Elevation in the Andes

31 Jul 2010

Along the coastline of South America there are a series of well known stranded beaches which indicate changes in sea level - or seismic shifts. The idea that mountain building in the Andes was recent was popular at one stage because Lake Titicaca, for example, was formerly located at a much lower altitude, it was thought, as it had been a coastal lagoon. A few years ago, Lonnie Thompson took cores from the Quelcaya mountain glacier (see earlier post on In the News) in Peru and discovered that in 3200BC the region was suddenly elevated and plants that normally grow way below the current horizon were frozen in situ instantly, as when the same glacier began to recede as a result of modern warming the plants emerged intact. A connection was then made with Oetzi in the Alps. He was suddenly frozen and preserved in the ice - at precisely the same point in time. The question is - was there a mountain building episode at this time? The geology is explicit - no, there was not. What other explanation can there be? The clue may lie in Paul Dunbavin's book, Atlantis in the West (available from the SIS book service) in which a small change in the axis of rotation may have occurred around 3200BC, and the Mid Holocene Warm Period may correspond with a slightly more upright tilt of the earth. Hence, the initial change may have occurred at around 6200BC (as an even bigger change in sea level is associated with that date), and the raised beaches around the coastline of South America may simply mark the change in geoid as the ocean waters realigned themselves. The simplicity of the explanation is what is attractive as it does not require massive tectonic forces at work. It also indicates Lake Titicaca was a coastal lagoon before 6200BC. Changes in the geoid result in some regions becoming submerged (sinking) and other regions rising up (elevation), so while South America appears to have been rising up out of the sea the continental shelf system around NW Europe and NE America was drowning. What about the Alps? Now, it just so happens they occur on the line Dunbavin draws between areas sinking and those that are rising, with four quarters. Hence, one part of the northern hemisphere sank and another part of it rose up, and the same would be true in the southern hemisphere where in fact the process is more pronounced as the continental land mass is spread out and smaller overall in comparison to the ocean. In SE Asia for example, the Sunda and Sahul continental shelf systems were submerged - at what appears to be roughly the same time the stranded beaches occur in South America. This post was prompted by the previous one in which tectonic processes appear to be inadequate to explain the raised beach phenomenon. It is of course not something that fits too easily into the idea of a smooth sea level curve, a gentle raising and lowering of sea level due to small processes at work in the natural world. However, it is a process that may account for the more rapid and dramatic rises in sea level as proposed by Rhodes Fairbridge.