The geology of Machu Picchu, Inca citadel in the Andes Mountains.

20/09/2020

Machu Picchu, iconic symbol of the Inca Empire, is one of the most widely recognized and admired archaeological sites on Earth. The Incas ingeniously integrated their buildings into the natural environment, taking advantage of the unique geology of this part of the Andes Mountains in southern Peru. Built over 500 years ago, it is in a spectacular location on a narrow ridge between two mountain peaks. 

Author: Dr. Roseanne Chambers (2020).

Although no one knows for certain, Machu Picchu was likely an important religious center and place of retreat for the royal Inca family and high priests. Machu Picchu is recognized as a UNESCO World Heritage Site, and in 2007 it was voted one of the New Seven Wonders of the World in a global internet poll.

Machu Picchu citadel, with the mountain Huanya Picchu behind the site. (Photo: Roseanne Chambers)
Machu Picchu citadel, with the mountain Huanya Picchu behind the site. (Photo: Roseanne Chambers)

The expansive terraces, elegant stone buildings, and grand plazas, all framed by the dramatic backdrop of steep-sided Huayna Picchu mountain, are a testament to the impressive engineering and artistic talents of the Incas. The unique geologic history of the challenging site, combined with exceptional design and construction, have made Machu Picchu what we see today. We can gain a greater appreciation of the site by understanding the geologic setting of the region and how this made the construction of the royal estate possible.

Geology of Machu Picchu

Along the western edge of South America, from the humid tropics of the Caribbean on the north to the ice fields of Patagonia on the south, the Andes Mountains extend for more than 7,000 km (4,000 mi). The procession of majestic snow-covered peaks and deep valleys forms the longest continental mountain chain on Earth. Dozens of Andean peaks exceed 5500 m (18,000 ft) in height, and only the Himalayas, including Mount Everest, are higher.

Elevation map of Peru.
Elevation map of Peru.

The Andes are arranged in a series of distinct mountain ranges. Machu Picchu is located within the Cordillera Oriental, or Eastern Cordillera, at an elevation of 2400 m (7,800 ft). The site is surrounded by lush green tropical rainforests that merge with the jungles of the Amazon Basin below. The narrow ridge occupied by the city connects two peaks, Machu Picchu and Huayna Picchu. The slopes of the ridge drop off steeply on both sides to the Urubamba River, which surrounds the site on three sides, a dizzying 500 m (1640 ft) far below.

Gray and white granitic rocks underly Machu Picchu and are used for all the Incan stonework at the site. These rocks are estimated to have formed around 250 million years ago, when South America was joined to Africa as part of the supercontinent Gondwana. They originated from hot magma in a large reservoir located 5 to 10 km (3 to 6 mi) deep underground. Over millions of years the magma slowly cooled, forming the distinctive quartz, feldspar, mica, and other distinctive crystals that can be seen in this rock today.

Fast-forward two hundred million years and the Andes Mountains began to rise as tectonic plates collided along the western edge of South America. Extreme compressive forces build mountain ranges by essentially squeezing the rock and forcing it upwards into mountains. Push the edge of a rug and watch wrinkles develop and grow taller - a similar process happens when mountains are built.

Rocks can deform up to a point, but after that limit is reached, the built-up stress is violently and instantaneously released in an earthquake. As the stored energy that has been locked in the rocks is released, an earthquake will propagate along a fault, somewhat like unzipping a zipper or collapsing a line of dominoes. In the Andes, a series of earthquakes on two faults, named the Machu Pichu fault and the Huayna Picchu fault, uplifted the steep peaks with the same names and formed the ridgetop where Machu Picchu is located. During those earthquakes, a large section of rock was down-dropped between the two faults in a geologic structure known as a graben. Machu Picchu is built in this graben.

The folding and faulting that formed the mountains in the region also created cracks, or joints, along zones of weakness in the rock. These occur in two major systems that intersect in an "X" shape beneath Machu Picchu. The set that includes the Machu Picchu and Huayna Picchu faults is oriented northeast-southwest; the second set is oriented northwest-southeast. These joint sets, together with the Machu Picchu fault, were critical factors in the site selection and successful construction of Machu Picchu.

Another important geologic process occurring in the region is a tendency for landslides, as the steep mountain slopes in the region are unstable. High seasonal rainfall (nearly 2,000 mm; 79 in) is slowly breaking down the minerals in the granitic rocks, providing material susceptible to landslides. By locating Machu Picchu on a ridge, the Inca builders protected the city from these dangers. The ridgetop setting also provided excellent natural defenses.

Springs, Quarries and Hidden Construction

The Inca builders must have examined many mountain sites before selecting the narrow ridge for Machu Picchu. A reliable water source for the city was an essential requirement and it is unusual to find one this high in the mountains. Fortuitously, the fractured rock along the Machu Picchu fault allows rainwater to infiltrate and be stored underground, and then flow out onto the surface as a perennial spring. The reliable yield of this spring was enhanced by a well-engineered stone block water collection system that is still operational today. A series of long canals, and an impressive sequence of 16 stone-lined fountains, provided the ceremonial and domestic water that cascaded through the royal estate. The purest water, at the top of the series of fountains, was located adjacent to the residence of the Inca ruler.

An enormous amount of high-quality stone was essential to build the city. Granitic rocks are extremely hard and strong, so they are ideal for construction. Again, fortuitously, the joint sets formed during mountain building have extensively fractured the rock, creating a convenient "natural quarry" of granitic blocks, with many "pre-cut" into useful sizes. Without such a source of suitable rock on the ridgetop, construction would have been exceedingly difficult.

Stone quarry at Machu Picchu, with naturally fractured, 'pre-cut' stones. (photo: Bobistraveling / CC BY 2.0)
Stone quarry at Machu Picchu, with naturally fractured, 'pre-cut' stones. (photo: Bobistraveling / CC BY 2.0)

There is much more than meets the eye at Machu Picchu. One of the most surprising facts about the city is that an estimated 60 percent of the total construction effort is hidden underground. The steep mountain slopes were stabilized with extensive terrace construction that opened the space for agricultural use and building construction. The terraces were carefully assembled behind strong walls of rock blocks, and with a lower layer of boulders, a middle layer of gravel, and a top layer of soil. These deep foundations, and an extensive drainage system built into the terraces, were essential to accommodate frequent heavy rainfalls. Rainwater percolating through the terraces could drain down the mountainside with minimal soil erosion. Without this site preparation, deterioration would have occurred rapidly as terrace fills settled and buildings crumbled.

Integrating Rocks and Mountains

The Incas had a deep respect for the dynamic world that surrounded them, and they recognized many natural features as sacred and powerful, including rock outcrops, mountains, rivers, caves, and springs. By deliberately integrating important buildings, gardens, and shrines into the natural setting of the landscape, the Incas expressed relationships to their gods and their kinship groups.

Sacred features of the landscape were highlighted by a variety of building strategies. At Machu Picchu, views of sacred mountains are framed in doors and windows. In some areas, carved rocks mimic the shape of the distant mountains. Integrating a natural rock outcrop with contoured masonry in a rock wall was practiced, notably in a temple shown in the photo below. Rock outcrops were also carved with steps, platforms, niches, and channels and other forms. These carvings distinguish a sacred rock from its setting.

In-situ natural outcrop integrated into a temple building. (photo: Roseanne Chambers)
In-situ natural outcrop integrated into a temple building. (photo: Roseanne Chambers)

For hundreds of years after Machu Picchu was abandoned by the Incas, it remained hidden from view. Jungle vegetation enveloped the city, and it was known to only a few local inhabitants, including some who cultivated crops on the ancient terraces. After Hiram Bingham, an American historian, stumbled across the walls of the city in 1911, it was finally brought to the attention of the world. For all visitors to the site, the spectacular setting and exquisite stonework of Machu Picchu inspire awe at the formidable power and resources of the Inca ruler responsible for the city. The Incas built this important city to last - and it has.

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If you enjoyed reading this article and would like to learn more about the Andes, the Incas, and their ancestors, plus volcanoes, earthquakes, and other geologic topics, please visit Roseanne's website: https://roseannechambers.com/

Sources

Menegat, R., 2019, September. How Incas used geological faults to build their settlements. In Geological Society of America Abstracts with Programs (Vol. 51, No. 5).

Wright, K.R. and Zegarra, A.V., 2000, October. Machu Picchu: A civil engineering marvel. American Society of Civil Engineers.

Title photo at the top: Machu Picchu stone wall, by McKay Savage / CC BY 2.0.