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:
i. Continental crust
ii. Oceanic 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.
Mantle is 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:
Chemical composition of atmosphere:
Component Volume Percentage
Carbon dioxide 00.033
Others (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.
a) The temperature becomes constant for a height of 20km and then starts increasing.
b) It 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.
c) The stratosphere itself has a layered structure and there is no significant mixing or turbulence of gases in this layer.
The ozone 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.