The earth is almost a spherical body, made up of concentric zones. The most important zones include outer layer of the earth is known as the crust. Its thickness ranges from 16 to 40 km. The crust is thicker beneath the continents than beneath the oceans.
CRUST
The crust is made up of two layers; an upper lighter layer called the Sial (Silicate+Aluminium) and a lower denser layer called Sima (Silicate + Magnesium). The average density of the earth’s surface is less than 3 gm/c.c. The inner part of lithosphere is partly molten.
LITHOSPHERE
Lithosphere refers to the strong and rigid portions of the earth including the solid crust and upper mantle in about 280 million years ago, the entire landmass formed which different forms of landforms are found. Beneath the lithosphere lies the asthenosphere, which is a weak and soft layer.
THE MANTLE
Below the crust of the earth is a thick layer called Mantle. This layer extends up to a depth of 2900 km. The mantle consists predominantly of solid rock made up of silicates of magnesium and iron and display plastic properties.
THE CORE
Beyond a depth of 2900 km. lies the core of the earth. This is made up of dense material consisting of iron and nickel (Nife). This metallic core of the earth has an aver- age density of 11 gm/c.c. This core of the earth is believed to be a reason for the earth’s magnetism. The physical state of matter in the earth’s interior is very different form that on the surface of the earth.
Temperature increases at an average rate of 1°C for every 32 meters of depth below the earth’s surface. The temperature at the center of the earth may be around 6,000°C. The pressure is so high but even at this temperature the metals in the core remain solid and rigid. Several Posts and Blogs are available in internet for further information.
MOHOROVICIC DISCONTINUITY
The line of sepation between the mantle and the crust is known as the Mohorovic discontinuity. GUTENBERG-WIECHERT DISCONTINUITY The line of separation between the mantle and the core is called Gutenberg-Wiechert discontinuity.
CONTINENTAL DRIFT
The theory of continental drift expounded by Alfred Wegener in 1915 holds that portions of the original continent which comprised the entire landmass of the world underwent a series of horizontal displacement before the present continents were formed. According to this theory, one super continent, called Pangea.
According to Wegner, after the breaking of the super continent Pangae, the movement of the continents took place in two directions-on towards the equator due to centrifugal force, of the earth which gave rise to fold mountains like the Himalayas, the Alps, etc and another towards west due to tidal force of Sun and the Moon which gave rise to Andes and Rockies.
Various evidences show that the continents or the land areas of the globe have not been the same in the geological past as they are today. Latest geological investigations show that the crust of the earth consisting of plates are drifting away along the mid-oceanic ridges.
A glance at the world map shows that S. America particularly Brazil can be fitted into the gulf of Guinea of Africa. Antarctica can roughly be fitted into 8. Australian coast and S.E. African coast.
Similarly NW. Australian coast and E-Indian coast are liable to fit. After the drifts some water bodies developed between them Geological evidences said that S. America and Africa were probably joined together till the Upper Triassic. Biological history of certain animals like marsupials and placental mammals also throw significant light on the continental drift.
The rate of drift ranges from 1 cm. per year in N Atlantic to about 2 cm per year in Central Atlantic. It varies from 16 cm. per year in East Central Pacific to about 2 cm. per year in S. Pacific. In the Indian ocean, the drift varies from 1.5 cm. per year in Arabia to 0.3 cm. per year in South Indian ocean.
PLATE TECTONICS
Plate tectonics deals with rock structures which are in the form of the Plates and it is not only the continents which are in motion but the oceans as well. These plates include not only the earth’s upper crust but also the part of denser mantle below.
They have an average thickness of 100 km. They float on the upper mantle called ‘Asthenosphere’ and carry the continents and oceans on their back. The edge of the plates are designed as boundaries and margins, where movements occur.
Major plates of the world are : 1. American Plate 2. Pacific Plate 3. Antarctic Plate 4. African Plate 5. European Plate and 6. Australian Plate. Some minor plates are: 1. Carribean Plate. 2. Cocas Plate 3. Nazca Plate 4. Juande Fuca Plate 5. Philippine Plate, etc.
All these plates are in constant motion both in relation to each other and with regard to the earth’s motion. Some movements are responsible for the volcanic activities, seismic and other plate disturbances on the margins of the plates.