MadSci Network: Earth Sciences
Query:

Re: Do mountains play a part in stabilizing the earth's crust?

Date: Wed Jan 2 10:42:29 2008
Posted By: David and John Free, Post-doc/Fellow, MFA, MFA
Area of science: Earth Sciences
ID: 1197759178.Es
Message:

Hi Ahmed

Your interesting question is a bit like "Do the wrinkles on an old apple 
affect how it shrinks" and the answer is the same - Very little!

The wrinkles are there BECAUSE the apple has shrunk: the mountains are 
there because of the impact between two massive plates. They are 
compression damage (buckling) at the edge of the plate.

The COLLISIONS do stablise the drift of the plates - by limiting the 
drift and causing earthquakes when the plates "stick slip" unstably as 
the plates slide past or over/under one another.

The earth's crust is very thin (about, to scale, like the skin on an 
apple) and the mountains tend to be of lighter material (less dense) 
than the rest of the crust. So the weight of the mountains near the edges 
of the crust is negligible compared to the total weight of the crust. For 
this reason the mountains tend to grow - rather than shrink as they would 
do if they were stabilising the earth's crust. 

In a VERTICAL sense mountains do a bit of stablising, and the crust can 
be said to be floating on the mantle beneath. But the weight of the 
mountain is negligible compared to both the crust beneath it and the 
convection forces of rising magma from depths that far exceed the crust's 
thickness. These thermal convection currents are due to the instabilities 
in the rising mantle material (it's not actually liquid, but soft and plastic and
yields slowly over time..typically at the rate of inches per year. It does not
actually melt until it gets pretty near the surface where the pressure is lower). 
You see these cell-like instability patterns on the surface of a pan of soup no
matter how evenly heat is applied from below. 
Hot mantle MUST rise - so in other places it falls. This is presumably 
what causes comtinental drift as well as upheaving forces that exceed the 
weight of the crust (plus mountains). 

The crust is thinnest under the sea and there, if weight was to be a 
stablising factor, is where you would see the effect. The mountain plus 
the crust it is sitting on is far thicker than the sub-sea crust. The 
vents at the mid-ocean depths (where plates are moving apart) are there 
because the crust there is thinnest (most fragile) not because the total 
vertical weight is less there.

John




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