Aconcagua

At the beginning, the confusion might have aroused from the appearance of the Colossus, especially from the north, when the wind rising from the southern wall causes fumaroles. The idea persisted even after the arrival of the first explorations. Paul Güssfeldt was the first one to write a report saying that Aconcagua was not a volcano. However, after analyzing some rocks in his 1883 summit attempt and discovering that they were volcanic, he took back his own words and said that Aconcagua was a volcano. It was Dr Walter Schiller who finally proved it otherwise. Some of his research published in 1907 and 1912, and later ratified by further field work, indicates that the lower third of the massif is made up of sedimentary rocks, Schiller’s first piece of evidence against the theory of the volcano. Another key point was the absence of an opening to let magma out after the Andean folding, together with the apparent absence of lava remains.

Finally, the volcanic rocks found on the two upper thirds of the mount (the very same that Güssfeldt had analyzed) were not local; that is, they had been carried from west to east by powerful tectonic events and had not been the result of Aconcagua’s volcanic activity. Therefore, Mt. Aconcagua’s shape is not that of a volcano, but rather the result of the overall rising movement of the mountain range.

Rocks in the Aconcagua region can be classified in three major groups:

First group: pre-Andean basement

This basement is made up of very old metamorphic, volcanic and other rocks from the lower Paleozoic, and some from the upper Precambrian. There are also igneous and magma rocks.

Second group: sedimentary deposits from the Mesozoic

A series of deposits settled on this pre-Andean basement from the beginning of the Jurassic to mid-Cretaceous periods. Such sedimentary deposits are the result of the back and forth movements of the sea on these lands. Six cycles of such movements have been determined, with their corresponding transition periods.

“Lotena”

Formation: First, a basal conglomerate can be distinguished, followed by sandy limestone, underlying, in turn, reddish conglomerate.

“La Manga”

Formation: Fossilized, compact limestone is observed.

“Auquilco”

Formation: Overlying this limestone are thick layers of plaster stones, probably the result of volcanic eruptions. Sulfur trioxide and dissolved-in –the-sea sandstone combined and precipitated, forming important deposits.

“Tordillo”

Formation: Sediment deposits of continental origin are found. They are made up of a mixture of conglomerates and sand.

“Mendoza”

Group: Next, sea sediments are found with abundant fossil fauna (ammonites and bivalves), where well stratified sandstones appear. The great number of fossils evidences the rising of the continent and the slow withdrawal of water, giving way to transition deposits of sandstone, as well as fine and regular sand.

“Huitrin”

Formation: Plaster stone is this formation’s main component, but it is scarce in the Aconcagua region.

“Diamante”

Formation: Sediments of this formation mainly contain sandy clay, limestone, fine sand, some schistous materials and some reddish conglomerates. They are the last stage in the sedimentary process.

Third Group: volcanic rocks (andesites)

Overlying the sedimentary rocks are the volcanic rocks, which are mainly andesites and porphyrites. It is the Formación Abanico (fan formation) and includes an important part of the upper third of Aconcagua.