The jerky growth of the AndesFormed by the tectonic compression and volcanism that accompany the sub

The jerky growth of the AndesFormed by the tectonic compression and volcanism that accompany the subduction of the Pacific, Antarctic and Nazca oceanic plates under the South American continental plate, the ranges of the Andes extend 7,000 km north to south through seven countries down the west coast of the continent. Split into several ranges, this chain is the longest and second highest on Earth. There are several high plateaus (uplifted flat areas) between the ranges, including the Altiplano (the world’s second highest after Tibet )at the range’s widest point, the Bolivian flexure. The chain’s name is derived from the Quechua word anti (meaning high crest).They started to rise during the Triassic, and are still growing today. Darwin famously witnessed the rise of the land during a quake during his visit aboard the Beagle, and used this observation to argue for the current shape of the world being the result of forces still observable today, a theory known as uniformitarianism. While not entirely true (we fortunately don’t experience dinosaur killing asteroid impacts every week), this way of thinking is a fundamental tenet of geology. As the compression increases, the crust reacts by folding, faulting and metamorphosing as is shortens, thickens rises and heats up.The Andes have grown at different rates in different areas (and been eroded, due to varying climate zones along the range), and a long standing debate is ongoing over their rate of uplift. A new paper just published in latest Earth and Planetary Science Letters suggests that the Altiplano at least, and probably the entire range, grew in a series of brief spurts rather than a gradual continuous rise. Now before you start worrying that they may suddenly leap up a mile, rapid in geological terms means a kilometre rise over several million years.In past papers the same team came up with the first estimates for the rate of uplift, surprising the geological world with their suggested rapidity for the process. They used measurements of surface temperatures and rainfall composition taken from the geochemical analysis of rocks as a proxy record of past altitudes, assisted by the abundant layers of volcanic ash that allow for precise dating.They suggested that a mechanism known as delamination was responsible, whereby the deep dense keel of the continent periodically detaches into the mantle as the thickened crust heats up. This happens as the overlying weight of thickened crust pushes the basaltic continental keel through a depth where a pressure based mineral phase transition takes place, at which the basalt converts to a very dense rock called eclogite and sinks into the mantle. The remaining crust then has a lower density and rebounds up quickly as its buoyancy increases, making it float higher on the underlying mantle.Their recent paper adds more evidence supporting the hypothesis, studying two further regions using a new technique to see whether rapid uplift occurred there. They analysed the bonding of carbon and oxygen isotopes in rainwater precipitated calcite in ancient soils, preserved at both high (where climate gradually cooled as the altitude increased) and low altitudes (that remained at their previous warm climate). They then compared them, to assess the temperatures at varied places at different points in time, using it as a rough proxy for altitude. Different patterns of bonding form at different temperatures, giving an estimate that the southern Altiplano rose 2.5km between 16 and 9 million years ago. Such pulses seem widely separated in time, sometimes by several tens of millions of years.The team also suggest many ranges worldwide rise in a similar manner, so the race is now on to apply similar methods elsewhere, and gain a deeper understanding of the extent to which delamination is responsible for the beautiful peaks we regularly share on this page.LozImage credit: Noumenonhttp://www.sciencedaily.com/releases/2014/04/140421135926.htmOriginal paper, paywall access: http://www.sciencedirect.com/science/article/pii/S0012821X14001101 -- source link

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