Figure 4.2: lithospheric columns B and C. Difference of GPE.
Collision zones and the Gravitational force
* When collision produces Mountain belts or uplift, gravitational potential
energy is being growing in the uplifted area.
* In the Figure 2 column C could be the area that has been uplifted during
collision and column B the undeformed lithosphere. As you can see the
uplifted column C has a higher GPE.
* As the Mountain belt grows the difference between the Gravitational Potential
Energy of the Mountain belt and the undeformed area grows.
* As the difference in GPE grows the Gravitational force will become a higher
and higher negative number.
* This negative gravitational force as described above produces a force that
* This gravitational force must be less that the tectonic driving force and
Lithospheric strength to maintain crustal thickening. (the tectonic forces
and the Lithospheric strength are the forces that allow the Mountain belt
to be built, the gravitational force opposes these forces)
* Eventually the gravitational force will become equal to the tectonic driving
force. The Gravitational force produced by the difference in GPE will balance
the force applied due to the collision of the plates.
* Therefore the height of a Mountain Range is restricted by the force of
the collisional tectonic driving force and by the plates densities (a denser
material needs a larger force to be elevated a set height).
* When the tectonic driving force is balanced by the gravitational force
the crustal thickening will extend laterally (forms plateau).
* If the collision tectonic driving force was to reduce or cease the
gravitational force would be greater, therefore the Mountain belt would experience
Figure 4.3: Collision zone and mountain building.
There are two main modes of Gravitational collapse:
1) Fixed boundary divergent collapse, where the lithosphere surrounding the
unlifted area is fixed. The gravitational potential energy is transferred
from the uplifted area to the lowlands. In collision zones where Mountain
building has occurred this would involve lateral growth of the Mountain belt
(the surrounding lithosphere thickens).
2) Free boundary divergent collapse, where the surrounding lithosphere is
not fixed. Thickened crust thinned without gravitational potential energy
transfer into the surrounding area.