Right off the main road, Chanka was formed on the northwest slopes of Azufre Volcano, and just north of San Pedro. Like most of the lava domes, Chanka is primarily dacitic in composition, and is extremely crystal rich (30%-40% crystals). The crystal rich nature of the lavas that formed Chanka, along with its dacitic composition, lead to incredibly viscous lavas. Due to this incredible viscosity, many of the domes take on a standard "torta" or cake shape, having steep sides and flat top.
While the exact date of Chanka is unknown, it is probably post glacial in age.