Pamukkale: Hot Springs and White Travertines

Pamukkale, located in Denizli Province in southwestern Turkey, is a prominent example of active travertine deposition. Its name, meaning “Cotton Castle,” refers to the extensive white carbonate terraces formed by mineral‐rich thermal springs. The site, together with the adjacent archaeological remains of Hierapolis, was inscribed as a UNESCO World Heritage Site in 1988.

Pamukkale's white travertine terraces and natural hot spring pools.

The Travertine terraces at Pamukkale are created through the precipitation of calcium carbonate (CaCO₃) from thermal spring water. This process unfolds as follows:

Thermal water, containing dissolved calcium bicarbonate (Ca(HCO₃)₂), emerges from springs at elevated temperatures.

As the water flows downhill and cools, carbon dioxide (CO₂) is released into the atmosphere.

The loss of CO₂ shifts the chemical equilibrium, prompting calcium carbonate to precipitate and form solid, layered deposits.

Over time, these deposits accumulate into the stepped terraces that define Pamukkale’s landscape. The site lies within the Aegean extensional province, along the northern margin of the Denizli Basin, where active faulting facilitates the circulation of geothermal waters. Fissure networks channel these waters to the surface, contributing to the ongoing formation of travertine.

Pamukkale: Amazing Geological Wonder. Photo: Brocken Inaglory

Hydrothermal System and Water Chemistry

Pamukkale’s hydrothermal system comprises seventeen principal springs, with surface temperatures between 35°C and 56°C, though some reach up to 100°C at greater depths. These waters are alkaline and rich in carbon dioxide (CO₂), maintaining equilibrium with deep carbonate host rocks. The water ascends through karstified Mesozoic limestone aquifers, dissolving significant quantities of calcium carbonate under high CO₂ partial pressure. Upon surfacing, rapid CO₂ degassing alters the carbonate equilibrium, prompting the precipitation of calcium carbonate as travertine.

Quantitative analysis reveals the water’s mineral content shifts as it flows across the terraces:

• CO₂ decreases from 725 mg/l at the source to 145 mg/l.
• Calcium carbonate drops from 1200 mg/l to 400 mg/l.
• Calcium reduces from 576.8 mg/l to 376.6 mg/l.

For each liter of water, 499.9 mg of calcium carbonate is deposited. With an average flow rate of 465.2 liters per second, substantial daily deposition occurs, theoretically capable of whitening up to 13,584 square meters per day, though actual coverage is lower due to practical constraints. The mineral-rich springs, containing calcium and magnesium, have historically been linked to therapeutic effects for ailments such as arthritis and skin disorders.

Historically, the mineral content of these springs has been associated with therapeutic effects for conditions such as arthritis and skin disorders. These associations stem from traditional accounts rather than contemporary scientific evidence. Chemical analyses reveal that the water’s composition changes as it flows, with calcium carbonate levels decreasing significantly, contributing to the substantial deposition observed at the site.

Historical Significance of Hierapolis

Adjacent to the travertine terraces lies Hierapolis, established in the 2nd century BCE as a Roman spa city. This ancient settlement capitalized on the therapeutic properties of the thermal springs. Key structures include:

• A Roman theater with a capacity of 15,000 seats, notable for its preservation.
• A necropolis extending over 2 km, featuring a variety of sarcophagi and tomb designs.
• An archaeological museum housed within a restored Roman bathhouse.
• The Plutonium, a cave historically associated with the god Pluto due to its CO₂ emissions.