Travertine Terraces: Pamukkale - Turkey

Pamukkale's terraces natural site in Denizli Province in southwestern Turkey are made of travertine, a sedimentary rock deposited by water from the hot springs.

The name Pamukkale itself means "cotton castle" in Turkish, and aptly describes the appearance of the terraces. They are formed by mineral-rich thermal waters that cascade down a steep hillside, depositing calcium carbonate over thousands of years. This process creates a series of cascading pools with a dazzling white travertine surface.

The terraces are formed by mineral-rich hot springs that have been cascading down the mountainside for millennia. The water is saturated with calcium carbonate, which hardens as it cools and creates a travertine deposit. Over time, these deposits have built up to create the stunning terraces we see today.

In this area, there are 17 hot water springs in which the temperature ranges from 35 °C (95 °F) to 100 °C (212 °F). The water that emerges from the spring is transported 320 metres (1,050 ft) to the head of the travertine terraces and deposits calcium carbonate on a section 60 to 70 metres (200 to 230 ft) long covering an expanse of 24 metres (79 ft) to 30 metres (98 ft).

When the water, supersaturated with calcium carbonate, reaches the surface, carbon dioxide de-gasses from it, and calcium carbonate is deposited. The depositing continues until the carbon dioxide in the water balances the carbon dioxide in the air. Calcium carbonate is deposited by the water as a soft jelly, but this eventually hardens into travertine.

This reaction is affected by the weather conditions, ambient temperature, and the flow duration. Precipitation continues until the carbon dioxide in the thermal water reaches equilibrium with the carbon dioxide in the atmosphere. Measurements made at the source of the springs find atmospheric levels of 725 mg/l carbon dioxide, by the time this water flows across the travertines, this figure falls to 145 mg/l. Likewise calcium carbonate falls from 1200 mg/l to 400 mg/l and calcium 576.8 mg/l to 376.6 mg/l. From these results it is calculated that 499.9 mg of CaCO₃ is deposited on the travertine for every liter of water.

This means that for a flow rate of 1 ı/s of water 43191 grams are deposited daily. The average density of a travertine is 1.48 g/cm3 implying a deposit of 29.2 dm3. Given that the average flow of the water is 465.2 l/s this implies that it can whiten 13,584 square metres (146,220 sq ft) a day, but in practice this area coverage is difficult to attain. These theoretical calculations indicate that up to 4.9 square kilometres (1.9 sq mi) it can be covered with a white deposit of 1 millimetre (0.039 in) thickness.