volcanic origin...
with parallel etymological
Etymology
Etymology is the study of the history of words, their origins, and how their form and meaning have changed over time.For languages with a long written history, etymologists make use of texts in these languages and texts about the languages to gather knowledge about how words were used during...
origins.
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 settings. 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 petrographicallyPetrography
Petrography is a branch of petrology that focuses on detailed descriptions of rocks. Someone who studies petrography is called a petrographer. The mineral content and the textural relationships within the rock are described in detail. Petrographic descriptions start with the field notes at the...
). Pedley (1990) provides an extensive classification system, which includes the following classes of fluvial tufa:-
- Spring - Deposits form on emergence from a spring/seepSpring (hydrosphere)A spring—also known as a rising or resurgence—is a component of the hydrosphere. Specifically, it is any natural situation where water flows to the surface of the earth from underground...
. Morphology can vary from mineratrophic wetlands to spring aprons (see calcareous sinterCalcareous sinterCalcareous sinter is a freshwater calcium carbonate deposit, also known as calc-sinter. Deposits are characterised by low porosity and well-developed lamination. Calcareous sinter should not be confused with siliceous sinter, which the term sinter more frequently refers to...
) - Braided channel - Deposits form within a fluvial channel, dominated by oncoids (see oncoliteOncoliteOncolites are sedimentary structures formed out of oncoids, which are layered spherical growth structures formed by cyanobacterial growth. Oncolites are very similar to stromatolites, but instead of forming columns they form approximately spherical structures...
) - Cascade - Deposits form at waterfalls, deposition is focussed 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 build up phytoherms (freshwater reefs) and stromatoliteStromatolite
Stromatolites or stromatoliths are layered accretionary structures formed in shallow water by the trapping, binding and cementation of sedimentary grains by biofilms of microorganisms, especially cyanobacteria ....
s. Oncoids are also common in these environments.
Other deposits
While fluvial and lacustrine systems make up the bulk of tufa systems worldwide, there are several other important tufa environments.Calcareous Sinter
Although sometimes regarded as a distinct carbonate deposit, calcareous sinterCalcareous sinter
Calcareous sinter is a freshwater calcium carbonate deposit, also known as calc-sinter. Deposits are characterised by low porosity and well-developed lamination. Calcareous sinter should not be confused with siliceous sinter, which the term sinter more frequently refers to...
formed from ambient temperature water can be considered a sub-type of tufa.
Speleothems
Calcareous speleothemSpeleothem
A speleothem , commonly known as a cave formation, is a secondary mineral deposit formed in a cave. Speleothems are typically formed in limestone or dolostone solutional caves.-Origin and composition:...
s may be regarded as a form of calcareous sinter
Calcareous sinter
Calcareous sinter is a freshwater calcium carbonate deposit, also known as calc-sinter. Deposits are characterised by low porosity and well-developed lamination. Calcareous sinter should not be confused with siliceous sinter, which the term sinter more frequently refers to...
. They lack any significant macrophyte
Macrophyte
A macrophyte is an aquatic plant that grows in or near water and is either emergent, submergent, or floating. In lakes macrophytes provide cover for fish and substrate for aquatic invertebrates, produce oxygen, and act as food for some fish and wildlife....
component due to the absence of light, for this reason they are often morphologically closer to travertine
Travertine
Travertine is a form of limestone deposited by mineral springs, especially hot springs. Travertine often has a fibrous or concentric appearance and exists in white, tan, and cream-colored varieties. It is formed by a process of rapid precipitation of calcium carbonate, often at the mouth of a hot...
or calcareous sinter
Calcareous sinter
Calcareous sinter is a freshwater calcium carbonate deposit, also known as calc-sinter. Deposits are characterised by low porosity and well-developed lamination. Calcareous sinter should not be confused with siliceous sinter, which the term sinter more frequently refers to...
.
Tufa columns
Tufa columns are an unusual form of tufa typically associated with saline lakesSalt lake
A salt lake or saline lake is a landlocked body of water which has a concentration of salts and other dissolved minerals significantly higher than most lakes . In some cases, salt lakes have a higher concentration of salt than sea water, but such lakes would also be termed hypersaline lakes...
. They are distinct from most tufa deposits in that they lack any significant macrophyte
Macrophyte
A macrophyte is an aquatic plant that grows in or near water and is either emergent, submergent, or floating. In lakes macrophytes provide cover for fish and substrate for aquatic invertebrates, produce oxygen, and act as food for some fish and wildlife....
component; this is due to the salinity excluding mesophilic organisms
Mesophile
A mesophile is an organism that grows best in moderate temperature, neither too hot nor too cold, typically between 25 and 40 °C...
. Some tufa columns may actually form from hot-springs and therefore actually be a form of travertine
Travertine
Travertine is a form of limestone deposited by mineral springs, especially hot springs. Travertine often has a fibrous or concentric appearance and exists in white, tan, and cream-colored varieties. It is formed by a process of rapid precipitation of calcium carbonate, often at the mouth of a hot...
. 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.
Biology
Tufa deposits form an important habitat for a diverse flora. BryophyteBryophyte
Bryophyte is a traditional name used to refer to all embryophytes that do not have true vascular tissue and are therefore called 'non-vascular plants'. Some bryophytes do have specialized tissues for the transport of water; however since these do not contain lignin, they are not considered to be...
s (non-vascular land plants) and diatom
Diatom
Diatoms are a major group of algae, and are one of the most common types of phytoplankton. Most diatoms are unicellular, although they can exist as colonies in the shape of filaments or ribbons , fans , zigzags , or stellate colonies . Diatoms are producers within the food chain...
s are well represented. The porosity of the deposits creates a wet habitat ideal for these plants.
Geochemistry
Modern tufa is formed from supersaturated alkaline waters, with raised pCO2. On emergence, waters degas CO2 due to the lower atmospheric pCO2 (see partial pressurePartial pressure
In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....
), 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 pCO2, 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
Biofilm
A biofilm is an aggregate of microorganisms in which cells adhere to each other on a surface. These adherent cells are frequently embedded within a self-produced matrix of extracellular polymeric substance...
is present, despite supersaturation.
Calcite
Calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
is the dominant mineral precipitate found; however, the polymorph aragonite
Aragonite
Aragonite is a carbonate mineral, one of the two common, naturally occurring, crystal forms of calcium carbonate, CaCO3...
is also found.
Occurrence
Tufa is common in many parts of the world. Some notable deposits include:-- Pyramid Lake, Nevada, USA - tufa formations
- Mono LakeMono LakeMono Lake is a large, shallow saline lake in Mono County, California, formed at least 760,000 years ago as a terminal lake in a basin that has no outlet to the ocean...
, California, USA- tufa columns - Trona PinnaclesTrona PinnaclesTrona PinnaclesDesignationNational Natural LandmarkLocationCaliforniaNearest CityTrona, CaliforniaCoordinatesArea Date of Establishment1968Governing BodyBureau of Land Management...
, California, USA - tufa columns - North Dock TufaNorth Dock TufaThe North Dock Tufa is a small dome-shaped petrified well, with stalactites and stalagmites, tucked away behind the Marine Activities Centre at Sunderland marina...
, United Kingdom - Plitvice Lakes National Park, CroatiaCroatiaCroatia , officially the Republic of Croatia , is a unitary democratic parliamentary republic in Europe at the crossroads of the Mitteleuropa, the Balkans, and the Mediterranean. Its capital and largest city is Zagreb. The country is divided into 20 counties and the city of Zagreb. Croatia covers ...
- Various parts of ArmeniaArmeniaArmenia , officially the Republic of Armenia , is a landlocked mountainous country in the Caucasus region of Eurasia...
, such as AshtarakAshtarakAshtarak is an industrial city in Armenia, on the left bank of Kasagh River along the gorge, approximately 20 km north-west of the capital Yerevan, at a height of 1110 meters above sea level. It is the administrative capital of Aragatsotn province...
Some sources suggest that "tufa" was used as the primary building material for most of the châteaux of the Loire Valley
Loire Valley
The Loire Valley , spanning , is located in the middle stretch of the Loire River in central France. Its area comprises approximately . It is referred to as the Cradle of the French Language, and the Garden of France due to the abundance of vineyards, fruit orchards, and artichoke, asparagus, and...
, France. This results from a mis-translation of the terms "tuffeau jaune" and "tuffeau blanc", which are porous varieties of the Late Cretaceous
Late Cretaceous
The Late Cretaceous is the younger of two epochs into which the Cretaceous period is divided in the geologic timescale. Rock strata from this epoch form the Upper Cretaceous series...
marine limestone known as chalk
Chalk
Chalk is a soft, white, porous sedimentary rock, a form of limestone composed of the mineral calcite. Calcite is calcium carbonate or CaCO3. It forms under reasonably deep marine conditions from the gradual accumulation of minute calcite plates shed from micro-organisms called coccolithophores....
.
Uses
Tufa is today occasionally shaped into a planter. Its porous consistency makes tufa ideal for alpine gardens. A concrete mixture called hypertufaHypertufa
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 favorable for plant growth.Hypertufa is popular for making...
is used for similar purposes.