Andean supervolcano cluster.
High up in the Eduardo Avaroa area of the Puna Altiplano, shared between Argentina, Chile and Bolivia, lurks a remote supervolcano, whose past eruptions have been on a similar vast scale to Yellowstone. While it is far from any population centres, an eruption there would almost certainly have a global effect on climate. It isn't very well researched, due to its remoteness and altitude, which make fieldwork very challenging. The rocks are poorly exposed, since the region is one of the driest on Earth, so there are no convenient rivers to erode the cuttings rockhounds love. It is also quite flat, depriving us of the types of landform that show us a slice of the world's innards. Parts of the area are also strewn with landmines that follow the entire Chileno border, a legacy of Pinochet's paranoia and insanity (last year the main road border crossing between Chile and Peru was closed when rain exposed and moved some of them).
Research by geologist Miguel Soler, of the Universidad Nacional de Jujuy in Argentina, revealed that the Vilama caldera was formed during a single event that erupted an estimated 2,000 cubic km of silica rich ash in a huge pyroclastic explosion, one of the largest recorded in the pages of geological history.
The magma came from a complex series of tectonic melting processes, as the now near defunct Nazca plate subducted under the South American one. As one plate overrode another and the crust thickened, the heating and squishing of the rocks and falling of the Nazca plate into the mantle resulted in extensive melting and a series of large eruptions throughout the region.
As the roof of the magma chamber collapsed at Vilama like a piston down a series of faults, the ash erupted out 8.4 million years ago, leaving behind a 65x30 km caldera. Yellowstone in contrast erupted its ignimbrites (deep volcanic ash deposits, emplaced burning hot and sometimes molten) in three events, each producing a successive caldera over a period of 1.4 million years, the last being 600,000 years ago.
The explosion is thought to have been triggered by tectonic movements collapsing the roof rather than inner pressure, as the ignimbrites are crystal rich and seem to have lost the gases before eruption (being poor in pumice) that usually propels such explosions, implying a cooling magma chamber erupted by sudden pressure release. The chamber was probably a granitic batholith in the making, gently cooling when it was suddenly unroofed.
The Vilama caldera may not be the only such beastie in the region. Eduardo Avaroa consists of a complex of similar calderas, whose eruptive history remains poorly understood. Other likely supervolcanoes in the area include Cerro Guacha, Coruto and Capina. The region has even been dubbed a supervolcano nursery. The whole burst of volcanic activity they represent, triggered by the death of the Nazca plate, may be one of the biggest paroxysms in the recent history of our planet. Researchers estimate that between 10 and 1 million years ago, over 30,000 cubic km of volcanic products were erupted in the region.
Loz
Image credit of Volcan Zapaleri by the tri state border: Mquarg/Wikimedia Commons
http://www.sciencedaily.com/releases/2006/03/060328183109.htm http://www.geosociety.org/news/pr/06-13.htm
Original paper, paywall access: http://www.sciencedirect.com/science/article/pii/S0377027307000844
