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Evidence from the coast of Devon Looking for the Ice Age Shore Line Magnetic Reversals and Crustal Overturn Risk of another overturn is still high! Evidence from the coast of Devon
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Crustal Overturn and Volcanic Island ChainsOne of the few objections that have been put forward to the theory of crustal overturn are a few lines of volcanoes under the Pacific Ocean. This is based on the theory that volcanoes form above rising plumes of magma. As the crustal plates drift over the rising plumes, the rising magma burns new holes, and the ne volcano errupts at the end of a long chain of dead volcanoes. But there are no chains of volcanoes showing increasing age for most volcanic systems. The Island of Mull, a fossil volcano, sits on its own off the west coast of Scotland, but there is no chain of younger volcanoes leading to currently active ones. The volcanoes of the Canary Islands do not form a chain that errupts afresh over a static plume. They have errupted through the same crustal structures for many millenia and this drifts with the crustal plate. The volcanoes of Germany seem to have errupted recently as a result of some transient event. Most volcanic structures sit on the margins of the crustal plates - and move with the crustal plates. The volcanic island chains of the Pacific may have much more to do with stresses in a very large oceanic plate that result from a major crustal movement event. When the plate is stressed, cracks propagate fom the ends of existing cracks and provide a fresh path for molten magma to reach the surface and create a new volcano. Where the older parts of the crack have been filled with injections of magama, this may relieve the stress, and remain more plastic, effectively sealing the route for further volcanic activity. If the theory of crustal overturn is correct, then a major period of volcanic activity would be expected along the edge of crustal plates immediately following the overturn. many of the volcanoes that errupted would be small, and rapidly return to dormancy. There should be a marked decline in the activity of small volcanoes over the last 10 000 years. This activity and its decline should be most marked in the zones of maximum stress during the overturn : a line through Central and South America, and the regions close to the Philipines and Japan. Volcanic systems outside this main stress zone, and even many of those inside it need not have been triggered by the crustal overturn. There are plenty of examples of severe earthquakes in volcanic systems not triggering erruptions. This information is copyright Peter Thomson 2001-2004 The following files on this site match your querySearch for any topic in this forumMore pages or to contribute to the discussion- look at the list in the main index.
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