The rocks of Machu Picchu and other Inca sites to see in Peru
Intro: When in Peru, do what the tourists do, right? Well, most tourists aren’t on the tail end of a several week intensive geology short course on ore deposits and mines of Chile and Peru I guess… but I still had to do it. Machu Picchu is considered one of the 7 Wonders of the World, and it is consistently on the top of any travel’s list. While the unprecedented preservation of the Inca culture’s architectural icon is mostly a lure on itself, there is a neat geological underpinning to this lost city. This blog post is about the rocks that make the famous stones of Machu Picchu and the geologically unstable faults that bound the city ruins! Plus, a list other spectacular Inca site that are definitely worth the visit while in the Cusco and Lima area of Peru.
Science Spiel: Seismic faults and tight fit (Geology, Geomorphology)
Machu Picchu is located high in the Peruvian mountains. The underlying rocks are ~246 million year old granite (Lancelot et al., 1978), with local dyke bright-green serpentine that outcrops along the Inca trail. The well-developed crystals within the granite (derived from its slow-cooling history as a magma) makes it an ideal rock for carving, hammer and making monumental Inca cities out of! The craftsmanship of the Incas was so good that the stones of granite fit together without any cement or binding agent.
The lost city of Machu Picchu is positioned between two large mountains known as Huayna Picchu and Cerro “montaña” Machu Picchu (both of which are great 1 – 3 hour day hikes from Machu Picchu that give you stunning views of the city). The position where the Machu Picchu is in-between these two taller mountains is a large down-dropped block of the mountain-side known as a graben. This is because there exists two parallel faults proximal to the two surrounding mountains, and seismic activity has lead to the wedge-like subsidence of the block where Machu Picchu is located (Carlotto et al., 1999; Carlotto and Cárdenas, 2001; Vilímek et al., 2006).
The Inca’s were smart, however, and appeared to have constructed the granite-stoned building and walls in such a way to withstand seismic activity! The region isn’t extremely seismically active, but historical there have been earthquakes in the region near Cusco (USGS, 2015).
Outside of Machu Picchu, there are plenty of other Inca ruins that are well-worth a visit. These include Moray, Sal Minas, Ollyamtambo, Pisac, Sacsayhuaman, Valley de Sur, and Pachacamac:
- Moray (Cusco region), spectacular circular Inca agricultural sites. The surrounding landscape is made of salty evaporate rocks, so these circular depressions are sinkholes caused by the dissolution of soluble rocks. The Incas utilized these depressions in the landscape and sculpted stunning symmetrical circular agricultural terraces where growing temperature vary up to 15°C from top-to-bottom!
- Sal Minas (Cusco region), hundreds of salty water pools that, when the water evaporates, residual salt is harvested.
- Ollanyaytambo (Cusco region), an ancient Inca military city with strategically high-built royal buildings and walls overlooking the town.
- Pisac (Cusco region), a site of large Inca agricultural terraces northeast of Cusco city.
- Sacsayhuaman (Cusco), one of the best examples outside of Machu Picchu of the wonderfully-carved stonemasonry of the Incas.
- Valley de Sur (Cusco region), a pre-Inca historic site with city streets that go on for miles!
- Pachacamac (Lima region), buildings and temples to the sun-god, built before the Inca Empire, but maintained during their reign. These buildings are made out of mud-bricks, and the brick themselves are made of mudstone!
Final Thoughts: It’s hard to imagine anyone being disappointed in a visit to Machu Picchu. No matter which direction you look, everything is truly awe-inspiring! The Incas were very smart and had an understanding of the geology. They built their lost city in a hidden place, aided by the graben geomorphology, but knew about the seismic activity in the area… thus, they skillfully constructed their buildings with carefully-carved granite in order to withstand small tremors. Machu Picchu fully deserves its spot as a Wonder of the World, and it, along with the many other areas I mentioned, are true testaments to the lost Inca civilization and well-worth traveling to. Cheers!
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