Geography for Form 3: STRUCTURE OF THE EARTH

The earth is a system which is composed of outer and inner zones. The outer zones of the earth include the atmosphere, the hydrosphere and the barysphere. The inner zones of the earth include the crust, mantle and the core.


The inner zones of the earth constitute the internal structure of the earth. The inner zone/ internal structure of the earth consists of three zones consist of:

  1. Crust (Lithosphere)
  2. Mantle (Mesosphere)
  3. Core (Barysphere)
  4. CRUST

Is the outermost and thinnest zone of the earth which found between 8-50 km or 5-30 miles. It is known as Lithospere, It is largely composed of igneous rocks. Other types of rocks also exist as a result of changes on the earth’s surface.

The rocks are crystalline, hard and brittle. Bacause of being brittle they tend to break when subjected to stress or forces especially the compressional forces.

The crust also consist of two layers, sial and sima.

  1. Sial

This is the outer layer of the crust which rich in Silica and Aluminium minerals. The sial form the basis of the continent.

  1. Sima

This is the layer which is found beneath the sial. It is the inner layer of the crust which is separated from sial layer by the zone called Conrad discontinuity line. The sima layer is composed of silica and magnesium. It forms the basis of ocean floor.


Mesosphere or mantle is found between the crust and core. It lies beneath the crust. It is separated from the crust by the zone of separation called Mohorovic discontinuity line. It extends downward to about 2900 km (1800 miles) where the temperatures may reach about 50000 degrees centigrades. It consist of pale green minerals called Olivine (Ferromagnesium silicate) in form of ultra basic rock.

It consists of the lower and the upper mantle. The upper mantle is rigid and crust to form a large layer called lithosphere.

The lower mantle is less rigid and forms the molten layer within the earth’s interior called asthenosphere.

Asthenosphere is the molten layer layer which responsible for the balancing movement of the earth’s material called isostatic readjustment. Asthenosphere is found between 100 to 200 km below the upper surface.

  1. CORE

The core is the innermost zone of the internal structure of the earth. It is also called barysphere or centrosphere. It has diameter of about 6900 km (4300 miles) density of about. The core is separated into two parts i.e. the outer and inner core. It separated from the mantle by zone of separation called Gutenberg discontinuity.

  1. The Outer Core

It is liquid in nature due to high temperature of up to 37000c, and consist of nickel and iron (NIFE) . It is estimated to be 2100 km with density of about 10.5 gms/cc.

  1. The Inner Core

It is thought to be solid in nature because of high pressure exerted from different parts toward the center – It composed mainly by iron. It has diameter of about 2600- 2700km. (1600-1700 miles).


External structure of the earth consists of four main layers. These are Atmosphere, Hydrosphere, Lithosphere and Biosphere.

It will continue next week, don’t miss.

Teacher: Kim Ismail

Contact: 0672900666.


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