When we arrived at the Village of Turi, it was the first time we had seen naturally occurring water since Antofagasta - and we were anxious to jump in and wash the dust off!
Looking west, we could still see the smelting plume from the Chuquicamata mine over sixty kilometers away.
Continuing our voyage through the APVC we traveled south from the San Pedro/San Pablo area, through the wonderful village of Turi, to the slopes of mighty Chao. Located between the composite cones of Leon and Paniri volcanoes, Chao is the second major lava dome we have come across, and the largest we will see in our time in the APVC. At over 14km long and 53km2 in total area Chao is the largest silicic lava flow known on earth - roughly 26km3 of erupted volume.
Dating of Chao has proven difficult, but due to deposits from recent glaciations it is certain that Chao is at least older than 11,000 years. Ar-Ar dating of Chao has put its age at 420,000 years +/- 100,000, however this is probably an overestimate. What is known is that the formation of Chao was not one discrete event but rather Chao was constructed in at least three stages.
Stage 1 was a mildly explosive stage, which produced coarse non-welded pumice and some block and ash flows later. Produced by this activity were also two overlapping cones on the northern flanks of Chao. Overall this activity resulted in roughly 1km3 in erupted material.
Built on top of stage I pyroclastics, stage II was the major phase in the construction of Chao. In contrast to stage I, stage II was primarily effusive and resulted in over 22.5km3 of erupted material. This enormous volume and the steep slope that Chao is formed on resulted in stage II stretching 14km with flow fronts of over 400m. As the lava flowed, it also formed impressive ogives, reaching heights of 30m, seen clearly in the satellite image.
Stage III was the final stage in the formation of Chao, built on top of the stage II flow its 6km long and 3km wide. The total volume is roughly 3km3.
All three stages of Chao are mineralogically similar, with minor variations in mafic enclave content. Phenocrysts of Plagioclase, Quartz, hornblende, biotite, and sphene are seen in a friable glassy matrix. The friable nature of the matrix is due to microvesiculation that took place in the lava.
A few things to note, getting to Chao can be difficult. To get to the base of Chao, as with everything on the APVC, 4WD is required. The road that runs to the south of Chao, between Chao and Leon, is washed out in a few places. However, great camping does exist along this road with some amazing views.
Beware of Vescatchas!... Ok, not really.
Driving along the Rio Salado Norte leads to another wonderfully exposed section of the Sifon Ignimbrite. Although this stop is along the main road, it is slightly inconvenient because just beyond the exposure cars are instructed to turn around due to land mines. The ignimbrite, however, is roughly 80 m thick in this part of the APVC.
The Sifon is crystal-rich, including large quartz crystals, plagioclase, biotite, hornblende, abundant pumice clasts and few lithics. The Sifon within the San Bartolo Group is a generally at is well to moderately indurated and has a fine-grained, crystal-rich (~65-70%) matrix.
The pumice rafts are not all connected throughout the outcrop, suggesting there may be a fault in the area resulting in differential flow movement between the blocks of flow.
The Village of Tocance sits along the south end of the Tocance river canyon.
The United Nations World Heritage entry for Tocance reads as follows:
It follows an agglutinated pattern of typical Andean architecture based on stone masonry and roofs of Cactaceae wood covered by vegetable fibers. It is worth mentioning that the space is divided according to some characteristic Andean criteria based on the principles of duality and tripartition, which are a manifestation of typically Andean cultural features. In fact, the villagers distinguish three sectors: the "Toconce town", the highest area near the church, "Katunmarca", to the center and West of town, and "the Chaco town" in the lowest access area. Toconce is part of, and associated with, important archeological sites, bearing witness to the presence and continuity of the Andean people's culture. Toconce is a highly traditional community with regard to its economy, architecture and craftsmanship, cultivating a close relation with neighbor communities which intensifies on the occasion of both religious and economic ritual festivities, such as the ceremonies of the "Cleaning of Irrigation Channels", and the "Cattle Adornment".
We were also able to buy a Coke for the first time here... there was great rejoicing!
For more information on the history and culture of the area see:
THE CASPANA IGNIMBRITE
This location reveals a great exposure of the Caspana Ignimbrite, the uppermost unit of the Toconce Formation that is unique from the other ignimbrites that comprise the APVC. It has a distinctive orange color that makes it extremely identifiable and explains its name as the “Orange Sillar”. The Caspana contains rhyolitic and andesitic pumice clasts as well as mixed clasts, making it compositionally heterogeneous.
The Caspana plinian deposit contains little pumice and only minor amounts of crystals, specifically quartz and biotite. The plinian and the ignimbrite have the same source, which is likely an ancestral volcano that is now covered by Leon or Tocanao. At this location, lake sediments are exposed underlying the ignimbrite.
The Caspana lies between the Linzor tuff, 5.77 Ma and the Puripicar ignimbrite, 4.18 Ma. The best age estimate of the Caspana is somewhere between these dates.
CASTLE ROCK AND THE TOCONCE FORMATION
Castle Rock is a formation comprised of layers of ignimbrite that have been eroded into the shape of a castle. Most ignimbrites are units of the Toconce Formation and the identifiable pink layer at the bottom is Sifon Ignimbrite, 8.1 Ma.
Just over the pass near Linsor dome and to the east of the Chao complex and Cerro Lean, we come across Chillahuita, yet another large silicic lava dome. Like Chanka and many of the domes yet to come, Chillahuita has the “torta” shape often exhibited by these large domes - flat tops and steep sides with some talus around the flanks.
Along with close proximity, Chillahuita has many similar characteristics to Chao, it is thought to be nearly contemporaneous in age and contains similar texture and mineralogy. It does, however, differ to Chao in that it was build on a much more gentle slope, and thus is smaller in size – 5km3 in volume.
Click for a high resolution image
Like most of the domes we have seen, and will see, Chillahuita is dacitic in composition and extremely rich in crystals (50%+). Mineralogically, the phenocrysts consist of plagioclase, quartz, hornblende, and sphene. The texture, like Chao, is friable due to microvesiculation of the matrix.
As an interesting note, many boulders that have fallen down the steep sides of Chillahuita show evidence of the primary wind direction through the products of wind erosion. Saltating sand grains hit these large boulders and overtime have formed many small holes that almost resemble a honeycomb texture.
Traveling south from Chillahuita, near the Tatio geothermal fields, we come across Copacoya, yet another large silicic dome of the APVC. Like its brethren, it takes on the typical “torta” shape with steep sides and a flat top.
Also like its brethren, Copacoya is dacitic in composition and crystal rich (40%+). Mineralogically it contains phenocrysts of plagioclase, quartz, hornblende and some devitrified glass in a glassy matrix. The matrix is much denser than seen at previous domes, due to the lack of microvesiculation within the lava. Mafic enclaves are also seen frequently – more andesitic, finer grained.
There are lines of evidence pointing to this dome being around the same age and close to the source of the Sifon Ignimbrite as the Sifon is greater than 300m think in this area.
To the southwest of the Tatio geothermal fields lies the Tocopuri volcanic complex. The name Tocopuri refers to the large composite cone that sits on the border of Chile and Bolivia. To the west of this cone lies La Torta, which was the focus of our stop here. True to its name, La Torta is in fact another “torta” shaped lava dome with steep sides, a flat top and small talus field surrounding the base. La Torta is moderately sized at 5km3 in volume. An age estimate of less than 1 million years has been reported to the Tocopuri system with La Torta being slightly younger.
La Torta is dacite to rhyolite in composition and like all the lava domes of the area is extremely crystal rich, generally over 60%. Mineralogically, La Torta contains phenocrysts of Quartz, Plagioclase, hornblende and biotite in a glassy matrix.