Travertines of Pamukkale, Turkey

Background

In Turkey’s inner Aegean region, in the Denizli basin, within the Menderes valley, lays Pamukkale, or ‘Cotton Castle’. The ancient Greco-Roman and Byzantine city of Hierapolis is located on top of Pamukkale. Hierapolis means ‘sacred city’ and was founded by the prominent historical figure Apollo. This landform is a very popular tourist destination, as it has a very unique feature – terraces of carbonate minerals, travertine, which is caused by the flow and evaporation of hot water springs. This gives the whole area a distinct white colour, hence the name Cotton Castle. Each year thousands of people visit this area, making it a very well-known landform/historical site. This article will be discussing the composition and structure of this landform, the process by which it was formed, how it interacts with other landforms or systems and finally will discuss how prevalent this type of landform is around the world.

Location

Pamukkale is located around 20km north of the town of Denizli, which is located in the Denizli basin. The Denizli basin is approximately 70 km long and 50 km wide, within the fault bounded Neogene–Quaternary depression in the Western Anatolian Extensional Province, at the junction of the Büyük Menderes Graben and Gediz Graben. This area is one of the most seismically active areas in the world and diverging plate tectonics has this area moving by around 30-40mm per year 1). This tectonic movement has been going on since the Early Miocene era. It is reported that the travertine masses in the basin have formed where dip-slip normal fault segments show step-over zones along the fault-strikes. The thermal springs of the Pamukkale area begin in the Çürüksu graben, which belongs to the Menderes graben system. The travertines at Pamukkale are on a plateau which is 300m high, and can be seen from up to 40km away due to its snow white appearance. They are formed in cascading pools which step down in a balcony like shape. Metamorphic basement rocks surround the plateau, however other rock types can be found in the area. They are neogene clastic sedimentary rocks, with associated shelly limestone, colluvial and fluvial quaternary sedimentary rocks and quaternary travertine 2).

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Pamukkale travertines, Turkey, eastern face Source: Author

Formation

There are also many ridge-fissure travertines in the same area; however these fissures do not create the same terraces that are seen in Pamukkale. Tectonic movement during the Upper Miocene and Quaternary periods are thought to have created the ridge fissures and rock forms found in the area. The Hellenic shear zone has transformed from the North Anatolian transform, which stops the west movement of the Anatolian plate causing an east-west compression in the West Anatolian and Aegean regions. The subduction of the East Mediterranean lithosphere under the Anatolian plate caused the uplift of the Menderes massif which allowed magma to flow into the massif. This resulted in the formation of the graben systems. Nearly all the faults in the area which are peripheral to the grabens are listric faults which cause greater lateral extension than what would be found in normal faults. It is estimated that the lateral extension is around 50% 3).

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The Denizli Basin, Turkey Source:Author

The travertine terraces of Pamukkale have been forming for the last 400,000 years. Many definitions have been given to describe travertine, with “formed by evaporation of spring and river waters” and “a kind of calc-tufa deposited by certain hot springs” 4). There are five types of morphological travertine masses at Pamukkale, terraced-mound travertines, fissure-ridge travertines, range-front travertines, eroded-sheet travertines; and self-built channel travertines 5). The aquifers which are multi-layered, which are made up of carbonate layers from different eras, such as Paleozoic marble, Pliocene limestone and Quaternary travertine. The groundwater is generally recharged by water which is meteoric 6).

Travertine is a terrestrial sedimentary rock, which is a form of limestone, and is mainly composed of calcium bicarbonate (CaHCO3). It is usually fibrous in appearance and is mainly light in colour, such as white, light tan or cream. These travertines are formed by hot water which comes from an open fault fissure. The temperature of the water can vary from between 35 degrees Celsius to 60 degrees Celsius 7). The water, which is super-saturated with calcium carbonate, flows to the surface where it undergoes a rapid loss of carbon dioxide due to evaporation, which then causes the deposition of the travertine. Initially the travertine is soft; almost jelly like in form, but eventually hardens into travertine. If the flow of water ceases, the travertine will start to take on a greyish-black colour 8). To avoid such occurrences, during times of reduced water flow, local authorities will block some channels in order to continue the flow into certain terraces. This process is then rotated through the different areas on the plateau, in order to keep all of the area in its uniquely white colour.

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Travertine deposits at Pamukkale, Turkey Source: Author

Travertine has long been used as a building material, thought to have been used for over 2000 years. Although there is now a sound understanding as to how travertine is formed, little information can be found on the subject before the 17th century. In the first half of the 19th century research concluded that the amount of calcium carbonate dissolved by carbon dioxide could be measured by simple equations which remained constant at given pressures and temperatures 9). It is believed that there are four chemical processes that lead to travertine formation. There needs to be a degassing of the groundwater which is rich in carbon dioxide, and generally the calcium levels need to be greater than 80 parts per million. In order for groundwater to deposit travertine, dissolved carbon dioxide (carbonic acid) reacts with carbonate rocks which results in the formation of a solution which contains bicarbonate and calcium ions (calcium bicarbonate). The first process equation is as follows –

CaCO3 + CO2 + H2O = Ca2+ + 2(HCO3)-

The above process must be reversed for deposition of travertine. Some travertines are formed by a reaction between hyperalkaline groundwater and atmospheric carbon dioxide as follows-

CaOH2 + CO2 = CaCO3 + H2O

The above process is usually observed in areas that are either undergoing serpentinazation or areas that have contact with calcium hydroxide. This process is thought to be widely distributed but uncommon.

The process which is described as groundwater alkalisation can occur when groundwater which is rich in calcium reacts with surface water which is alkaline. This reaction is as follows-

Ca (HCO3) + OH- = CaCO3 + HCO3- + H2O

The fourth process is called the ‘common ion effect’. Groundwater is infiltrated by gypsum or anhydrite (CaSO4). When combined with Ca-bicarbonate water, the Ca is elevated to surpass the solubility of calcite which leads to the precipitation process 10). This process then leaves deposits of calcium carbonate or calcium bicarbonate which harden to form the travertine that we witness.

Other Examples Worldwide

There are many examples of travertines around the world. In Italy, there are two large travertine quarries in Tivoli and Montecelio. Cascading travertines or travertine dams can also be found in such places as Huanglong in the Sichuan province of China, Mammoth Hot Springs at Yellowstone National Park, USA, Abbass Abad, Atash Kooh, and Badab-e Surt in Iran, Egerszalok, Hungary, Lagunas de Ruidera, Spain and Band-I-Amir, Afghanistan 11). Whilst many of the above locations boast large deposits of travertine, none seem to rival the visual appearance of Pamukkale, which would explain the vast number of tourists and historical enthusiasts that visit the location each year. Perhaps the closest resemblance comes from the travertine deposits at Mammoth Hot Springs at Yellowstone National Park, USA, where the deposits appear to have taken on the form of a natural waterfall.

Conclusion

This article has discussed the travertine deposits at the site in question, Pamukkale, Turkey. The geological location of the deposits, which is in the Denizli basin, is shown to be an area which is very active seismically. The various rock forms which are found in the area were discussed, which are made up mainly of sedimentary rocks, limestone and travertine. Various tectonic movements over the years have shaped the different valleys, basins and grabens in the area and the many faults contribute to the flow of calcium rich groundwater. The reduction of groundwater flow can lead to the darkening colour of the travertine, and certain measures have been taken at Pamukkale in order to avoid this. Travertine deposits can be formed in various ways, however there needs to be certain factors to be present, such as calcium. The evaporation of the groundwater via degassing of carbon dioxide leaves deposits of calcium carbonate or calcium bicarbonate which hardens into what we know as travertine. There are many other deposits of travertine around the world, and examples of locations were given in the discussion. Sites such as Pamukkale are important in order to gain knowledge about landforms and earth sciences/systems. It is vital that such sites are kept in their natural form as possible, so that the future generations can appreciate and learn from them.


Earth Sciences | Geology | Turkey

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11)
Ford, T.D. & Pedley, H.M. 1996, ‘A review of tufa and travertine deposits of the world’, Earth-Science reviews, vol.41, pp.117-175

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