Pamukkale, Turkish for "cotton castle", is a hillside of white travertine in Denizli Province in south-western Turkey, built by calcium-rich hot springs that rise along active faults on the north side of the Çürüksu (ancient Lycus) valley. The deposit is about 2,700 m long, 600 m wide and 160 m high, white enough to be seen from Denizli 20 km away, and it is fed by 17 springs between 35 °C and 100 °C. The Greek and Roman spa city of Hierapolis was built on its upper surface, and the two were inscribed together as a UNESCO World Heritage Site in 1988. Everything at Pamukkale, from the shape of the terraces to the reason a city stood there, follows from one sequence: a rift opened, faults let hot water up, and the water turned into rock.
The Büyük Menderes Graben and a Rift Opened in the Miocene
The story starts with western Turkey being pulled apart. Since the early Miocene the crust of the Aegean region has been stretched north to south, a process geologists connect to the rollback of the Hellenic subduction zone south of Crete, where the African plate sinks beneath the Aegean at roughly 40 mm a year. Stretched crust breaks into long blocks: some drop to form valleys called grabens, while others stand up as ridges called horsts. Western Anatolia is ribbed with them, and many of the faults that bound them trend roughly east to west.
The largest of these troughs near Pamukkale is the Büyük Menderes Graben, an active rift about 140 km long and 2.5 to 14 km wide that formed in the early Miocene and is still moving. Its faults produced the magnitude 7.5 East Smyrna earthquake of 1653 and the 1899 Aydın–Denizli earthquake. Through it flows the Büyük Menderes, the classical Maeander, a river so sinuous that its name became the English word for a winding bend. Pamukkale sits inland of it, on the Çürüksu, the ancient Lycus, a tributary of the Büyük Menderes. The Çürüksu basin is itself a down-dropped block, with Denizli and the ruins of Laodicea on its southern side and the travertine shelf on its northern edge, where the valley floor meets the slopes of Çökelez Mountain.
Faults Under Hierapolis and the Gas at Pluto’s Gate
Where a graben margin fault cuts the surface it opens a path for groundwater to sink deep, heat up and return. At Pamukkale the water that comes back has been loaded with carbon dioxide. That gas is the key ingredient: water charged with CO₂ is a weak acid, and on its way up it dissolves calcium carbonate from the limestone and marble it passes through. Geologists call the rock that results thermogene travertine, because its carbon dioxide comes from deep sources such as heated rock or magma rather than from soil.
The fault line itself can still be found inside the ancient city. In 1998 Luigi Piccardi, a geologist with Italy’s National Research Council, showed that the Ploutonion of Hierapolis and the nearby oracle of Apollo were deliberately built on the surface trace of a seismic fault. The Ploutonion, or Pluto’s Gate, is a small cave over a cleft through which hot water runs, and it still releases enough carbon dioxide to kill. Italian archaeologists, who found the site in 1965 and dug it again in 2013 under Francesco D’Andria of the University of Salento, recorded birds suffocating as they flew into the warm air. Ancient writers including Strabo and Cassius Dio described the same effect, and the priests of Cybele were believed to be the only people who could enter and survive.
Seventeen Springs and a 320-Metre Channel
Seventeen hot springs feed the site today, ranging from about 35 °C to 100 °C. They do not all emerge at the cliff edge. The main flow is carried 320 m to the head of the terraces, and from there it spreads out and drops down the slope. The water that bathers use in Cleopatra’s Pool, inside the Hierapolis grounds, gives an idea of its chemistry: 36 to 57 °C, a slightly acidic pH of 5.8, rich in bicarbonate, sulphate and dissolved carbon dioxide, with traces of iron and radon.
Other springs along the same fault zone behave differently. At Karahayıt, about 5 km north of Pamukkale, the thermal water runs red rather than white. At Kaklık Cave, roughly 30 km east of Denizli, travertine has formed underground around a small thermal lake, and locals call it "small Pamukkale". Springs and deposits repeat along the valley because Pamukkale is one outlet of a regional plumbing system, not an isolated fountain.
How Calcium Carbonate Becomes the Cotton Castle
When the water reaches the surface it is supersaturated with calcium carbonate. As it flows out in thin sheets, carbon dioxide escapes into the air, the water can no longer hold its dissolved load, and calcite comes out of solution. It is laid down first as a soft gel that later crystallises into hard travertine. Wherever the flow slows at a lip or a ridge, more mineral is deposited there, so the rims grow upward and trap shallow pools behind them. That feedback produces the stepped basins and frozen-looking cascades that give the site its name.
Active deposition at any one time is concentrated on a strip 60 to 70 m long and 24 to 30 m wide, not across the whole 2,700 m front. Because only part of the front receives water at any one time, some terraces are brilliant white and water-filled while others stand dry. Travertine of this kind forms wherever deep CO₂ meets carbonate rock: Mammoth Hot Springs in Yellowstone, Huanglong in China and Badab-e Surt in Iran are other examples. Turkey also quarries the stone commercially. Of 17,808 tonnes of travertine imported by the United States in 2019, 12,804 tonnes came from Turkey.
Building on the Travertine: Hierapolis from the 2nd Century BC
The hot water and the gas drew people long before the Greeks. The Phrygians built a temple to the mother goddess Cybele over the gas cave, probably in the first half of the 7th century BC, and Iron Age round huts found in 2016 show a settlement on the slopes of Çökelez above the Lycus plain. Hierapolis was founded by the Seleucid kings, traditionally in the reign of Antiochus I (281–261 BC), and it was a working spa by early in the 2nd century BC. After the Roman victory over Antiochus III at Magnesia in 190 BC and the Treaty of Apamea, the region passed to Eumenes II of Pergamon, and in 133 BC Attalus III bequeathed his kingdom to Rome, making Hierapolis part of the province of Asia.
The city’s layout followed the geology. Its main street ran about 1,500 m from north to south, 13.5 m wide, close to the cliff edge of the travertine, and the Frontinus Gate near its northern end was built by the proconsul Julius Frontinus in AD 84–86. The city lay on the road from Iconium to Ephesus, about 10 km north of Laodicea across the valley, and it became known for wool, dyes and the cures offered at more than fifteen baths. Its necropolis of about 1,200 tombs, most built of local limestone, stretches for more than 2 km along the roads into the city, a sign of how many patrons came to the springs and retired or died there. The philosopher Epictetus was born there as a slave around AD 50, and Christian tradition places the death of the apostle Philip in the city around AD 80.
Earthquakes of AD 17, AD 60 and the 7th Century
The same faults that supply the water also shake the city. A major earthquake destroyed Hierapolis in AD 17, under Tiberius, and an even more severe one in AD 60, under Nero, left it in ruins. Rebuilt with imperial money, the city reached the form visible today: a theatre with a 91 m façade and 50 rows of seats, built for Hadrian’s visit in 129 and renovated under Septimius Severus; two Roman baths; a gymnasium; and a fountain at the hot spring. Caracalla granted the city the title of neocoros in 215, and its population is thought to have reached about 100,000.
In the early 7th century Persian armies and another destructive earthquake broke the city again. That quake threw the marble Ionic portico of the sacred spring into the water, creating the Antique Pool. Hierapolis never fully recovered. The Seljuks took the area in the 12th century, a castle was built in the 13th, and the settlement was abandoned by the late 14th century. Over the following centuries the springs kept running, and travertine slowly crept over the ruins, burying parts of the city under a crust of its own building stone.
Hotels, Closures and the 1988 UNESCO Listing
The German archaeologist Carl Humann made the first excavations in June and July 1887, and an Italian mission has worked at Hierapolis since 1957, when Paolo Verzone began systematic digging. The bigger threat came with tourism in the 20th century. Hotels were built on top of the travertine and among the ruins, drawing off spring water for their pools and damaging both the ancient city and the white terraces. Those buildings have since been removed. The hot-water pool of one hotel was kept, and visitors now pay to swim among the fallen stonework of the Antique Pool.
Today more than two million people a year visit the terraces, bathing only in designated thermal pools. Water flow is regulated and sections are periodically closed so that new travertine can cover stained or worn surfaces. The arrangement is a deliberate copy of the natural process: by moving the outflow, managers let the springs do what they did for thousands of years before the hotels arrived. For comparison with another spring whose colours come from biology rather than chemistry, see our account of Grand Prismatic Spring in Yellowstone; for the basics of how groundwater is heated and returned, see what a hot spring is; and for the city at the Aegean end of the Iconium road, read about Ephesus.