How mountains grow

In the Lake District of Cumbria, asking older local farmers how the fells grew will often get the response that they started out as pebbles.  The justification of this seemingly implausible hypothesis is that once a field is cleared of boulders, about 20 to 30 years later new ones have appeared and the clearing has to start again.  Geologists have their own ideas.  Compressive deformation of continental crust will thicken it, and gravity acting on this low-density material will ensure that its surface rises.  Counter-intuitively, the action of erosion can cause mountains to rise as well.  Debris flushed from deep valleys lessens the load on the underlying crust, so that it continually rises to drive up the elevations of the remaining ridges and peaks.  The compressional origin of the Himalaya is hard to dispute, yet they bounced up quite quickly, long after they began to form.  Current ideas, backed up by a variety of evidence, suggests that a lump of the dense lithosphere beneath the India-Asia collision zone fell off (delaminated) and sank in the mantle.  That reduced the mass of the lithosphere beneath and the gravitational field, so that the surface rose.  The second highest mountains, the Andes, offer no such mechanism, for they are not products of compression associated with collision.  Dense Pacific Ocean lithosphere subducts beneath them and the forces involved are insufficient to raise the Andes to even half their present elevation.  Simon Lamb of the University of Oxford and Paul Davies of the University of California, Los Angeles have attempted an explanation for the anomalously high Central Andes (Lamb, S. & Davies P. 2003.  Cenozoic climate change as a possible cause for the rise of the Andes.  Nature, v. 425, p. 792-797).  Their idea is that sediments that pour into subduction-related trenches from rising arcs, to form part of the accretionary prism where lithosphere starts to go down, lubricate subduction because of the pore water in them.  If there is little sediment supply from the rising crust, then frictional forces build up along the line of the subduction zone.  That focuses the plate boundary stresses over a narrow zone, thereby giving sufficient force to drive the crust higher and higher.  Today the cold northward ocean current along western South America provides little rainfall to the Central Andes, so erosion is much slowed.  Episodic global cooling since the Mid-Eocene probably reduced erosion there several times during the Cenozoic.  So for long periods the worlds largest subduction zone would have been starved of lubricants, thereby driving up the Andes.  The mountains themselves, by forcing maritime air upwards, would also starve the rising peaks and the great Altiplano plateau of rainfall, further influencing sediment supply to the trench system.  Lamb and Davies reckon that the Andes are fortuitous results of a N-S subduction zone at a continental margin, combined with its development during a period of global cooling and tropical drying.


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