Cerro Blanco lies on the southern rim of the Robledo caldera, and is surrounded by an extensive apron of pumice fall deposits. It is unusual in that the lava is of high silica rhyolite composition, isotopically distinct from the dacites of the near-by Cerro Galan caldera (Francis et al., 1989). It also differs from the other two centers in the well preserved pyroclastic flows were erupted prior to the extrusion of the dome. These are best exposed on the floor of the Robledo caldera. Satellite geodetic surveys by Pritchard and Simons (2002) showed subsidence of Robledo caldera in the 1990s.
Francis, P.W., Sparks, R.S.J., Hawkesworth, C.J., Thorpe, R.S, Pyle, D.M. Tait, S.R. Mantovani, M.S. and McDermott, F., 1989. Petrology and geochemistry of volcanic rocks of the Cerro Galan caldera, north west Argentina. Geol Mag. 126, p515-547.
Pritchard, M., and Simons, M., 2002. A satellite geodetic survey of large-scale deformation of volcanic centres in the Central Andes. Nature, 418: 167-170.