The recent eastward cruise of the MV Discovery across the South Pacific provided an unparalleled opportunity to view a total solar eclipse (April 9, 2005) from an oceanic vantage point north of Pitcairn Island. This 600-passenger ship was packed with amateur astronomers, many of whom set up an amazing array of telescopes on deck to capture images before, during and immediately after the 31 seconds of totality. Professional astronomers Ed Krupp and Rick Fienberg, in collaboration with astrophotographer George Keene, offered a series of excellent lectures before and after we collectively witnessed this picture-perfect eclipse.

To provide a little diversion from this astronomical theme during the balance of our days at sea, Dr. Suzanne Connolly of the Mayo Clinic was aboard to give lectures on the hazards of sun exposure (and to review the latest information on how best to avoid UV overexposure); I provided four lectures on the geology behind the spectacular landscapes seen during our 16-day voyage from Tahiti to Lima, Peru, where 150 passengers disembarked for a two-day post-cruise tour to Cusco and Machu Picchu. Because many of the tour participants had attended my talks at sea, I was frequently asked if I might provide some follow-up information about these fascinating terrestrial destinations and the remarkable Inca-built stone structures of the region, I gladly offered to post a commentary on this website within a few weeks of returning to Ottawa. Accordingly, here is the information I’ve since gleaned from geological maps and scientific reports viewed at the Geological Survey of Canada, supplemented by my own impressions, plus some digital images.


Classic example of the built environment blended artistically with the natural environment. Like Zen Masters, the Incans appreciated their geoheritage.

Most rocks underlying Cusco, the Urubamba River Valley and Machu Picchu are igneous, comprising parts of several vast Andean plutons (intrusions) similar to those that form the principal core complexes within the Coast Range of North America. Although many guidebooks describe the intrusive rocks of Peru as “granites”, most appear to be diorites, quartz diorites and granodiorites (plagioclase feldspar almost universally predominates over K-feldspar, and the quartz content is generally less than 10 per cent). A distinct yellowish tinge is imparted to many blocks by the mineral epidote, probably developed by hydrothermal alteration of the plagioclase feldspar during the late stages of intrusion. Many blocks at Machu Picchu also display faint foliation (planar alignment of elongate mineral grains), indicative of incipient metamorphism, perhaps concurrent with deformation related to uplift of the Andean Mountains.

Orthogonal sets of joints are pervasive throughout these intrusive rocks, and provided natural pathways for Inca craftsmen to free the naturally formed blocks from their source outcrops by inserting wooden wedges along the joints. Experiments by archaeological teams have demonstrated that, if the wedges were first soaked in water before being pounded into slots chipped into the joints, the wedges would swell sufficiently to split the blocks free (probably after many repetitions of this wedge-expansion routine).


Tour guide explaining the method of utilizing wooden wedges pounded in along joints used by Inca artisans to split blocks from an outcrop of diorite.

But this was only the start, because tedious episodes of shaping with harder stone tools (chert and ironstone) followed by polishing with sand would be required to modify the natural faces sufficiently to attain remarkablly near-perfect fits between adjacent blocks, as are still preserved in so many places.

Twelve-sided block of diorite, representing 12 Inca kings. Fashioning this large block involved considerably more effort than would be required to prepare the adjacent rectangular blocks bounded by planar joints. The remarkably tight fits with adjacent stones is particularly impressive at this locality. The uniform yellowish grey color of all blocks reflects provenance from a single quarry of epidotized diorite. Note exfoliation at the left and upper right sides of the 12-sided block (has obviously occurred after shaping and emplacement).

I was struck by the frequent incorporation of fine-grained massive green rocks into some of the highest stone stairways at Machu Picchu. Most of these stones appeared to be slightly metamorphosed basalt, although a few darker ones may be andesite. Many were rounded, indicating abrasion by water transport, so undoubtedly they were manually carried from the river valley far below. There must have been some good reason to justify this effort, in view of the abundant on-the-spot supply of uniformly colored, medium- to coarse-grained intrusive rocks. In fact, many of the latter incorporated in the stairs apparently have been oriented to display dark inclusions (such inclusion-rich stones appear to be in greater abundance in the stone stairways than in nearby walls). Perhaps the Incas attributed some religious significance to the darker stones and inclusions, or did they simply appreciate incorporation of a little geological variety in their stairways up the mountains? Dark fine-grained volcanic rocks appear not to have been used in most of the vertical walls, although one doorway is capped by a slab of schist, a metamorphic rock possibly derived from a large inclusion or from a fault zone (or was it also hauled up from far below?).

Angular inclusion of gray volcaniclastic rock in a block of diorite built into a stone staircase. Note incipient fracture with patches of brown staining due to oxidation of iron-bearing minerals in the diorite.

Slickensides on a fault surface coated with the alteration minerals, epidote and serpentine. With six sides to the block, it is interesting to see that the artisan who placed this block chose to display a geologically significant surface.

According to geological maps of Peru, the geological age of most intrusions extending from Cusco to Machu Picchu and beyond is late Cretaceous (final period of the Mesozoic Era -- the time of the dinosaurs, terminating around 65 million years ago).

On our return trip by train to Cusco, I noted and photographed some dark green boulders in and bordering the Urubamba River. These appeared to contain pillow structures, much like those typical of submarine flows in greenstone belts in all Precambrian Shields of the world. Sure enough, a relevant Peruvian map indicates that a sliver of Precambrian basement rock (likely more than 2.5 billion years old) crosses the Urubamba River just upstream from the site where these boulders were seen and photographed.


Subangular blocks of fine-grained greenstone in Urubamba River. Note suggestion of pillow structures (use magnifying tool). These blocks probably were derived from Precambrian (Archean?) outcrops upstream from this site.


Subround greenstone cobble incorporated in stone staircase. Coin for scale. Presumably carried up from the river below.

In addition, light grey to buff carbonate rocks are well exposed along several stretches of this river (and atop several flanking mountains). These stratified sedimentary rocks, either limestone or dolostone of probable early Cretaceous age, record shallow-water marine deposition, and likely contain coral reefs comparable to the older Capitan Reef of Permian age in Texas that I offered in one of my talks as a classic analogue to modern-day reefs (although built by different assemblages of organisms).

Rock slab that has started to move downslope along a joint surface. The valley below will be a hazardous place to hang out during the next earthquake.

Finally, there was one segment of the Urubamba River in which extensive potholes and polished surfaces were displayed in carbonate outcrops along both shores, attesting to the erosive power of this river during floods. Flat pebbles and cobbles also showed upstream imbrication in several places along the river banks. So, lots of great geology seen in but a few hours. I look forward to returning, and possibly walking the Inca trail between Km 101 and Machu Picchu. For those similarly impressed with the wealth of human history and geohistory to which we were exposed, I’ve listed below some websites that provide interesting reading to supplement what we observed and absorbed during our brief visit to this remarkable part of the world:

http://www.reganimages.com/machupicchuhistory.htm

http://www.shastahome.com/machu-picchu/ruins.html

http://news.nationalgeographic.com/news/2002/04/0415_020415_machu.html

And here are some excellent books to futher explore the geology behind the landscapes you have already seen, and will undoubtedly see during future cruises and land excursions:

Ballard, Robert. 1983. Exploring Our Living Planet. National Geographic Society
Bryson, Bill. 2003. A Short History of Nearly Everything. Doubleday
Cloud, Preston.1988. Oasis in Space. Norton & Company
Gould, Stephen Jay. 1989. Wonderful Life. Norton & Company
McPhee, John. 1980. Basin and Range. Farrar, Straus & Giroux
Winchester, Simon. 2001. The Map that Changed the World. Viking
--------------------------. 2003. Krakatoa. Harper Collins

Moai who has joined the eclipse-chasing crowd. Photo courtesy of C. Bruce.

Close-up view of a rock pillar in our hotel in Cusco. This appears to be a welded tuff formed by compaction of a superheated gas-charged flow. The black streaks are obsidian (volcanic glass) representing flattened vesicular fragments

Close-up view of bedding in airfall tuff composed mostly of sand-size fragments of sideromelane (altered volcanic glass) . The Polynesian artisans carved their Moai parallel to this layering to increase integrity of the medium. Like the Incas, they understood some basic geological principles. Because Easter Island is relatively young geologically speaking ( 3 million years or less) the layering has not been disturbed by folding, so remains subhorizontal. This explains why one unfinished statue, carved in situ, lies with its long axis almost horizontal.