The Incan sanctuary of Machu Picchu -- considered one of humanity's greatest architectural achievements -- was intentionally built in a location where tectonic faults meet, according to a study. Built in a remote Andean setting atop a narrow ridge high above a precipitous river canyon in Peru, The UNESCO World Heritage Site is renowned for its perfect integration with the spectacular landscape.
However, the 15th-century sanctuary's location has long puzzled scientists who wonder why the Incas built their masterpiece in such an inaccessible place.
New study presented at the Geological Society of America (GSA) Annual Meeting in Arizona, US suggests the answer may be related to the geological faults that lie beneath the site.
According to Rualdo Menegat, a geologist at Brazil's Federal University of Rio Grande do Sul, the Incas intentionally built Machu Picchu -- as well as some of their cities -- in locations where tectonic faults meet.
"Machu Pichu's location is not a coincidence. It would be impossible to build such a site in the high mountains if the substrate was not fractured," said Menegat.
Using a combination of satellite imagery and field measurements, Menegat mapped a dense web of intersecting fractures and faults beneath the site.
His analysis indicates these features vary widely in scale, from tiny fractures visible in individual stones to major, 175-kilometre-long lineaments that control the orientation of some of the region's river valleys.
Menegat found that these faults and fractures occur in several sets, some of which correspond to the major fault zones responsible for uplifting the Central Andes Mountains during the past eight million years.
Because some of these faults are oriented northeast-southwest and others trend northwest-southeast, they collectively create an "X" shape where they intersect beneath Machu Picchu.
Menegat's mapping suggests that the sanctuary's urban sectors and the surrounding agricultural fields, as well as individual buildings and stairs, are all oriented along the trends of these major faults.
"The layout clearly reflects the fracture matrix underlying the site," said Menegat.
Other Incan cities, including Ollantaytambo, Pisac, and Cusco, are also located at the intersection of faults, said Menegat.
"Each is precisely the expression of the main directions of the site's geological faults," he said.
Menegat's results indicate the underlying fault-and-fracture network is as integral to Machu Picchu's construction as its legendary stonework.
This mortar-free masonry features stones so perfectly fitted together that it's impossible to slide a credit card between them.
As master stone-workers, the Incas took advantage of the abundant building materials in the fault zone, said Menegat.
"The intense fracturing there predisposed the rocks to breaking along these same planes of weakness, which greatly reduced the energy needed to carve them," he said.
In addition to helping shape individual stones, the fault network at Machu Picchu likely offered the Incas other advantages, according to Menegat.
Chief among these was a ready source of water.
"The area's tectonic faults channeled meltwater and rainwater straight to the site," he said.
Construction of the sanctuary in such a high perch also had the benefit of isolating the site from avalanches and landslides, all-too-common hazards in this alpine environment, Menegat said. The faults and fractures underlying Machu Picchu also helped drain the site during the intense rainstorms prevalent in the region.
"About two-thirds of the effort to build the sanctuary involved constructing subsurface drainages," said Menegat.
"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," he said.