Thursday, June 6, 2013

Cerro Tronador, San Carlos de Barilloche

This extinct horseshoe shaped basaltic stratovolcano straddling the Chileno border is a perfect example of what remains after a lateral blast eviscerates a perfect cone, in an explosion similar to the event at Mt St Helens in 1980. New magma intrusion combines with the weakness of the mountain (made of layers of lava and ash that has been weakened by hydrothermal alteration from circulating magma heated groundwater cells) to cause the whole cone to explode sideways. The area is sits in is a post glacial landscape of U shaped valleys, glacial lakes (such as Lago Barilloche itself) and fyords. It became extinct 300000 years ago, as the volcanic front from the subduction of the Pacific plate under the South American one had one of its periodic westwards shifts.

The mountain is glaciated, with eight blue white glaciers radiating from the top, one of which feeds a black glacier within the crater by falling into it. The black glacier, coloured by volcanic mud, then calves as it melts into a river below within the crater. Ice can be seen crashing down with binoculars, and waterfalls plummet down the vertical layered inner sides of the horseshoe. Despite the high accumulation rate from the moist Pacific winds, the ice is retreating fast, like most mountain glaciers worldwide.

The name comes from the Spanish for thunderer, from the rumbling noise the seracs make as they crash into the old crater. The cliffs surrounding it give an excellent impression of the layered nature of stratovolcanoes, with different colours of lava and ash and intruded dykes tickling the eye as it roves around the walls. At 3470 metres, it towers over the surrounding mountains, frequently developing orographic clouds like the one in the picture, due to rising air condensing as the winds pass over the peak.

The drive from Barilloche is 90Km, half of which is by a spectacular ruta ripio (unpaved road, but no need for 4WD) that takes you through fields of lava, welded ignimbrite deposits with columnar joints (the remnants of huge pyroclastic flows) and the typical hummocky deposits of blast induced debris landslides. To the seeing eye, these quiet and cold rocks record moments of fire and destruction that flit into the mind's eye as one drives along. Entering the horseshoe through the gap, a tall spine of old lava blocking the conduit becomes visible, that was once extruded red hot like toothpaste before freezing in the air. There are plentiful trekking and climbing opportunities, and the glaciers are accessible from a refuge or hikes. A beautiful campsite with cafe within the crater offers spectacular breakfast views of the guts of a volcano.

Image credit: Anonymous on Wikimedia Commons.

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