PLATE TECTONICS
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From the initial knowledge of the structure of the Earth,it was thought that the whole Earth was rigid and rigidly fixed on the ocean floor. But later and more detailed scientific knowledge proved the rigid theory wrong. The Earth was thought to be rigid because of the following structures
•Upper continental crust,also known as the 'Sial' because it consists of silica and alumina averaging a density of about 2.8g/cm^3
•Lower oceanic seafloor which is a thinner portion of the continental crust but constitutionally different. Made up predominantly of silica and magnesium. It's also known as the 'Sima', averaging a density of about 3.0g/cm^3.
The Sima is darker and more basaltic and denser and younger while the Sial is lighter in colour and more granitic,lies above the Sima. Both were earlier thought to be fixed in place with the oceanic basin as the crust.
The upper crust was thought to be subject to dynamism and modification by the sea level and other modification by changes and periods of mountain building orogenies and isostasy. The rigid earth Isostasy principle and theory of the Earth crust and oceanic basin principle also held sway until the mid 20th century. After the 21st century,more detailed understanding of the Earth structure revealed the lithosphere and more details of the upper mantle,the Earth crust,oceanic and continental crust are known to constitute the lithospheric plates. It may also be referred to as the CRUSTAL CONTINENTAL LITHOSPHERIC PLATES. It's now understood that the lithospheric continental landmass split because they are brittle. They move and migrate. As they do this,new oceanic floors are created while older oceanic floors are carried along sea trenches. This new information led to the birth in 1968 of the knowledge and aspect of continental drift, seafloor spreading. 1968,marked the change from the theory of rigid Earth to the theory and concept of plate tectonics.
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DEFINITION
Plate tectonics can be defined as the scientific study of the rigid continental landmasses,called lithospheric plates,their movements,collision,over weakly or partially molten layer called the 'ASTHENOSPHERE' i.e of the upper mantle and their resultant effects on the entire dynamism.
CLASSICAL DEFINITION
Plate tectonics can be defined as a synthesis of the concept of continental drift and seafloor spreading and as the composite of a variety of ideas and scientific knowledge that explain the observed motion of the Earth's lithosphere through the mechanism of subjunction and seafloor spreading,which in turn generates Earth's major features, structures,continents,oceanic basins and is the basis of the most geological processes. For example,metamorphism,orogenesis and volcanism.
SCOPE
It entails the study of the structure and the Dynamics of the Earth's interior and the forces within and outside that propels the plate movement,collision and generate other tectonic processes,the effects and interaction with one another. It covers the study and understanding of their nature, response to pressure,stresses and plate margins,the sea and oceanic floors,sea and crustal topography,their structures,fractures, subduction,divergences,volcanic and earthquake activities. The scope also uses the principle of stress and strain distribution. Geophysics, paleontology, geography,remote sensing, computer iteration which is used extensively in juxtaposition of continental landmasses to fit continental margins. Also radiometric dating and radio sensing help to decipher the conditions of the Earth in the past, present and future.
