Machu Picchu; Stable as a Rock
The diagram shows photos taken by the author of the south facing wall of the Principal Temple at the Sacred Plaza, Machu Picchu in 1965 and again in 2006. Noticeable change in the separation of the granite blocks is not evident in the 41 years represented by the photos.
The world famous Inca site Machu Picchu is built upon a large wedge shaped fault block called a graben that was formed by a gradual subsidence downward of the block created by two parallel running faults, one near (north of) Cerro ( hill or peak)Huayna Picchu and the other near (north of) Cerro Machu Picchu. This, along with erosional processes over the past several million years created the familiar ”two peaks with a saddle between “topography that the Inca carefully selected for the most important of their sacred ceremonial centers.
The nearby Urubamba River contributed by carving a deep entrenched meander around the site as continuing tectonic forces slowly lift the mountain mass of the Andes. Numerous lesser faults and subsurface factures resulting from mountain building pressures are also present. As base rock is exposed by erosion, these fractures offer zones of weakness subject to ground water penetration and other surface forces that create fragmentation and disintegration into blocks and pieces and eventually into mixed mineral-organic residue soil.
These loose boulders and rocks were the material that supplied building stone for Machu Picchu. The regional mountain base is part of an uplifted Paleozoic era (250 million years old) intrusive igneous feature classified geological as a batholith. These are massive upward traveling bodies of molten material (magma) that penetrate the upper layers of the earth’s surface before stopping short of the surface. Upon cooling they typically leave a shield or stock shaped mass of igneous rock more than 40 square kilometers in area.
Here, the rock type is mainly a resistant fine grained ( small crystallized ) white granite which proved excellent for hammering (the pecking method) into finely shaped blocks and sharply defined angles. So is the Machu Picchu site about to now fall apart having survived in its present configuration, the past million or so years that we H. Sapiens have been walking about?
A recent report by a Japanese funded study suggests that just this event may soon take place based upon discovery of several additional faults. The faults may indeed be there (we all have faults) but given the regional geological environment, I suggest that Machu Picchu is not likely to fall apart in the near future and may well outlast the human race.
The key words in the formation of the fault block described above are “gradual subsidence” This means in geological terms, several million years. Photos taken by the Hiram Bingham expeditions in the early years of the last century compared with recent Machu Picchu photos show no change or settling of Inca structures since Bingham's investigations. (with the exception of the Tourist hotel which seems to be sagging a bit).
This is not to say that catastrophic geological events have not happened in the nearby Andes. Mountain slopes regularly slide away during the intense rainy season. Flooding has destroyed the railroad below Machu Picchu and wiped out entire towns. But there are few examples of a massive, solid granite feature like the Machu Picchu-Huayna Picchu mountain breaking apart within the time frame of human history.
The Inca located Machu Picchu squarely upon the ridge plateau between the two mountains realizing that the location is not subject to seasonal land slides as there are few steep slopes to utilize the lubrication of heavy rains and the pull of gravity. Inca builders also engineered excellent drainage systems to protect buildings and terraces. Of course some lower outlying areas of the site have been subject to slides but not the principal high status temples and buildings that the world recognizes as Machu Picchu.
Finally I should say that the area seems to be outside the zone of earth quakes and tremors that have so threatened Cusco and other Peruvian regions. The several major quakes that affected Cusco during the last century were not felt at Machu Picchu. Another major source of geologic catastrophe, active volcanics is far removed from this central-southern region of Peru.
I respectfully Submit that the magnificently engineered Inca works at Machu Picchu will be around intact much longer that the Cassandran Japanese team who predict its eminent destruction.
Gary R. Ziegler