Compositional Layers Of Earth


The chemical differentiation which began early in the earth’s history is still occurring but on a much smaller scale

Because of the chemical differentiation  earth’s interior is not homogeneous

It consists of three major regions that have different chemical compositions
The principle division of Earth includes:

The Crust, Earth’s comparatively thin outer skin ranges in thickness from 3 kilometers (2 miles) at the oceanic ridges to over 70 kilometers (40 miles) in some mountain belts

The Mantle, a solid rocky shell that extends to a depth of about 2885 kilometers (1800 miles)

The Core, which can be further divided into the Outer Core and Inner Core
The Outer Core, a molten metallic layer some 2270 kilometers (1410 miles) thick

The Inner Core, a solid iron rich sphere having a radius of 1216 kilometers (756 miles)


The crust, the rigid outermost layer of Earth is further divided into:

1. Oceanic crust
2. Continental crust

Oceanic crust range from 3 to 15 kilometers in thickness and is composed of dark igneous rocks called Basalt

The upper continental crust consists of a large variety of rock types which have an average composition of a granitic rock called granodiorite.

The rocks of the oceanic crust are younger (180 million  years or less) and more dense having density about 3 gm/cm3

 2.7 Continental rocks have an average density of about gm/ cm3

types of crust

the crust

Some have been discovered that exceed four billion years of  age


The three important features of the oceanic crust are

Mid Oceanic Ridges
Oceanic Trenches
Island arcs
features of oceanic crust

           A mid-ocean ridge is a general term for an underwater mountain system that consists of various mountain ranges (chains), typically having a valley known as a rift running along its spine, formed by plate tectonics.

           This type of oceanic ridge is characteristic of what is known as an oceanic spreading center, which is responsible for seafloor spreading.
           The uplifted seafloor results from convection currents which rise in the mantle as magma at a linear weakness in the oceanic crust, and emerge as lava, creating new crust upon cooling.

           A mid-ocean ridge demarcates/separates the boundary between two tectonic plates, and consequently is termed a divergent plate boundary
mid oceanic ridges

           Seafloor spreading is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge.

           Seafloor spreading helps explain continental drift in the theory of plate tectonics.
sea floor spreading

           An island arc is a type of archipelago composed of a chain of volcanoes which alignment is arc-shaped, and which are situated parallel and close to a boundary between two converging tectonic plates.
           Most of these island arcs are formed as one oceanic tectonic plate subducts another one and, in most cases, produces magma at depth below the over-riding plate.
Island arcs

           The oceanic trenches are hemispheric-scale long but narrow topographic depressions of the sea floor. They are also the deepest parts of the ocean floor.
           Trenches are found at the boundary between two lithospheric plates.

There are three types of lithospheric plate boundaries:

divergent (where lithosphere and oceanic crust is created at mid-ocean ridges)

convergent (where one lithospheric plate sinks beneath another and returns to the mantle)

transform (where two lithospheric plates slide past each other).
Oceanic trenches

The ocean basins are known to contain the most prominent mountain ranges on Earth, the Mid 

Oceanic Ridge    

The Mid Atlantic Ridge and East Pacific Ridge are parts of
this system

They formed a continous belt of more than 70,000 kilometer around the globe
  Rather than consisting of highly deformed rocks the oceanic ridge system consists of layer upon layer of once molten rock that has been fractured and uplifted

     The ocean floor also contain deep graves that are occasionally more than eleven thousand feet deep called Oceanic Trenches

Ocean basins


 Over eighty two percent of the earth’s volume is
contained in the mantle

  It is a rocky shell which is 2900 kilometers thick

The boundary between the crust and mantle reflects a change in composition   
The mantle behaves like a solid while transmitting
earthquake waves

Mantle rocks are able to flow at an incredible slow rates
The mantle is divided into the Lower Mantle or   Mesosphere which extends from the core – mantle
boundary to a depth of 660 kilometers

The Upper Mantle continues to the base of the crust


The core is composed mostly of iron with lesser amounts of nickle and other elements

At the extreme pressure found in the core, this iron rich    
material has an average density of about 11 gm / cm3 and have 14 times the density of water at Earth’s centre

The inner core and outer core are very similar

Their division is based on differences in mechanical strength

The outer core is a liquid capable of flow

The inner core, despite its high temperature is stronger than the outer core and behaves like a solid.