Notes on Crust,
Mantle, Core, Stratosphere and Atmosphere
Mohorovicic.He noted that in
measurements of seismic wave arriving from an earthquake, those focus lay
within 40km of the surface, seismographs within 800 km of the epicenter.
Recorded two distinct sets of P and S-waves. He concluded that one par of waves
must have travelled from
the focus to the station by a
direct path whereas the other pair of waves had arrived slightly later because
they had been refracted.
There are two types of crust:
The continental crust consists of
two layers separated by a well-defined discontinuity known as Conard
discontinuity. The layers have been defined on the basis of seismic waves
velocities and densities.
In the upper layers the velocity
of seismic waves corresponds to the velocity found by experimental to be
characteristic of granite. Hence they are called as Granitic or silica layer.
The earths crust beneath the
oceans consist of a low velocity layer of deep sea sediments about 300-400m
thick in pacific and 600-700 m in the Atlantic.
The Layer of intermediate
velocity called basement about 0,8 km thick, composed of compacted and
indurated sediments and lave flows.
The third layer is called the
oceanic layer about 4.1 to 5.8 km thick and certain composition. This
three-layered oceanic crust is generally 5 to 8 km thick.
Materials making the earth become
quite different in properties at the base of the crust. This depth below the
surface of the earth at which a striking change in the properties of the
materials is observed has been named as Mohovorovicic discontinuity. In
geological literature it is often referred as M-discontinuity or simply as
Moho.Hence mantle is that zone within the earth that starts from
M-discontinuity and continues up to a depth of 2900km.
made up of extremely basic material called aptly ultra basic that is very rich
in iron and magnesium but quite poor in silica. The material of the mantle is
believed to be variably viscous in nature .
It is the third and the innermost
structural shell of the earth as conclusively proved by the seismic evidence.
It starts at a depth of 2900 km below the surface and extends right up to the
centre of the earth, at a depth of 6370km.
The core remains a mystery in
many ways. Within the core the physical nature ands composition of the material
is not uniform throughout its depth. It has a very high density at mantle core
boundary above 10g/cc.The outer core behaves lime a liquid towards the seismic
waves. The inner core starting from 4800km and extending up to 6370 m is of
unknown nature but definitely of solid character and with properties resembling
top a metallic body.
The outer gaseous part of the
earth starting from the surface and extending as far as 700km and even beyond
is termed atmosphere. It makes only about one million part of the total mass of
the earth. The homogenous structure as follows:
composition of atmosphere:
Component Volume Percentage
(Ne, He, Kr, Xe) 00.003
Thermally speaking the atmosphere
shows an interesting height from the surface. Based on thermal characters the
atmosphere is divided into following layers: troposphere, Stratosphere,
Mesosphere and Thermosphere.
It is the second layer of the
atmosphere starting from the tropopause and extending up to san average height
of 50km.The stratosphere differs from the lower layer in following respects.
temperature becomes constant for a height of 20km and then starts increasing.
contains almost the entire concentration of OZONE GAS that occurs above the
earth in the form of a well-defined envelope distinguished as the Ozone layer.
stratosphere itself has a layered structure and there is no significant mixing
or turbulence of gases in this layer.
layer starts at a height of 9km above the surface and continues up to 35km.The maximum
concentration of ozone in this layer is estimated at a height of 20-25km. The
upper boundary of the stratosphere is called Stratopause.