Covering a steep hillside in  inland Turkey, visible from space, is a bright white blanket of rock  more than a kilometer across.

But get closer, and you can see that it’s made of intricate rock formations, filled with warm milky white waters, like a partially petrified waterfall. This is Pamukkale.

Its striking appearance, along  with the supposedly curative waters of the hot springs that feed it,  have attracted people for millennia. But this place has more to offer  than just a few pics for the ‘gram. Scientists are now teasing out  Pamukkale’s petrified secrets, to look hundreds of thousands of  years back into Earth’s history. [♪INTRO] The unique site of Pamukkale  can be found in western Turkey, about 200 kilometers inland  from the Mediterranean coast.

You can spot it from more than 30 kilometers out. Its Turkish name translates to ‘cotton castle’, thanks to the snow-white terraces,  pools and wall-like structures that are stacked up the hillside  like a castle or opulent palace. Despite appearances, this  palace isn’t made from cotton, but rather a material called travertine.

Limestone rock typically forms  at the bottom of tropical oceans, and you often find fossils  of long dead sea creatures. Chemically, travertine is  the same basic substance. But this particular type of  limestone forms in the open air, and typically much faster than  your usual seafloor strata - accumulating up to several millimeters per day, depending on the location.

This hard, compact limestone rock is deposited when freshwater springs emerge from underground, carrying a high concentration of  dissolved calcium and carbonate ions. As the water evaporates, calcium  carbonate is left behind. Travertine is bright white  when it’s first deposited, but it turns a buff color when it dries out, and eventually weathers to gray or black.

It’s most often hot springs  that produce travertine, and Pamukkale is no exception. The site is located on the northern  margin of the Denizli Basin, which is an active fault zone. The floor of the basin to the  south has dropped along a series of geological faults, leaving a  roughly 100 meter-high escarpment.

Because of the stress and cracking of the rock, groundwater in the area  penetrates a long way down, passing through limestone  rocks that supply calcium, and coming close to molten magma deep underground. This magma not only heats  up the subterranean water, but it also supplies a ton  of extra dissolved carbonate. Magma itself is usually pretty gassy, and carbon dioxide dissolves  into the passing water.

And when limestone is heated up, extra carbon dioxide is baked out of the rock, and also dissolves in groundwater. So when the water makes its way back to the surface it’s about 35 degrees Celsius, just a little less than body temperature, and it’s packed with the  ingredients to make travertine. It emerges as springs from  fissures along the fault-line, and flows down the escarpment to the valley below, creating some pretty special  travertine formations along the way.

There are six main types of travertine structure, five of which can be found  at Turkey’s cotton castle, and all of which have been built  up over thousands of years. Let’s take a look at three of the coolest. Terraced mounds and pools are the  most visually striking formations, which draw people to this site from all over.

They’re created as water  emerges from individual springs and flows over the surface of a mound. Travertine growth starts around tiny changes in slope, twigs, stones and  other travertine structures. The springwater collects, and limestone  is deposited around its edges, creating and building a rim that encloses a pool.

These pools range from centimeters  to a few meters across. Water continuously flows from the springs, building a series of terraces made  from overlapping cup-shaped pools, often with stalactites like columns around their walls from the overflowing water. Next up, fissure-ridge  travertines form when springwater emerges from an elongated fissure  rather than a single point.

The limestone builds up in wedge-shaped layers, creating ridges either side of the fissure, and also filling up the fissure itself. Despite not being much to look at on the surface, they represent large, pretty consistent  deposits of the limestone rock, and they’ve been quarried for building and decorative stone since ancient times. Finally, self-built channels are a  structure that’s unique to Pamukkale, and in their own way they’re as  spectacular as the terraced pools.

They start as springwater  flowing in a narrow stream, which can either be natural, like the overflow from an existing travertine  pool or mound, or man-made, like the channels cut to  supply water for irrigation. In the stream, water flows  more slowly and turbulently at the edges compared to the middle, leading to continuous travertine  deposition on the channel sides. Over time, these elevated rims grow, creating steep-sided raised  channels up to 10 meters high.

They’re often wavy, following  the natural flow of water, but otherwise look just like the defensive walls of some ancient ruined castle. [midroll: now I don’t wanna interrupt  ‘flow’ too much, but here is a quick break.] Pamukkale is undoubtedly beautiful, but its intricate travertine formations also have a lot to tell us about the past. We know that the limestone has been accumulating here for at least 400,000 years. Tiny amounts of uranium trapped  among the calcium carbonate give scientists a way to determine  the age of its layered deposits, as they measure how much of the  radioactive element has decayed.

This then allows them to track physical and chemical changes in the rock through time, giving insight into the ancient Earth environment, including paleoclimate and  past earthquake activity. Travertine is made from calcium carbonate, which contains carbon and oxygen. Both of these elements have  more than one stable isotope, meaning there are both heavy  and light versions of carbon and oxygen floating around in  Earth’s air, water, and crust.

Now, the ratios of heavy to  light isotopes are known to vary in the environment depending  on the ambient temperature. You get more lightweight carbon  and oxygen when it’s warm and wet, and more heavy carbon and  oxygen when it’s cold and dry. That’s because the heavy  elements are… well, heavier.

So when it’s cold and there’s not  much energy to evaporate water in the environment, these isotopes  are preferentially left behind. So by measuring the isotope ratios  in successive layers of travertine, you can use them to track temperature and rainfall over hundreds of thousands of years. And analysis of thick travertines  nearby in the Denizli basin has revealed some pretty big climate fluctuations, with three sharp drops in temperature  within just 100,000 years.

But while these shifts were  taking place in the atmosphere, there were changes happening  beneath the surface, too. Turkey sits on the Anatolian tectonic plate, which is being squeezed out of the way as Africa and Arabia crash northwards into Eurasia. As a result, the country  experiences complex tectonic forces; it is riddled with faults and  has been host to some of the most devastating earthquakes since records began.

And travertines are a great way to track earthquake activity through prehistory. The limestones are most often found along faults and fractures in extensional tectonic settings. In other words, they crop up when the crust breaks because it’s being pulled apart.

And the shape of the travertine formations depend on the stiffness and movement  of the underlying rock. For instance, fissure-ridge  travertines tend to form between two fault lines that don’t quite line up, and the angles of the travertine layers can be used to figure out local stretching directions. Then, combining this with  the radioactive element dates allows geologists to figure out if the stretching happened gradually or in big  earthquake-causing ruptures.

Not only that, but self-built  travertine channels can get fractured during earthquakes, causing vertical  steps of more than half a meter, and sideways offsets of up to a meter. Since bigger displacement  usually means bigger earthquakes, these offsets can be used to estimate the size and destructive power of past seismic events, as well as how often destructive earthquakes can be expected in the region. One study found that a magnitude  6.5 earthquake is likely to strike western Turkey and  Pammukale roughly every 130 years.

And since there was a pretty  big one in the area in 1899, we might expect to see another one fairly soon. So, not only is Pamukkale a  uniquely beautiful geological and archeological wonder, but  it contains a valuable petrified slice of our planet’s  climatic and tectonic history, allowing us to understand and  prepare for the future, too. [♪OUTRO]