travertine

Travertine terraces at Mammoth Hot Springs, Yellowstone National Park

Calcium-carbonate-encrusted, yet growing moss, early stage of porous travertine formation.

Travertine is a terrestrial sedimentary rock, formed by the precipitation of carbonate minerals from geothermally heated hot-springs. Similar (but more porous) deposits formed from ambient temperature water are known as tufa.

Features

Travertine forms from geothermal springs and is often linked to siliceous systems which form siliceous sinter. Macrophytes, bryophytes, algae, cyanobacteria and other organisms often colonise the surface of travertine and are preserved, giving travertine its distinctive porosity.

Some springs have temperatures high enough to exclude macrophytes and bryophytes from the deposits, consequently, deposits are generally less porous than tufa. Thermophilic microbes are important in these environments and stromatolitic fabrics are common. When deposits are apparently devoid of any biological component, they are often referred to as calcareous sinter.

Geochemistry

Modern travertine is formed from geothermally heated supersaturated alkaline waters, with raised pCO₂ (see partial pressure). On emergence, waters degas CO₂ due to the lower atmospheric pCO₂, resulting in an increase in pH. Since carbonate solubility decreases with increased pH,^[1] 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,^[2] as may photosynthesis.^[3] Precipitation may also be enhanced by evaporation in some springs.

Both calcite and aragonite are found in hot spring travertines, aragonite is preferentially precipitated when temperatures are hot, while calcite dominates when temperatures are cooler.^[4][5] When pure and fine, travertine is white, but often it is brown to yellow due to impurities (other than carbonate minerals).

Travertine forming at Jupiter Terrace, Fountain Geyser Pool, Yellowstone National Park. Photo by Ansel Adams, 1941.

Travertine may precipate out directly onto rock and other inert materials as in Pamukkale or Yellowstone for example. Travertine may also precipate out onto growing moss as happens a great deal in Plitvice Lakes.

Stalagtites and Stalagmites are similar to travertine in the way they form and in their composition but there are differences. [2]

Occurrence

Travertine waterfalls exist not only in the U.S. in Oklahoma and Texas but most famously in Tivoli, Italy and Guidonia Montecelio, near Rome. Travertine derives its name from this town. Tivoli was known as Tibur in ancient Roman times. The ancient name for the stone was lapis tiburtinus, meaning tibur stone, which has been corrupted to travertine. Detailed studies of the Tivoli and Guidonia travertine deposits revealed diurnal and annual rhythmic banding and laminae, which have potential use in geochronology.^[6] These formations commonly occur in tropical and subtropical karst areas around the world.

In Central Europe's last postglacial palaeoclimatic optimum (Atlantic Period, 8000-5000 B.C.), huge deposits of tufa formed from karst springs. Important geotopes are found at the Swabian Alb, mainly in valleys at the foremost northwest ridge of the cuesta, in many valleys of the eroded periphery of the karstic Franconian Jura, at the northern Alpine foothills, and the northern Karst Alps. On a smaller scale these karst processes are still working. Travertine was a very important building material for housing and representative buildings since the Middle Ages.

Travertine has formed sixteen huge, natural dams in a valley in Croatia known as Plitvice Lakes National Park. The travertine clings to moss and rocks in the water, and has built up over several millennia to form waterfalls up to 70 m in height.^[7]

Other beautiful cascades of natural lakes formed behind travertine dams can be seen in Band-i-Amir (Afghanistan), HuangLong Valley (Sichuan, China), Semuc Champey (Guatemala), and Pamukkale (Turkey). Many geyser fields also have colorful travertine deposits.

In the U.S., the most well-known place for travertine formation are the geothermal areas of the Yellowstone National Park, which are rich in travertine deposits. In Oklahoma, there are two parks dedicated to this natural wonder. Turner Falls, the tallest waterfall in Oklahoma and one of the tallest in the U.S., is a 77-foot turquoise cascade of spring water flowing over a travertine cave under which one can swim. Honey Creek feeds this waterfall and creates miles of travertine shelves both up and downstream. There are many small waterfalls upstream in the dense woods, which repeat the travertine formation effect. The city of Davis now owns thousands of acres of this land and has made it a tourist attraction. Another travertine visage lies in Sulphur, Oklahoma, just 10 miles east of Turner Falls. Travertine Creek flows through a spring water nature preserve within the boundaries of the Chickasaw National Recreation Area. In Texas, the city of Austin and its surrounding "Hillcountry" to the south is built on limestone and is home to many travertine formations such as those found at Gorman Falls within Colorado Bend State Park, the nature preserve known as Hamilton Pool, and a nature spot in Spicewood known as Krause Springs.

Uses

Europe's longest castle in Burghausen, 1000 years old and mainly composed of travertine

Travertine in a 400 year old wall.

A carving in travertine

Travertine is often used as a building material. Rich deposits of aged, dried and hardened travertine were mined by the Romans. The largest building in the world constructed mostly of travertine is the Colosseum in Rome. Other notable buildings using travertine extensively include the Sacré-Cœur Basilica in Paris and the Getty Center in Los Angeles, California. The travertine used in the Getty Center construction was imported from Tivoli and Guidonia.^[8]

Travertine is one of several natural stones that are used for paving patios and garden paths. It is sometimes known as travertine limestone, sometimes as travertine marble; these are the same stone, even though it is neither limestone nor marble. The stone is characterised by pitted holes and troughs in its surface. Although these troughs occur naturally, they suggest to some eyes that considerable wear and tear has occurred over many years. Some installers use a grout to fill these holes, whereas others leave them open — travertine can even be purchased "filled" or "unfilled." It can be effectively polished to a smooth, shiny finish and comes in a variety of colors from grey to coral-red. Travertine is most commonly available in tile sizes for floor installations.

Travertine is one of the most frequently used stones in modern architecture, and is commonly seen as façade material, wall cladding, and flooring. The lobby walls of the modernist Willis Tower in Chicago, Illinois are travertine.^[9] Architect Welton Becket was one of the most frequent users of travertine, incorporating it extensively into many if not most of his projects. The entire first floor of the Becket-designed UCLA Medical Center has thick travertine walls.

The relative softness of the stone, combined with the holes and troughs, make travertine flooring particularly difficult to finish and maintain. Aggressive grinding — sometimes called honing — can reveal previously hidden air pockets that significantly change the look of the floor.

There are two or three small travertine producers in the western United States. U.S. demand for travertine is about 0.85 million tons, almost all of it imported. Most of the imports come from Turkey, Mexico, Italy, and Peru. A decade ago, Italy had a near monopoly on the world travertine market.

See also

• Calc-sinter
• Karst topography

References

External links

• New Mexico travertine study
• Minerals.net Aragonite

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