The geological source of Chivay obsidian is above 4800 masl among the lava flows from two Barroso (Pliocene) volcanic vents named Cerro Ancachita and Cerro Hornillo. The discussion here focuses on the geographical context of Survey Block 1, and the geology of these volcanic features will be explored in more detail in Section 4.3.3, below.

In the high altitude portions of the Upper Colca survey, most notably in the Chivay obsidian source area above 4800 masl, the local temperature and climatic exposure is strongly affected by the lack of vegetative cover and the katabatic (mountain breeze) and anabatic (valley breeze) winds. The winds were a daily feature during the Block 1 fieldwork in the months of August and September 2003, and the winds are most notable in the mornings and evenings when the temperature differential between the high altitude areas and the warmer Colca Valley are greatest. Extrapolating the local lapse rate from Colca meteorological stations suggests that the obsidian source, at around 5000 masl, would have mean temperatures of 0.5 to 1.0° C year-round (

Figure4-4), not accounting for the effects of wind in this exposed area. In the course of fieldwork daytime high temperatures were approximately 5° C, and nighttime temperatures were commonly -9° C with the coldest night measured at -12° C. These data largely corroborate the estimates derived from the local adiabatic lapse rate discussed above.