The influence of volcanism on Andean Rivers

The Andean Cordillera spans more than 60° of latitude along the western margin of the South American continent. The climate, tectonics, and morphology of the range are known to vary considerably and predictably with latitude (Montgomery et al., 2001). The southern limit of the highest peaks in the Cordillera is ~35° S; between here and 33° S, recent volcanism is common and diverse. This portion of the range is also narrow and rivers originating at the range crest quickly descend either to the Pacific Ocean or the Atlantic Ocean. Rivers flowing eastward from the range crest into the broad, arid Argentine plain provide examples of the interaction between rivers and active volcanism. Google Earth imagery in the region is excellent. Copying and pasting geographic coordinates given herein into a Google Earth browser will take the reader to a bird's eye view of the locality being discussed.

Explosive silicic volcanism provides enormous (and instantaneous) sediment loads to rivers and permanently alters drainage patterns. Deposits of volcanic ash associated with explosive volcanism also provide essentially isochronus horizons which can serve as chronometers of geomorphic processes. The ~450 ka eruption of the Diamante Caldera (S34.2° W69.7°) emplaced 450 km³ of volcanic ash in the surrounding drainages and resulted in debris flow deposits >30 m thick as far away as Santiago, Chile (Stern et al., 1984). Three river drainages were variably affected by this event, and each records a unique history of this eruption and the ensuing half-million years (Figure 1).

Near the confluence of the Río Papagayos and Río Yaucha drainages a 15 m thick deposit of volcanic ash from the Diamante eruption is quarried for commercial uses (S33.9713° W69.0643°). The ash is loosely consolidated and has a bright white appearance (Figure 2). Above this eruptive deposit lies another 15 m of reworked volcanic material (primarily pumice and welded tuff cobbles) from the eruption of the Diamante Caldera. Presumably this reworked material is a result of headward erosion into the eastern scarp of the Diamante Caldera (S34.215° W69.600°). Geologic evidence indicates that a paleo-river flowing northward at the time of eruption was overwhelmed with eruptive material resulting in widespread fluvial deposits of Diamante ash and associated material (Polanski, 1962). These deposits are currently 20-30 m above the modern river level.

Many deposits of the Diamante ash are known from the geomorphic record of the Río Diamante. One of the most obvious deposits is a 40 m thick pyroclastic flow deposit ~ 100 m above the modern river level near the range front (S34.6872° W69.5025°). This deposit has a different appearance than most other occurrences of the Diamante ash, but its geochemical composition is identical to other deposits (Baker et al., 2009). Approximately 5 km downstream the Diamante ash is exposed in a gully ~ 200 m above the modern river (S34.7084° W69.4455°). Deposits of the Diamante ash are also documented 15 km further downstream as the Río Diamante flows through a canyon (Baker et al., 2009). The extensive deposits of the Diamante ash along the Río Diamante demonstrate that amount of incision during the intervening 0.5 Ma has varied significantly along its course.

Nearly 100 km south of the Diamante caldera a thick deposit of the Diamante ash is observed in the Río Atuel drainage at the range front (S35.0955° W69.6292°). This deposit is exposed along the shores of a small lake, and sits 20-30 m above the level of the modern Río Atuel, an observation that is consistent with what we observed at the first locality and a stark contrast to the Río Diamante. Together these localities demonstrate the utility of identifying a widespread and isochronus horizon. At each locality the Diamante ash is interbedded with fluvial deposits and it has a known elevational relationship with modern rivers. Combining this information allows estimation of long term average rates of fluvial incision. Because many geomorphic processes other than incision (i.e. aggradation of fluvial deposits above the level of the Diamante ash deposit and at other intermediate levels) have occurred in the last 0.5 Ma, these rates should be viewed as absolute minimum estimates. Still, these estimates successfully identify differences in the rate of downcutting by rivers at the range front and also identify variable amounts of incision along the course of a single river (see Baker et al., 2009 for a thorough discussion).

It has been suggested that the caldera forming eruption of the Diamante ash had significant impacts on the geologic and geomorphic history of rivers draining the Andean Cordillera. Abundant geologic evidence in the Río Papagayos and Río Yaucha drainages supports this notion (Polanksi, 1962). More cryptic, but equally intriguing evidence exists in the Río Atuel. This evidence comes in the form of large Diamante pumice (>8 cm in length) in the Río Atuel deposit nearly 100 km south of the caldera. Pumice of that size and density has a limited range of dispersal during explosive eruptions. An extremely violent eruption with a column height of 40 km and a strong southerly cross-wind is capable of ejecting such pumice ~ 20 km from the caldera (Carey and Sparks, 1986). These discrepant observations can be reconciled if the pumice was transported most of the distance by the paleo-Río Atuel. This, however, also requires that the headwaters of the paleo-Río Atuel were 20–25 km closer to the Diamante Caldera than present. If the headwaters of the Río Atuel have indeed been rearranged significantly in the last 450 ka; caldera forming volcanic eruptions are a plausible mechanism for such changes (Figure 1).

As a matter of comparison, the reader is encouraged to investigate the interaction between recent basaltic-andesite lava flows and the nearby Río Salado (S35.1855° W69.7866°). Field investigation suggests that the flow may have dammed the Río Salado, resulting in a short-lived lake upstream (Figure 3). The impact of this flow on the long term geomorphic history of the Río Salado is difficult to estimate, but long term effects of basaltic volcanism at Volcan Payún Matru (S36.4010° W69.2889°) are clear. The eruptive volumes of this shield volcano are much larger and the geomorphic record along the volcano's western margin indicates a long-lived influence on the southward flowing Río Grande.

• Baker, S.E., Gosse, J.C., McDonald, E.V., Evenson, E.B., and Martinez, O., 2009. Quaternary history of the piedmont reach of Río Diamante, Argentina. Journal of South American Earth Sciences 28, 54–73.
• Carey, S. and Sparks, R.S.J., 1986. Quantitative models of the fallout and dispersal of tephra from eruption columns. Bulletin of Volcanology 48, 109–125.
• Montgomery, D.R., Balco, G., and Willet, S.D., 2001. Climate, tectonics, and morphology of the Andes. Geology: v. 29, p. 579-582.
• Polanski, Jorge, 1962. Estratigrafía, Neotectónica y Geomorfología del Pleistoceno Pedemontano entre los ríos Diamante y Mendoza (Provincia de Mendoza). Revista de la Asociación Geológica Argentina 17, 127–349.
• Stern, C.R., Amini, H., Charrier, R., Godoy, E., Herve, F., Varela, J., 1984. Petrochemistry and age of rhyolitic pyroclastic flows which occur along the drainage valleys of the Río Maipo and Río Cachapoal (Chile) and the Río Yaucha and Río Papagayos (Argentina). Revista Geológica de Chile 23, 39–52.