Nice question. There are many, many factors
that cause mountains to change with time, but
ultimately they can be broken down into the
factors that decrease the mountain altitude
versus those that increase it.
First the increase: basically there are
horizontal forces in the crust that cause some
areas to be uplifted. A good example at the
large scale is when two continental masses
collide with each other. An example of THAT is
where the subcontinent of INDIA has collided
with and continues to push up against Asia. So,
it is like having two rugs and pushing them
together. When they collide, the only place to
go is up provided one cannot go down beneath the
other. Since continents are made up of low dense
rocks, indeed, when they collide, this event
forces the land surface UP.
Now as far as the down part: Well, that is
mainly driven by erosion, mechanical erosion and
chemical erosion. An example of mechanical
erosion is say by freeze/thaw, water gets into
cracks of rocks; when it gets cold, water
freezes and expands and that cracks the rock and
creates particles. Gravity then takes these
particles DOWN HILL. If it rains, the rocks and
sands get carried by water downstream. Also
when it rains, the rocks dissolve a little bit
and the rivers wash the cations carried in
solution into the ocean.
According to all these events, the height of
a mountain is the result of the competition
between the forces of uplift through
DEFORMATION, and the forces of attrition or
weathering and transport by gravity (always
Finally, all these forces act upon each
other!!! So, contrast a flat coastal plain
versus a mountain top.
The rate of erosion is small on a plain
because gravity does not have much to work with.
On the other hand a steep mountain top is just
waiting for gravity to do its thing!
Naturally the climate plays into all this
also. The chemical weathering rates increase by
a factor of 2 for every few degrees of warming.
The warmer the faster rocks dissolve, but
temperature is related to altitude! Then again,
if temperature is low and glaciers forms, they
can be very erosive, even more so than water!
The processes are very complicated because
of the feedbacks.