World Geography Part 7 – Sea – floor spreading

Sea – floor spreading

  1. Propounded by the Harry Hess of the Princeton University in the year 1960.
  2. He propounded that mid – oceanic ridges were situated on the rising thermal convection currents, coming up from the mantle.
  3. The oceanic crust move in opposite directions from mid – oceanic ridges.
  4. This molten lava cool down and solidify to form new crust along the trailing ends of divergent plates (oceanic crust).
  5. Thus there is continuous creation of new crust along the mid – oceanic ridges and the expanding crusts (plates) are destroyed along the oceanic trenches.
  6. These facts prove that continents and ocean basins are in constant motion.

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Endogenetic forces

  1. The forces coming from within the earth are called as endogenetic forces.
  2. It causes two types of movements in the earth; (i) horizontal movements, (ii) vertical movements
  3. There is a lack of precise knowledge regarding the mode of origin of the endogenetic forces and movements.
  4. On the basis of intensity, these are divided into two categories
    • Sudden forces
    • Diastrophic forces

Sudden forces

  1. It causes rapid events and massive destruction at and below the earth’s surface.
  2. These force work very quickly and their results are seen within minutes.
  3. It’s the result of long period preparation deep within the earth.
  4. Because these forces create certain relief features on the earth’s surface, they are also termed as constructive forces.

Diastrophic forces

  1. These forces operate slowly and their effects become discernible after thousand and millions of years.
  2. They affect larger areas of globe and produce meso – level reliefs (e.g.) mountains, plateaus, plain, lakes, big faults etc.)
  3. It is divided into two groups
    • Epeirogenetic movements
    • Orogenetic movements

Epeirogenetic movements

  1. It is continent forming
  2. Acts along the radius of earth, therefore also called radial movement
  3. Direction may be towards (subsidence) or away (uplift) from the centre.
  4. Results are seen in relief i.e. sea beaches (Kathiwar, Orissa), elevated wave cut terraces, sea caves.

Example of Emergence

  1. Evidence of marine fossils above sea level in parts of Britain Norway is examples of epeirogenetic uplift

Example of Submergence

  1. Downward movements cause subsidence of continental masses.
  2. In 1819, Rann of Kachchh was submerged as a result of the earthquake.

Orogenetic movement

  1. It means mountain building
  2. It is caused due to endogenetic forces working in a horizontal manner.
  3. Thus these forces create rupture, cracks, fracture and faults in the crustal parts of the earth.
  4. When these forces work face to face these are called compressional forces or convergent force.
  5. It causes crustal bending leading to the formation of folds
  6. The crustal rocks undergo the process of crustal bending in two ways
    • Wrapping
    • Folding

Wrapping

  1. It affects the larger areas of the crust, wherein the crustal parts are either warped (raised) upward or downward.
  2. When it affects larger areas, the resultant mechanism is called broad wrapping.

Folding

  • Waves like bands are formed in the crustal rocks due to tangential compressive force.
  • It is the result of horizontal movement caused by the endogenetic force
  • Some parts of bent are in an upward direction, they are called anticlines, and they are unfolded rock strata in the arch-like form.
  • The down folded structure forming through – like features are called

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  • Anticlinorium represents a folding structure, where extensive anticline having numerous minor anticline and synclines.
  • Synclinoriu m represents a folding structure, where extensive syncline having numerous minor anticline and synclines.

 

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  • The two sides of folds are called limbs of the fold, the limb shared between anticline and syncline is called middle limb.
  • The plain line which bisects the angle between the two limbs of the anticline or middle limb of the syncline is called axis of fold or axial plane.

 

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  • It’s also necessary to understand dip and strike, in order to understand the structural form.
  • The inclination of rock beds with respect to the horizontal plane is called dip.
  • The angle of dip is measured with an instrument called clinometers.
  • The strike of an inclined bed is the direction of any horizontal line along a bedding plane.
  • The direction of dip is always at right angle to the strike.

 

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Types of folds

  • The elasticity of rocks largely affects the nature and the magnitude of the folding process.
  • The softer and more elastic rocks are subjected to more intense folding, while rigid and less elastic rocks are moderately folded.
  • The difference in intensity and magnitude of compressive forces also causes variations in the characteristics of folds.
  • Normally both the limbs of folds are having equal limbs, but mostly they are not.

 

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  • Based on the inclination of the limbs, folds are divided into five types.
    • Symmetrical folds – The limbs (both) of which incline uniformly, compressive force work regularly with moderate intensity.
    • Asymmetrical folds – characterized by unequal and irregular limbs, both the limbs incline at different angles, one limb is relatively larger than other.
    • Monoclinal folds – one limb inclines moderately with regular slope, while other limb inclines steeply at right angle and slope the is almost verticthe al.
    • Isoclinal folds – when both the limbs of fold become parallel but not horizontal.
    • Recumbent folds – when the compressive forces are so strong that both the limbs of the fold become parallel as well as horizontal.
    • The other types of fold are overturned fold (one limb of the fold is thrust upon another), plunge fold (when axis of fold become tilted), fan fold (extensive and broad fold consisting of several minor anticline and syncline), open folds (in which angle between the two limbs of the fold is more than 90 degree but less than 180 degree) and lastly closed folds (angle between two limbs of a fold is acute angle).

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