Structure and Evolution
A prolific lava producer, Volcan Puntas Negras is a collection of flows and domes forming a distinct large edifice (Figures 30.1; 30.2). The lower flanks consist of an extensive apron of older pre-Holocene lava flows which extend out to almost 15 km from the present volcano on its northern side. These early flows have morphologies typical of andesitic lavas but are relatively bright on the TM images and are somewhat "smoothed out", consistent with some degradation and aeolian cover. The younger, wholly Holocene volcano is built on this platform (Figure 30.2).
Several sets of young lava flows associated with this centre can be discerned but it is not possible to tell which of these is the most recent. They typically exhibit well developed flow ridges and are relatively short and thick, suggesting a more silicic composition than the older andesitic flows, possibly as silicic as dacite. A large mass of these lavas occurs on the northern flank of the volcano (Lo) and appears to have been erupted from a vent below the summit region. The summit region contains a conspicuous 500 m diameter crater (SC) around which several lava flows are distributed radially (L). One of these flows, the one on the southeastern side, displays excellent levées. Light gray coloured material on the eastern flanks (P) maybe pyroclastic material, possibly hot avalanche deposits?
Structurally, Volcan Puntas Negras is located on the junction between the E-W trending Cordon Puntas Negras and the N-S trending Cordon Chalviri. At the southernmost end of the Chalviri chain is another substantial center, Cerro Tuyajto (5,482 m), which may also be Holocene, though it is clearly overlain by younger lavas from the Chalviri complex.
No current activity is known.
The rocks of Volcan Puntas Negras are typical of the Cordons Puntas Negras and Chalviri (q.v.). Deruelle (1982) presents two analyses showing that both andesites (59.66% SiO2) and dacites (65.29% SiO2) have been erupted.
Deruelle, B., 1982. Petrology of the Plio-Quaternary volcanism of the south-central and meridional Andes. Jour. Volcanol. Geotherm. Res. 14:77-124.
Deruelle, B., 1994. Volcan Puntas Negras and Cordon Puntas Negras (Northern Chile: petrology and geochemistry of minor eruptive centres. In: VII Congreso Chileno-Concepcion, Acta II, pp. 1029–1033.