Machu Picchu, the urban citadel built high in the peaks of the Andes by the Inca civilization, has fascinated visitors and scholars alike. But the biggest question for most of them—especially after hiking for several days on the Inca Trail to reach the spot perched high in the mountains on a ridge overlooking a precipitous river valley—is why the Incas built the city in such a remote place. Now, a new study suggests it all has to do with geology; Machu Picchu, as well as other Inca cities, were deliberately built on fault lines.
Earlier this week, Rualdo Menegat, a geologist at Brazil’s Federal University of Rio Grande do Sul, presented the findings at the annual meeting of the Geological Society of America. Using satellite images and field data, Menegat was able to document a web of fractures underneath Machu, from small fissures running across individual boulders to a 107-mile-long fault responsible for the orientation of the rock in the river valley. Some of the faults have a northwest-southeast orientation while others have a northwest-southwest orientation. In the middle, where the faults intersect in the shape of an “X” is Machu Picchu.
It's not likely that the Inca selected the fault lines for any religious or symbolic reason. Rather, the faults produce chunks of granite that have already been cracked into pieces, making it possible to build the elaborate stone outpost of fitted rocks with minimal effort. The walls of the city are also oriented in the direction of the faults. “Machu Picchu’s location is not a coincidence,” Menegat argues in a press release. “The intense fracturing there predisposed the rocks to breaking along these same planes of weakness, which greatly reduced the energy needed to carve them. It would be impossible to build such a site in the high mountains if the substrate was not fractured.”
Besides allowing the Inca to more easily find and fit stones together without mortar, the faults provided other advantages. The fault lines running through the site probably directed melting snow and rainwater to the high-altitude outpost providing water. And the network of fissures below the site likely allowed it to drain, one of the reasons the city has lasted so long.
Menegat tells Aristos Georgiou at Newsweek that building Machu Picchu at that site was probably not an accident. “It seemed to me that no civilization could be established in the Andes without knowing the rocks and mountains of the region. Machu Picchu is not an isolated case of Inca survival strategy in the Andes,” he says.
Other Inca cities, including Ollantaytambo, Pisac and Cusco, are also built on similar fault intersections, as Menegat has found. This doesn’t necessarily suggest the Inca had a deep knowledge of plate tectonics. Rather, they may have sought out these areas, filled with a jumble of rocks fractured into shapes like triangles and rhombuses, which could be fit together to make walls.
“The Incas knew how to recognize intensely fractured zones and knew that they extended over long stretches. This is for one simple reason: faults can lead to water,” Menegat tells Georgiou. “So consider a fault that starts from the top of a snowy mountain and extends down to 3,000 meters [around 9,450 feet] to reach the deep valleys. The melting of spring and summer fuels this fault and changes the amount of water that flows through it. Faults and aquifers are part of the water cycle in the Andean realm.”
In Quechua, the language of the Inca, there is a word for large fractures, or faults, which is another indication that the Inca were aware of the faults running through their mountain domain.
Machu Picchu is believed to have been constructed around 1450 under the direction of Inca emperor Pachacuti Inca Yupanqui as something of a royal mountain estate. Residences for Inca elite and what’s believed to be a private residence for the emperor, including a private garden and his own toilet site, were built there. When the Spanish began invading South America, war and disease brought the Inca empire to an end, and the city on the mountains along with many others were abandoned. It was discovered by western science in 1911, when Yale professor Hiram Bingham III was tipped off to its existence by locals and led to the site, then overgrown with vegetation. Now Machu Picchu is a World Heritage site and a massive global tourist attraction. Today, it faces very modern threats including overtourism, a problem that will only be exasperated by plans for a new airport in the region.