UPPER COLCA PROJECT
Arequipa, Peru
ASTER SCENES OF BOTH THE CHIVAY
AND THE ALCA SOURCES
The following subsets of ASTER scenes are intended to show the geological context of two important obsidian sources in south-western Peru. In each case the red polygon indicates the area of highest grade obsidian (largest nodules, fewest bubbles and phenocrysts) observed in field visits. Scale: 1 cm = approx. 1.5 km.
CHIVAY SOURCE: The Chivay / Cotallaulli obsidian source in the Colca valley area. The town of Chivay is the circular gray area in the left-center half of the image, it lies on the Colca river at 3650 masl. The source area indicated with a box is at approximately 4900 masl.
ALCA SOURCE: The Alca obsidian source on Cerro Aycano in the Cotahuasi valley area. The town of Alca is on the river bend on the left-center edge of the image around 3400 masl. The western source area indicated is at approximately 4350 masl and was visited by Tripcevich at Jennings' suggestion in 2001. The large polygon on the east is at approximately 4600 masl and it shows an area with large nodules and quarry pits that is probably the primary depositional zone for Alca obsidian identified by Rademaker et al in 2004, indicated here.
Comment on these images from volcanologist Tobias Fischer (Univ of New Mexico). [I asked him if they look like collapsed rhyolitic domes.]
"The Colca image seems to show two vents (darker features) surrounded by viscous, possibly rhyolitic lava flows. The flows have the very characteristic pancake shape in aerial view that implies high viscosity, i.e. high silica content. So they could be rhyolites. Basalts would produce features that do not show such high relief. The feature due west from the northern vent looks much older and partially collapsed with the crater opening to the SW. It also looks like there are several layers of flows that have issued from both vents and were stacked on top of eachother. At least one flow broke out to the north from the northern vent.
In the Alca image, I am not so sure what it shows. Everything is much more eroded and nothing looks like a dome. The ridges may be very thick and eroded lava flows. Again just judging by the thickness (and I do not know the height of the ridges) I would expect them to be very viscous and probably rhyolites."