|Geology says high mountain peaks are formed in
subduction zones where a continental plate riding
on an oceanic crust meets a similar plate. This is
also as a result of the seafloor spreading that
has to be compensated for, due the fact that the
Earth has a unit Mass and Area. My question here
is: why has the mountain range (especially
Everest) not extended it's peak indefinitely,
since a new seafloor is created at the mid oceanic
Ridge? Thank you|
|Question Date: 2013-04-08|
Interesting question! You are correct that the
earth is not expanding or contracting, therefore
new crust formed at mid ocean ridges must be
balanced by recycling of other crust back in to
the mantle. This process occurs at subduction
zones. The Pacific Ocean is probably the best
example of this. The East Pacific Rise (the mid
ocean ridge in the pacific) spreads at a very fast
rate. The oceanic crust that is created at the
ridge spreads outward and is eventually recycling
back into the mantle in subduction zones, for
example, below the Andes, Cascades, Aleutians,
Japan, and New Zealand. Volcanoes that form above
subduction zones make up the Ring of Fire around
the Pacific Ocean.
The Himalaya and Mt. Everest is a slightly
different setting. This is an area of continent
collision. There was once a subduction zone in
this area, but it was basically “clogged up” when
India collided with Asia. India is made up of
continental crust, which is much less dense than
the underlying mantle. Therefore it cannot easily
be subducted into the mantle; it’s like trying to
get an ice cube to sink in a glass of water. India
and Asia continue to converge, but neither can
sink into the mantle, and that is why large
mountains form in such areas. The uplift of the
mountains is balanced by erosion. In fact, the
Himalaya are probably near a maximum height that
mountain ranges can reach. The faster that
mountains uplift, the faster they erode.
Mount Everest is actually still growing about
3-5mm per year! But perhaps eventually the plate
tectonics in the area will alter and it will not
continue to grow. Tectonic plates shift around due
to slow magma currents under the Earth's surface.
It is because of these currents that the surface
of the Earth is constantly changing.
That is a very good question. The reason that
mountains do not keep growing up indefinitely is
because of erosion. Although the mountain keeps
being uplifted and growing higher, the peaks are
eroded. This erosion takes places in different
ways. Two common examples are erosion from the
freeze/thaw cycle and erosion from rain. The
freeze/thaw cycle is that water gets into the
cracks between rocks, freezes, and expands. This
happens over and over and over and over...breaking
off chunks of the rocks over time. Due to gravity
these rocks will eventually fall. It might be hard
to imagine that erosion can keep the mountains
from growing up but both tectonic uplift and
erosion happen over thousands and millions of
years. When tectonic uplift rates exceed erosion
rates, the mountains grow higher (e.g. Himalayan
mountains), when erosion rates exceed tectonic
uplift rates the mountains grow smaller (e.g.
In one word: erosion.
The higher mountains get relative to
surrounding terrain, the faster they are eroded.
This prevents mountains from getting too high,
because eventually erosion catches up with rate of
uplift and the mountains stop growing.
Additionally, mountains that get high enough
tend to break up along fault lines that result
from the limited strength of rock. I believe it is
currently thought that the Himalaya are close to
this limit already, but that doesn't explain why
all mountain ranges aren't as high as the
Himalaya. The real reason, as I said, is erosion.
The collision of plates is indeed ongoing and
may create mountain ranges, but it exists in an
equilibrium with the effects of gravity and
erosion. Thus the Himalayas (and the Santa Ynez
Range) are both being simultaneously uplifted and
Erosion takes place through the action
of wind, water & freeze thaw, (and in the
Himalayas, water in the form of glaciers),
breaking rocks into smaller component parts that
then wash or roll down slope, answering to
gravity. This is why old mountain ranges that are
no longer being uplifted, erode to lower & lower
elevations over time.
Additionally, the Himalayas are
interesting in that it has been suggested that
they are also collapsing under their own weight,
sloughing off marginal units much as a tall
wedding cake might press down on its center and
slowly push the lower layers out laterally.
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