lunes, 15 de junio de 2009
Socompa is a stratovolcano at the border of Argentina and Chile. A large composite complex, Socompa is best known for its large debris avalanche deposit, widely accepted as the best preserved example of this type deposit in the world. The volcano is difficult to reach - either from the north along dirt tracks south of the Miscanti Pass, or from the west via the Escondida copper mine. Both routes require a full-day's driving and for any reasonable amount of time to be spent at Socompa would need significant planning.
The western rim of the volcano borders the Monturaqui Basin, which is draped with the large debris avalanche deposit. Escondida mining currently has a network of roads covering this area, from beneath which they pump ground water for use at the nearby copper mine. The southern margin of the deposit is bordered by the Antofagasta to Salta trans-Andean railway, although this is rarely used.
Socompa Debris Avalanche Deposit
The debris avalanche deposit is distinct in that its volume sets it apart from most other known terrestrial debris avalanches. Prior to the 1980 eruption of Mount St. Helens, many debris avalanches were miss-interpreted, and the Socompa deposit was initially linked to pyroclastic flow products of a cataclysmic eruption. It was first recognised as resulting from volcano collapse by Peter Francis and others in 1985, when they described the major features and reclassified it as a debris avalanche deposit. Subsequent works studied the deposit itself in more detail. It contains many features expected from a debris avalanche, including large-volume, rotated and slumped toreva blocks and hummocky topography. There is also evidence for a magmatic component (Bezymianny-type collapse) from the breadcrust texture of large dacitic blocks and a thin pyroclastic flow deposit. A large amphitheatre, open at 70° and with a width of 10 km at its mouth, marks the site of collapse on the remaining edifice. Since the failure, some 7000 years ago, this has been partially filled by subsequent lavas and pyroclastics.
The most striking aspects of the deposit are its volume, deposition and composition. The deposit has a volume of 25 km2 (10 sq mi) – around an order of magnitude greater than the Mount St. Helens collapse - in addition to 11 km2 (4 sq mi) of toreva blocks at the mouth of the amphitheatre. While a significant component clearly originated from the ancestral Socompa edifice, there are also large amounts of ignimbrite and gravels which have been shown to have come from the substrata immediately below Socompa. By volume, these make up the bulk (80%) of the deposit. Additionally, despite originating at the lowest part of the failure zone, these units travelled the furthest distance and are found at the base of the deposit. The avalanche travelled down the regional slope for part of its course before mounting at least 250 m (820 ft) of topography near to its distal end, suggesting a high speed of emplacement, low friction and great mobility. There was also considerable remobilisation of the deposits and secondary flowage after the initial deposition, creating the lobe which was channeled northwards under gravity towards the Monturaqui Basin.
The large volume and stratification of the deposit suggests that the failure was not merely the result, as at Mount St Helens, of slope failure of the volcanic cone. Structural evidence has recently been interpreted to suggest that prior to the failure the weak underlying substrata had been spreading under the load of the volcano. The remnants of thrust anticlines at La Flexura, west of the collapse amphitheatre, delineate the western edge of this spreading zone. The suggestion is that the deforming substrata suffered catastrophic failure as a result of gravitational spreading and was ejected to the northwest on the collapse of the basal anticlines. The substrata then formed the lower horizon of the debris avalanche, upon which the remainder of the edifice was carried and deposited. As a consequence, the large volume, high fluidity and stratification of the deposit can be explained.