Chile From Within

The Geology of the Juncal Valley

Parque Andino Juncal borders the Juncal River approximately 50 kilometers east of Los Andes, Chile (see area map below). The property extends across approximately 13,000 hectares, or, 26,000 acres in numerous valleys. This report describes some of the more interesting geologic and geomorphic features that can be seen in this unique area of the Andes. This geologic survey was written by John Rygh of Cordillera Consultants. He can be reached at jrygh at gmail.com

It is organized by the four significant geographic areas which are all within a day’s hike of the complex of buildings at Los Hornitos, at the end of the entrance road:

The Upper Juncal Valley is the main valley of the Park which extends south-east from the confluence of the Estero Navarro and the Juncal River, to the foot of the Juncal Glaciar.

The Cajon del Estero Monos de Agua extends east from the confluence of the Estero Monos de Agua and the Juncal River to Francisco Moreno Pass of the El Plomo Glaciar.

The Cajon del Estero Navarro extends east from the Juncal River at Los Hornitos along the Estero Navarro and to the bifurcation of the valley which occurs at the rock glaciar at the foot of the Cerro Tres Gemelos.

The Mardones Valley extends north east from Los Hornitos and terminates in the Mardones Pass, with extensive views of the Mt. Aconcagua. This valley is not touched upon in this report.

Regional Geologic Setting

The Andes mountains are a product of one of the greatest subduction zones on earth. The Peru-Chile trench on the ocean floor off the coast of Chile is where the Nazca tectonic plate is being driven underneath the South American plate. Over the course of 30 million years, this collision has resulted in the westward growth of the South American continent.

As the Nazca plate is subducted faulting scrapes the upper layers of marine sedimentary rock off and it become attached to the South American plate. At the same time, sediment which has eroded from the continent is deposited in basins on the westward-growing continental margin. When the Nazca plate dives to depths of 20-50 km. the rock close above this subduction zone begins to melt.

This molten rock then rises into the overlying rocks of the South American plate. Here it may solidify below the surface to form large bodies of rock known as plutons, or it may be erupted on the surface from volcanos. It is this continual process of sedimentary accretion, volcano building, and uplift due to compressional folding and faulting of the whole pile of rocks that gives the Andes their great height.

While these geologic processes are building the Andes up, the erosional processes of ice and water are tearing them down. The landscape seen in the Juncal Valley today is a demonstration of the dynamic balance of these competing forces. It is also a living registery of these complex interactions over the ages. This is what you see when you walk through Juncal.

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