11_geography_notes_02_the_origin_and_evolution_of_the_earth.pdf

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9This chapter deals with1.Origin of the earth2. Earlytheories3. Modern Theories4 .Big Bang theory 5. The starformation 6. formation of planets 7. Our solar system8. The moon9 Evolution of the earth10. Development of lithosphere 11. Evolution of Atmosphere andhydrosphere 12. Origin of lifeNebular hypothesis:There is evidence that the nebular hypothesiswas first proposed in 1734 byImmanuel Kant, who was familiar withSwedenborg's work, developed the theory further in 1755.[4]Heargued that gaseous cloudsnebulae, which slowly rotate, graduallycollapse and flatten due togravityand eventually formstarsandplanets. A similar model was proposed in 1796 byPierre-SimonLaplace. It featured a contracting and cooling proto solar cloudtheproto solar nebula. As the nebula contracted, it flattened and shedrings of material, which later collapsed into the planets. While theLaplacian nebular model dominated in the 19th century, it encountereda number of difficulties. The main problem wasangular momentumdistribution between the Sun and planets. The planets have 99% of theangular momentum, and this fact could not be explained by the nebularmodel. As a result this theory of planet formation was largely abandonedat the beginning of the 20th century.The fall of the Laplacian model stimulated scientists to find areplacement for it. During the 20th century many theories wereproposed including theplanetesimal theoryofThomas ChamberlinandForest Moulton(1901),tidalmodelofJeans(1917),accretion modelofOtto Schmidt(1944),protoplanet theoryofWilliam McCrea(1960) and finallycapture theoryofMichael Woolfson. In 1978Andrew Prenticeresurrected the initial Laplacian ideas about planetformation and developed themodern Laplacian theory.[4]None of these attempts was completelysuccessful and many of the proposed theories were descriptive. Sir Horald JefferyNebularHypothesis in its original form was proposed by Kant and Laplace in the 18th century.The initial steps are indicated in the following figures .Collapsing Clouds of Gas and Dust-A greatcloud of gas and dust (called anebula) begins to collapse because the gravitational forces thatwould like to collapse it overcome the forces associated with gas pressure that would like toexpand it (the initial collapse might be triggered by a variety of perturbations---a supernovablast wave, density waves in spiral galaxies, etc.).It is unlikely that such a nebula would be created with no angular momentum, so it is probablyinitially spinning slowly. Because of conservation of angular momentum, the cloud spins fasteras it contracts.The Spinning Nebula Flattens Because of the competing forces associated with gravity, gaspressure, and rotation, the contracting nebula begins to flatten into a spinning pancake shapeIn the Nebular Hypothesis, a cloud of gas and dust collapsed by gravity begins to spin faster becauseof angular momentum conservationCHAPTER-2THE ORIGIN AND EVOLUTION OFTHE EARTHFigure 6angular momentum7.NEBULA8ANGULAR MOMENTUM
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