Volcanoes past and present

The lake district is in the northwestern part of Patagonia, where the landscape has changed somewhat. In Argentina, the lakes are still on the dry side of the Andes, but the country border here jogs a little westward to capture more of the cordillera with its mountains and green forests. The nothofagus is still here—southern beech tree that evolved when the continents were together in Gondwana—but there are also abundant conifers. The towns of Bariloche and San Martín de Los Andes are the two main jumping-off points to explore this region.

The geology has changed too. If you look at the map in the 12 February posting—tectonic plates at the end of the world—you will see that the oceanic plates on the west side of South America change along the length of Patagonia. In the south (Austral Andes) the Antarctica plate is subducting at a rate of only 1–2 cm/year and there are a few scattered volcanoes that are not prominent features of the landscape. Farther north (around 46–49 degrees latitude) there is a volcanic gap where the Chile Rise (mid-ocean spreading ridge) is being subducted beneath South America. But here in the lake district (north of 49 degrees) we are now in a classic Andean volcanic area, where the Nazca plate is being subducted beneath South America at a rate of about 7 cm/yr and producing a chain of stratovolcanoes that evoke the conical shapes of Mount Fuji or Mount Shasta.

The currently active volcanoes are on the west side of the cordillera, in Chile, where volcanoes like Villarica are among the most active in all of the Andes. But, being in the zone of the westerly winds, Argentina receives much of their ash. Most recently, on 4 June 2011, Puyehue volcano erupted suddenly and spewed ash that covered Bariloche, San Martín, and the surrounding area with up to 0.5 meter (1–2 feet) of whitish, silica-rich ash that disrupted daily activities and closed airports throughout the northern part of Patagonia.

Much of the landscape between Bariloche and San Martín appears to be dusted with snow. All of the slopes in this photo are white because of their coating of ash.


Yesterday we traveled on a road that runs east–west directly west of Puyehue volcano. Even 9 months later, the effects of the ash were quite evident. A sign at the start of the road warned us of possible low visibility due to the ash (ceniza).


Along the river were large quantities of ash that had been formed into rippled bedforms, and the water was a whitish beige color from the ash it was carrying away from the surrounding slopes. In the background, the exposed bedrock is volcanic rock from a much older episode of active volcanism in the region. If you like volcanoes, this is a very good place to visit!


One thing that hasn’t changed from the south is a landscape that has been recently scoured by glaciers. The lakes are long, narrow “finger lakes” that were carved by ice and that are bounded by glacial deposits such as moraines.


Here, though, we are north of the Patagonian ice field and the ice forms only isolated glaciers. Even near the peaks, many of the glaciers have been completely melted, leaving behind the classic landscape of knife-edge ridges and elevated cirques.

The region is rich in national parks, the most popular of which is Parque Nacional Nahuel Huapi. Bariloche is at its edge and unfortunately has suffered from overdevelopment as a result. A redeeming feature, however, is the ability to take local buses to the park, where day hikes or multi-day backpacking trips can be made. The local alpine club (Club Andino) operates a series of refugios (mountain huts) that offer basic lodging and food. We did a day hike to Refugio Frey, at the base of Cerro Catedral. Like our Cathedral Peak, in the Sierra Nevada, this peak is made of Cretaceous granite that cooled deep in the earth, in this case when the magmatic arc (chain of active volcanoes) was located east of its current position.


A glacial erratic (large boulder dropped from ice) along the path has provided the roof for another, much smaller, refugio!


Another redeeming aspect of Bariloche was its celebration, on 28 February 2012, of the 200th anniversary of the national flag. In the main plaza, a military band played while a group of men and women danced. In the background is an example of the buildings in the Centro Civico (civic center). They are made of a distinctive green volcanic rock and local wood. Unfortunately, much of this charming architectural style has been replaced by less attractive modern structures.


Still, one can’t feel too badly about a place that is just a short bus ride from a national park with beautiful sedimentary rocks filled with angular metamorphic clastic (prominently displayed in photo below), abundant evidence of ancient and active volcanoes, and, of course, glacially-scoured landscapes!


Posted in ,

Subscribe to Blog

Enter your email address to subscribe to this blog and receive notifications of new posts by email.