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 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 spring or in a limestone cave. In the latter, it can form stalactites, stalagmites, and other speleothems. It is frequently used in Italy and elsewhere as a building material.

Travertine is a terrestrial sedimentary rock, formed by the precipitation of carbonate minerals from solution in ground and surface waters, and/or geothermally heated hot-springs.^[1][2] Similar (but softer and extremely porous) deposits formed from ambient-temperature water are known as tufa.

Features

Travertine forms from geothermal springs and is often linked to siliceous systems that 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. As a consequence, deposits are, in general, less porous than tufa. Thermophilic microbes are important in these environments and stromatolitic fabrics are common. When it is apparent that deposits are 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,^[3] precipitation is induced. Precipitation may be enhanced by factors leading to a reduction in pCO₂, for example increased air-water interactions at waterfalls may be important,^[4] as may photosynthesis.^[5] 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.^[6][7] When pure and fine, travertine is white, but often it is brown to yellow due to impurities.

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

Travertine may precipitate out directly onto rock and other inert materials as in Pamukkale or Yellowstone for example.

Occurrence

Badab-e Surt's stepped travertine terrace formations. The red color of travertine terraces are due to iron carbonate

Travertine waterfalls exist not only in the U.S. in Oklahoma and Texas but most famously in Italy – in Tivoli and Guidonia Montecelio – where we can find the most important quarries since Ancient Roman times like the old quarry of Bernini in Guidonia.^[8] The latter has a major historic value, because it was one of the quarries that Gian Lorenzo Bernini selected material from to build the famous (colonnato di Piazza S.Pietro ) The Colonnade of St. Peter's Square in Rome in 1500. Michaelangelo also chose travertine as the material for the external ribs of the dome of St Peter's Basilica.^[9] Travertine derives its name from the former town, known as Tibur in ancient Roman times. The ancient name for the stone was lapis tiburtinus, meaning tibur stone, which was gradually 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.^[10]

Pamukkale's limestone wall

Pamukkale, in Turkey, is a World Heritage Site. A few other places in the world resemble it, including the Mammoth Hot Springs in the USA, Egerszalók in Hungary, and Huanglong in Sichuan Province of China (another UNESCO World Heritage Site).

In Central Europe's last post-glacial 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 has been an important building material since the Middle Ages.

Travertine has formed sixteen huge, natural dams in a valley in Croatia known as Plitvice Lakes National Park. Clinging to moss and rocks in the water, the travertine has built up over several millennia to form waterfalls up to 70 metres (230 ft) in height.^[11]

Cascades of natural lakes formed behind travertine dams can be seen in Mahallat, Abbass Abad, Atash Kooh, and Badab-e Surt in Iran; Pamukkale, Turkey; Band-i-Amir, Afghanistan; HuangLong Valley, Sichuan, China; Lagunas de Ruidera, Spain; and Semuc Champey, Guatemala.

In the U.S., the most well-known place for travertine formation is Yellowstone National Park, where the geothermal areas are rich in travertine deposits. Oklahoma has two parks are dedicated to this natural wonder. Turner Falls, the tallest waterfall in Oklahoma, is a 77 feet (23 m) cascade of spring water flowing over a travertine cave. Honey Creek feeds this waterfall and creates miles of travertine shelves both up and downstream. Many small waterfalls upstream in the dense woods 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 resource is in Sulphur, Oklahoma, 10 miles (16 km) 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. The area has many travertine formations, such as those found at Gorman Falls within Colorado Bend State Park, the nature preserve known as Hamilton Pool, the West Cave Preserve, and Krause Springs in Spicewood.

In North East Sulawesi, Indonesia, there are Wawolesea Karst with a pond several meters from the beach which is formed by salty hot water fountain since Neogene period.^[12]

Uses

Europe's longest castle in Burghausen, 1000 years old and built mainly with travertine

Travertine in a 400-year-old wall.

A carving in travertine

Travertine is often used as a building material. The Romans mined deposits of travertine for building temples, aqueducts, monuments, bath complexes, and amphitheaters such as the Colosseum, the largest building in the world constructed mostly of travertine.

Other notable buildings using travertine extensively include the Sacré-Cœur Basilica in Paris and the 20th-century Getty Center in Los Angeles, California, and Shell-Haus in Berlin. The travertine used in the Getty Center and Shell-Haus constructions was imported from Tivoli and Guidonia.^[13]

Travertine is one of several natural stones that are used for paving patios and garden paths. It is sometimes known as travertine limestone or travertine marble; these are the same stone, although travertine is classified properly as a type of limestone, not marble. The stone is characterised by pitted holes and troughs in its surface. Although these troughs occur naturally, they suggest signs of considerable wear and tear over time. Some installers use a grout to fill the holes, whereas others leave them open — travertine can be purchased "filled" or "unfilled." It can also be 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. It is a commonly use for façades, wall cladding, and flooring. The lobby walls of the modernist Willis Tower (1970) (formerly Sears Tower) in Chicago are made of travertine.^[14] Architect Welton Becket frequently incorporated travertine into many of his projects. The first floor of the Becket-designed UCLA Medical Center has thick travertine walls. Architect Ludwig Mies van der Rohe used travertine in several of his major works, including the Toronto-Dominion Centre, S.R. Crown Hall and the Farnsworth House.

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

Two or three small travertine producers operate 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 Italy, Iran, Mexico, Turkey, and Peru. A decade ago, Italy had a near-monopoly on the world travertine market.

See also

• Calc-sinter
• Karst topography
• List of types of limestone

References

External links

Wikimedia Commons has media related to: Travertine

• New Mexico travertine study
• Minerals.net Aragonite
• Information about Turkish beige Travertine
• Catalogue of Turkish travertines