Montain [Electronic resources] نسخه متنی

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Mountain-Anchor of the earth and stable

A better understanding of the role of
mountains in creating equilibrium and
stability of the Earth requires a knowledge
of the essence and structure of different
components of the Earth's crust . These
components )like mountains,plains and seas)
are not in the form of irregular ridges and
depressions which are located in the upper
section of the crust all of the components
are however,in a relative equilibrium;this
state results from a decrease or increase in
the density and also from a change in their

According to isostasy hypothesis there is
another layer parallel and under the surface
of the Earth on which the pressure exerted
from mountains,plains,and seas is equal.This
layer is knows as equilibrium or isostasic
layer. To get a clearer image of isostasy,
a simple example would be to imagine
mountains as great pieces of ice floating in
the sea . As we know the excess weight of
the piece of ice above the surface of water
counterbalances the lost weight of the
portion under water . To explain isostasy
different hypothesis have been proposed,
among which the most important ones are :

1 - Pratt hypothesis , according to this
hypothesis the higher parts of the crust
have a lower density and the pressure
exerted from all parts of the crust is
equal on the equilibrium surface.It worths
mentioning that the specific gravity of the
upper portion of the crust of the Earth
which is mainly made of granite rock and is
called sial is less than that of the lower
portion which is made of basalt rock and is
called sima;it has also been determined that
the thickness of sial in continents is to its
maximum amount , and in most oceans it is
absent or scanty.

2 - Airy hypothesis , according to this
hypothesis the specific gravity of the
Earth's crust is equal under its components,
however the dimensions of its components are
in such a way the higher they are from the
surface of the Earth the more their depth
in the ground is.It's worth mentioning that
the thickness of the crust is between 30 to
40 km. in the plainareas and between 50 to
60 km. in mountain ranges.In fact mountains
have roots;the more their height is from the
ground,the more the depth of their root is.

3 - Heiskaen hypothesis,this hypothesis is
actually in the middle point of the two
aforesaid hypothesis , it states that the
specific gravity of the components of the
crust varies proportionately with their
depth.Considering the facts mentioned,it is
quite obvious that mountains are the most
important factors for creating a balance in
the Earth's movement and stability through
their lesser specific gravity ) in comparison
with the lower and shorter parts ( and also
their deeper roots in the Earth's crust.

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