Incas Built Machu Picchu Over Tectonic Faults on Purpose, Potentially Solving Mystery of Monument's Extreme Location

The Incas—a civilization that ruled over vast swathes of South America's Andean region in the 15th and 16th centuries—intentionally built Machu Picchu, and other cites, in a location where tectonic faults meet, research suggests.

One of the new Seven Wonders of the World, the majestic Incan citadel is located around 8,000 feet above sea level atop a narrow mountain ridge in the Andes. But the reasons why the Incas chose this remote and inaccessible location have long remained a mystery to experts.

Now, research presented by Rualdo Menegat, from the Federal University of Rio Grande do Sul in Brazil, at the Annual Meeting of the Geological Society of America indicates that the decision may have had something to do with the location of tectonic faults—fracture zones between two blocks of rock in the Earth's crust. These can range in length from a few millimeters to thousands of miles.

"It seemed to me that no civilization could be established in the Andes without knowing the rocks and mountains of the region," Menegat told Newsweek. "Machu Picchu is not an isolated case of Inca survival strategy in the Andes."

"It could not be built on a whim. It is part of a practice of building settlements in high rocky places. But what guides this practice? What knowledge of the rocks and mountains did builders need to know to succeed in building cities under these conditions? These questions had not yet been the subject of scientific research and I decided to investigate them," he said.

For his research, Menegat used satellite imagery and field measurements to map the network of faults—some of which measure up to around 110 miles long—in the area beneath Machu Picchu.

"Field investigations were carried out in four expeditions in 2001, 2006, 2010 and 2012," he said. "The analysis of satellite images was done in the laboratory. I also used various geological descriptions and studies of the Cusco region and the Sacred Valley."

"At each stage of the research I presented the results to Peruvian researchers from various fields of knowledge—geology, archeology, anthropology, architecture, urbanism, landscape ecology, and epistemology—and regions of Peru, so as to assure me of the correctness of data and I also evaluate the scope and importance of my findings for Andean culture researchers," he said.

Menegat's investigations showed that the UNESCO World Heritage Site sits right above the mutual intersection of three main fault directions, and two secondary directions that run north-south and east-west, which almost form an "X' shape. Furthermore, Menegat found that Machu Picchu's main sectors, buildings and stairs all appear to be oriented along the directions of these faults.

"The principal result was the discovery that Machu Picchu was built where geological faults intersect," Menegat said. "Geologic faults are breaks in rock masses, forming fractured rock strips that continue for large distances. The faults can occur as reticulated networks as in the region of Cusco where they are related to the geological processes of formation of the Andes Mountains."

"Chief among them is the compression generated at the edge of the continent by the clash between the South American Plate and the Nazca Plate. This produces several ruptures in the rock masses, fracturing them. Fault networks are responsible for the shape of the mountains, valleys and also blocks of rock that detach from the slopes," he said.

The research also showed that some other Inca settlements in the region—such as Ollantaytambo, Pisac and Cusco—were also built on top of fault intersections, like Machu Picchu.

In light of the results, he argues that the Incas—who were master stoneworkers—deliberately chose sites like this at the intersection of faults, in part because they offered plentiful building materials in the form of pre-fractured rocks.

"Where faults intersect, the rocks are even more fractured," he said. "Therefore, they are places that have more loose blocks on the surface, and also places where [the rocks] can be easily removed to build terraces and buildings. In addition, the blocks take on typical shapes such as triangles, hexagons and rhombohedra. These forms fit geometrically on the wall mosaics of buildings."

Menegat says that it would have been "impossible" to build such a site so high in the mountains if the rocks were not already fractured in this way. The Incas used these materials to create structures without mortar, featuring stones that fit together so perfectly, there are almost no visible gaps.

Furthermore, the area at the intersection of faults may have provided other advantages. For example, the faults could have acted as a source of water, channeling rain and ice melt directly into the site. This allowed the Incas to build away from the bottom of valleys, where any settlement would have been at risk from flooding and avalanches.

"The Andean world is inhospitable. Here, human life is possible only in a few places where water drips through fractures," Menegat said. "The Incas knew to follow this criterion, which allowed them to establish networks of settlements in this kind of oasis of habitability provided by the faults and fractures."

"Their cities and plantations were not large, but the little that was produced in one place made possible exchanges with other places, resulting in great diversity," he said. In this inhospitable environment, the Incas managed to sustain a culture of 10 million people "without hunger."

Conversely, building on top of faults could also have helped to drain the site after intense rainstorms—which frequently affect the region.

"About two-thirds of the effort to build the sanctuary involved constructing subsurface drainages," Menegat said. "The preexisting fractures aided this process and help account for its remarkable preservation. Machu Picchu clearly shows us that the Incan civilization was an empire of fractured rocks."

So did the Incas have any understanding of tectonic faults, or did they simply choose the site for Machu Picchu based on the fact that it was rich in fractured rock? Menegat thinks that while the Incas may not have known what the fractures were, they could have recognized them.

"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 said. "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 addition, there is a Quechua word for large fractures. As the great Peruvian writer José Maria Arguedas said, the Incas called the fractures that cross the mountains 'quijlo.' Geologists call them faults," he said.

The general consensus is that the Incas built Machu Picchu around 1450, however, the site was later abandoned during the Spanish conquest.

This article was updated to include additional comments from Rualdo Menegat.

Machu Picchu
Detailed mapping indicates Machu Picchu's location and layout were dictated by the underlying geological faults. Rualdo Menegat