Structure and Evolution
San Pedro (Figure 20.1, 20.2) is one of the few Central Andean volcanoes which have been studied in detail (Francis et al., 1974; O' Callaghan and Francis, 1986). It is a twin of the older Volcan San Pablo (SPA; 6,092 m). Both volcanoes are unusually large edifices, rising more than 2,500 m above local base level. San Pedro is itself a composite of two cones, each representing a different structural and geochemical stage of the volcano. Eruption of thin basaltic andesite lavas and beds of scoria built up an older cone (OC). This was subsequently modified by glaciation and a major collapse event which produced a large débris avalanche deposit forming a conspicuous feature on the pampa west of the volcano (DF). A major plinian pumice fall deposit which mantles the northern flanks may have been erupted at the same time. The Younger Cone (YC) which grew within the collapse amphitheater of the old cone is post-glacial and its lava flows (LF) all have pristine morphology. Four distinct phases of eruption were involved in the formation of the Younger Cone. These produced andesite and dacite lavas, pumice flows and hot avalanche deposits (HAD; Francis et al., 1974). An eruption taking place on a fault-controlled vent gave rise to youthful scoria cone, La Poruña (LP), and associated 8 km long lava flow. Although almost pristine, La Poruña appears to pre-date the most recent deposits from San Pedro proper. The most recent activity on San Pedro has been the eruption of a series of lobes of viscous dacite lavas (LD) from a vent high in the summit region of the young cone. Successive collapses of these lobes as they were extruded on to steep north-west facing slopes led to the extensive apron of hot avalanche deposits on the northwestern flanks (HAD). Casertano (1963) has reports of uncertain reliability of phreatic explosions taking place within the last century.
In favourable weather conditions, a feeble but persistent fumarolic plume can be seen emanating from the crater at the summit of the younger cone.
O'Callaghan and Francis (1986) described the geochemistry and petrology of the volcano. Compositions of the lavas are typically calc-alkaline, high-K basaltic andesites, andesites and dacites. There are significant differences between the Old Cone and the Young Cone lavas. YC samples are strongly enriched in MgO relative to those from the OC at similar SiO2 and slightly depleted in TiO2 and Al2O3. La Poruña is chemically related to the YC samples as are the samples from the southwestern dome (SWD). 87Sr/86Sr ratios of San Pedro-San Pablo rocks range from 0.70575 to 0.70717.
Casertano, L., 1963. Catalogue of the active volcanoes of the world, including solfatara fields: Part XV, Chilean continent, I.A.V.C.E.I. Naples, Italy, 55pp .
Francis, P.W., Roobol, M.J., Walker, G.P.L., Cobbold, P.R., and Coward, M., 1974. The San Pedro and San Pablo volcanoes of northern Chile and their hot avalanche deposits. Geol. Runds. 63, 338-357.
O'Callaghan, L.J., and Francis, P.W., 1986. Volcanological and petrological evolution of San Pedro volcano, Provincia El Loa, North Chile. J. Geol. Soc. Lond. 143, 275-286.