MadSci Network: Earth Sciences

Re: What plates are colliding to form the Andes Mountains?

Date: Thu Feb 14 17:38:43 2002
Posted By: David Scarboro, Faculty, Earth Sciences, The Open University
Area of science: Earth Sciences
ID: 1012226705.Es

Dear Ashley,

The plates whose collision is uplifting the Andes Mountains are the South 
American plate and the Nazca plate.  The Nazca plate extends eastwards 
from the East Pacific Rise, a constructive plate boundary at which new 
ocean floor is being formed.  The Nazca plate is moving eastwards from the 
East Pacific Rise.  The South American plate is moving westward from the 
Mid Atlantic Ridge.  Where these two plates meet the Nazca plate is being 
subducted beneath the South American plate.

The Andes Mountains lie above the subduction zone.  The subducting slab is 
heated by friction and by the hot mantle into which it is descending.  
This heating causes partial melting of the slab and the overlying mantle, 
generating magma that rises into the crust.  Most of the magma cools and 
solidifies in the lower crust, adding to its volume  a process known as 
underplating.  Some of it rises into the upper part of the crust and may 
erupt to form the volcanoes of the Andes range.  The high heat flow above 
the subducting slab also reduces the density of the crust, causing the 
crust to become isostatically buoyant.  Both underplating and the 
isostatic uplift of the region contribute to the formation of the Andes.

I hope this answers your question.

Best wishes,

David Scarboro

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