Introduction to Geology

Origin of earth There are various theories regarding the origination of earth and other planets They are as follows.1. Nebular hypothesis2. planetisimal hypothesis3. gaseous tidal hypothesis4. Binary star hypothesis5. Gas dust cloud hypothesis

NEBULAR HYPOTHESISThis is put forward by Kant, the germen philosopher in 1775, and Laplace, the French mathematician. It states that the solar system including sun has originated from Nebula (large gaseous cloud ball) which rotated slowly and than it gradually cooled and contracted. Due to contraction, the speed of rotation about its axis increased and the centrifugal force in the equatorial zone also increased. Its causes the nebula to bulged out and ultimately a gaseous ring was separated out. Later on the separated ring transformed into planet. This process repeated until nine planets are formed and the central mass of the nebula became the sun.PLANETISIMAL HYPOTHESISThis is proposed by Chamberlain in 1904 it states that, The sun existed before the formation of the planets. A very large star came close by the sun. Due to the strong disruptive forces and strong gravitational pull of the stars, a large number of small gas masses were torn and thrown away from the sun. These on cooling became “planetesimal” which continued to rotate round the sun in elliptical orbit to form planets. GASEOUS TIDAL HYPOTHESISThis hypothesis was put forwarded by Jeans and Jeffrey’s in 1925. It states that A very large star progressively came near by the sun. Due to gravitational pull large gaseous tides raised on the surface of the sun. As it began to move away, the gaseous tide was detached from the sun. it shapes like spindle which broke into ten pieces out which nine cooled to planets and the last pieces further broke into the number of small pieces called planetoids.BINARY STAR HYPOTHESISThis idea was developed by Lyttleton in 1938. it sates that. Before the formation of nine planets the sun has a companion star. Another star passed close to these double stars and dragged the companion star away. A gaseous filament was torn from the companion star and it remain close to the sun and than the planets are originated from this gaseous filament in the same way as described in the Gaseous tidal Hypothesis.GAS DUST HYPOTHESISThis is proposed by C. Von Weizsacker and O. J.Schmidt (1943). In this hypothesis the planets are thought to be evolved out of the cold cloud of gas and dust which was present around the sun. It was called “protoplanetary cloud”. It shapes like a disc. Planets were formed in this cloud by the gradual aggregation of the dispersed matter. -------------------------------

Age of the EarthSeveral attempts are made to determine the age of the planet earth.1. From History of the organic evolution.2. From rate of cooling of earth 3. From rate of formation of sedimentary Rocks4. From the Salinity of Sea waterFrom History of Organic Evolution: From the history organic evolution, the biologist have determined the age of earth to be about 1000 million years.From Rate of cooling earth: From the rate of cooling of the earth, Lord Kelvin determined the age of earth to be between 20 to 400 million years.From Rate of formation of sedimentary Rocks: By the knowledge of the thickness of sedimentary rocks and the rate of sedimentation, the age of the earth comes about 500 million years.From the Salinity of sea water: If the amount of the sea water is known it can be calculated that how much salt accumulate each year. By this method the age of the earth is about 120 million years. -----------------------------------

Figure

INTERIOR OF THE EARTH

Crust of the EarthThe outer most layer of solid rock, which lies above the “mohorovicic” discontinuity and has an average thickness of 35 Km, is known as crust of earth. Under the continents it is possible to distinguish two layers in the crust. The upper layer, which is less dense and granitic in character, is known as the “sial”. While the lower layer which is basaltic in character, is known as “sima”. The term sial represents rocks rich in silica and alumina, and term sima represents rocks containing silica and magnesia. Under oceans only sima layer is found and the sial layer appears to be absent.MantleThe mantle lies beneath the earth’s crust and extended down up to 2900 Km in depth. It has been divided into upper mantle and lower mantle. The upper mantle lies between 35 km and 100 km in depth, and is made up of ultrabasic rocks like peridotite. The lower mantle is between 1000 km and 2900 km and contains pallasite which is a mixture of basic rocks and metallic iron.Core The core is situated from 2900 Km depth up to the center of the earth (6370 Km). Since the sound waves do not pass through the core. It is concluded that it may be in the liquid state. The inner core begins at about 5000 Km depth. Here the P waves increased in velocity, which indicates that it may be in the solid state. Both the solid and liquid portion of the core is believed to consist of iron and nickel similar to iron meteorites, and has a specific gravity about 10.0

ISOSTACY: C. E. Dutton in 1889 suggested that” a gravitational balance exist between the elevated masses (continents) and ocean basins of the earth’s crust. The mountains stands high because they are composed of rocks of low density and oceans basin are low because they are made up of rocks of higher density. To this equilibrium the term is “ISOSTACY” is applied

CONTINENTAL DRIFTThe theory of continental drift was proposed by Wagener in 1912. He noticed that the east coast of South America fits like a jigsaw puzzle into the west coast of Africa. He therefore suggested that “all the continents of the world were together and formed a single landmass called “Pangaea”. Then this landmass broke into the small pieces which drifted away, just like the pieces of wood floating on water, these continents were drifted away under “Differential gravitational force”