Structure of Earth’s Interior, Crust of earth, Mantle, Core
The earthquake waves undergo changes at definite intervals during their propagation through the interior of the earth. They also undergo the action of reflection and refraction. Places on earth where seismic waves are not record are call as “Shadow Zones”. S-waves are not record beyond 103° angular distance from focus which indicate the outer core of earth is in molten or semi molten in which S-waves cannot propagate. As P-waves are not record between angular distances of 103° to 142°, it indicates that the core has different density, state and composition.
Structure of Earth’s Interior
From the analysis of the behaviour of these waves it is clear that the interior of the earth has a layered structure of different densities. With the help of earthquake waves, we can get the information about the exact location of the layers, their depth thickness and other physical and chemical properties. Based on the passage of these waves through different types of rocks and their behaviour we can conclude that the earth’s interior has three main layers.
These three layers are
It is the earth’s uppermost layer. Crust is solid, rigid and very thin compared with the other two. Like the shell of an egg, the earth’s crust is brittle and can break. The thickness of the crust is not same everywhere. Oceanic crust is thinner as compared to the continental crust. The main thickness of oceanic crust is 5 km whereas that of the continental is around 30 km. The continental crust is thicker in the areas of the major mountain systems. It is as much as 70 km thick in the Himalaya region.
Its two main parts are:
1. The uppermost thin layer- It is composed of such rock which contain a large proportion of Silca and Aluminium. it is called SIAL. The continents are mostly composed of SIAL. Its average density is 2.7 and thickness is about 28 km.
2. The lower layer- It is made of comparatively heavier rocks. Silca and Magnesium are the major constituents in it. This part is therefore, known as SIMA. The oceanic floor is also made of this rock strata. Its average thickness is 6 to 7 km and density of about 3.0.
The thickness of SIAL and SIMA put together does not exceed 70 km. Its volume is 1% of the total volume of the earth. In comparison to 6378 km radius of the earth, the thickness of 70 km is insignificant. However this cannot be over looked. This shallow crust is the ground of the nature’s wonderful activities.
It’s thickness is about 2900 km. It volume in 83% of the whole earth. Near the lower limit of the crust the velocity of P-waves increases from about 6.4 km per seconds to 8 km per second. This change in velocity of P-waves indicates the surface discontinuity between the crust and the mantle. It is popularly known as Moho or the Mohorovicic discontinuity.
The mantle is of dense and heavy materials such as oxygen, iron and magnesium. The average density of the materials in the mental varies between 3.5 g per cubic cm and 5.5 g per cubic cm. The temperature of this layer ranges between 900°C and 2200°C. The temperature is quite high and the hot rocks form magma in this layer. The pressure of the overlying layers, keeps the lower part of the crust and the upper part of the mantle in an almost solid state. If cracks appear in the crust, the pressure is released and the molten matter from inside the earth tries to reach the surface through volcanic eruptions.
The upper portion of the mental is call as asthenosphere. The word astheno means weak. It is consider to be extending up to 400 km. It is the main source of magma that finds its way to the surface during volcanic eruptions. The mantle plays an important role in all the happenings in the interior of the earth. It also gives to Convection Currents. These current supply energy for happenings like continental drift earthquake, volcanoes etc.
Structure of Earth’s Interior
Core extends from 2900 km depths up to centre of the earth. It the interior most part of the Earth. It begins from Gutenberg discontinuity. The mantle of demarcated from the core by Gutenberg discontinuity. The core is divide into two parts: (i) The outer core, (ii) The inner core.
The outer core is possibly in wholly liquid or semi-liquid state. The transverse or S-Waves of earthquakes, seem to disappear at the Gutenberg discontinuity. The outer core extends from the depth of 2900 km up to 5150 km. It has an average density of 10.
The inner core is believe to be solid. It extends from the depth of 5150 kilometre up to the centre of the earth. The velocity of P-waves increases at the boundary of outer and inner core. Its density is between 12 to 13. To volume of the entire core is 16% of earth as a whole. The mass of the core is 32% of the earth’s mass. The major part of the core is made of heavy metals like Iron and Nickel. This zone is therefore known as Nife (Ni= Nickel, Fe= Ferrous). It is also call as Barvsphere which means heavy metallic rocks.
Crust and Mantle vs. Lithosphere and Asthenosphere
Lithosphere, Asthenosphere and Mesosphere represent changes in the mechanical properties of the earth. Crust, Mantle and Core refer to changes in the chemical composition of the earth. The lithosphere is the strong, upper 100 km of the earth. The lithosphere is the tectonic plate. The asthenosphere is the weak and easily deformed layer of the earth that acts as a “lubricant” for the tectonic plates to slide over.
The asthenosphere extends from 100 km depth to 660 km beneath the Earth’s surface. Beneath the asthenosphere is the mesosphere, another strong layer.