Chapter 23: The Origin and Evolution of the Earth PDF

UNIT II THE EARTH This unit deals with Origin and evolution of the earth; Interior of the earth; Wegener’s continental drift theory and plate tectonics; earthquakes and volcanoes CHAPTER...

UNIT II THE EARTH This unit deals with Origin and evolution of the earth; Interior of the earth; Wegener’s continental drift theory and plate tectonics; earthquakes and volcanoes CHAPTER THE ORIGIN AND EVOLUTION OF THE EARTH argument. At a later date, the arguments D o you remember the nursery rhyme “…Twinkle, twinkle little star…”? considered of a companion to the sun to have been coexisting. These arguments are called Starry nights have always attracted us since binary theories. In 1950, Otto Schmidt in the childhood. You may also have thought of Russia and Carl Weizascar in Germany these stars and had numerous questions in somewhat revised the ‘nebular hypothesis’, your mind. Questions such as how many stars though differing in details. They considered that are there in the sky? How did they come into the sun was surrounded by solar nebula existence? Can one reach the end of the sky? containing mostly the hydrogen and helium May be many more such questions are still along with what may be termed as dust. The there in your mind. In this chapter, you will friction and collision of particles led to formation learn how these “twinkling little stars” were of a disk-shaped cloud and the planets were formed. With that you will eventually also read formed through the process of accretion. the story of origin and evolution of the earth. However, scientists in later period took up the problems of origin of universe rather than that of just the earth or the planets. EARLY THEORIES Origin of the Earth MODERN THEORIES A large number of hypotheses were put forth Origin of the Universe by different philosophers and scientists The most popular argument regarding the origin regarding the origin of the earth. One of the of the universe is the Big Bang Theory. It is also earlier and popular arguments was by German called expanding universe hypothesis. Edwin philosopher Immanuel Kant. Mathematician Hubble, in 1920, provided evidence that the Laplace revised it in 1796. It is known as universe is expanding. As time passes, galaxies Nebular Hypothesis. The hypothesis considered move further and further apart. You can that the planets were formed out of a cloud of experiment and find what does the expanding material associated with a youthful sun, which universe mean. Take a balloon and mark some was slowly rotating. Later in 1900, Chamberlain points on it to represent the galaxies. Now, if and Moulton considered that a wandering star you start inflating the balloon, the points marked approached the sun. As a result, a cigar-shaped on the balloon will appear to be moving away extension of material was separated from the from each other as the balloon expands. solar surface. As the passing star moved away, Similarly, the distance between the galaxies is the material separated from the solar surface also found to be increasing and thereby, the continued to revolve around the sun and it universe is considered to be expanding. slowly condensed into planets. Sir James Jeans However, you will find that besides the increase and later Sir Harold Jeffrey supported this in the distances between the points on the THE ORIGIN AND EVOLUTION OF THE EARTH 15 balloon, the points themselves are expanding. The expansion of universe means increase This is not in accordance with the fact. Scientists in space between the galaxies. An alternative believe that though the space between the to this was Hoyle’s concept of steady state. It galaxies is increasing, observations do not considered the universe to be roughly the same support the expansion of galaxies. So, the at any point of time. However, with greater balloon example is only partially correct. evidence becoming available about the expanding universe, scientific community at present favours argument of expanding universe. The Star Formation The distribution of matter and energy was not even in the early universe. These initial density differences gave rise to differences in gravitational forces and it caused the matter to get drawn together. These formed the bases for development of galaxies. A galaxy contains a large number of stars. Galaxies spread over vast distances that are measured in thousands of light-years. The diameters of individual galaxies range from 80,000-150,000 light years. A galaxy starts to form by accumulation Figure 2.1 : The Big Bang of hydrogen gas in the form of a very large The Big Bang Theory considers the cloud called nebula. Eventually, growing following stages in the development of the nebula develops localised clumps of gas. These universe. clumps continue to grow into even denser gaseous bodies, giving rise to formation of (i) In the beginning, all matter forming the stars. The formation of stars is believed to have universe existed in one place in the form taken place some 5-6 billion years ago. of a “tiny ball” (singular atom) with an unimaginably small volume, infinite A light year is a measure of distance and temperature and infinite density. not of time. Light travels at a speed of (ii) At the Big Bang the “tiny ball” exploded 300,000 km/second. Considering this, violently. This led to a huge expansion. the distances the light will travel in one It is now generally accepted that the year is taken to be one light year. This equals to 9.461×10 12 km. The mean event of big bang took place 13.7 billion distance between the sun and the earth years before the present. The expansion is 149,598,000 km. In terms of light continues even to the present day. As it years, it is 8.311minutes. grew, some energy was converted into matter. There was particularly rapid Formation of Planets expansion within fractions of a second after the bang. Thereafter, the The following are considered to be the stages expansion has slowed down. Within first in the development of planets : three minutes from the Big Bang event, (i) The stars are localised lumps of gas the first atom began to form. within a nebula. The gravitational force (iii) Within 300,000 years from the Big within the lumps leads to the formation Bang, temperature dropped to 4,500 K of a core to the gas cloud and a huge (Kelvin) and gave rise to atomic matter. rotating disc of gas and dust develops The universe became transparent. around the gas core. 16 FUNDAMENTALS OF PHYSICAL GEOGRAPHY (ii) In the next stage, the gas cloud starts in the same period sometime about 4.6 billion getting condensed and the matter years ago. Till recently (August 2006), Pluto was around the core develops into small- also considered a planet. However, in a meeting rounded objects. These small-rounded of the International Astronomical Union, a objects by the process of cohesion develop decision was taken that Pluto like other celestial into what is called planetesimals. objects (2003 UB313)discovered in recent past Larger bodies start forming by collision, may be called ‘dwarf planet’. Some data regarding and gravitational attraction causes the our solar system are given in the box below. material to stick together. Planetesimals are a large number of smaller bodies. Why are the inner planets rocky while (iii) In the final stage, these large number of others are mostly in gaseous form? small planetesimals accrete to form a fewer large bodies in the form of planets. The difference between terrestrial and jovian planets can be attributed to the following OUR SOLAR SYSTEM conditions: (i) The terrestrial planets were formed in the Our Solar system consists of eight planets. The close vicinity of the parent star where it nebula from which our Solar system is supposed was too warm for gases to condense to to have been formed, started its collapse and core solid particles. Jovian planets were formation some time 5-5.6 billion years ago and formed at quite a distant location. the planets were formed about 4.6 billion years (ii) The solar wind was most intense nearer ago. Our solar system consists of the sun (the the sun; so, it blew off lots of gas and dust star), 8 planets, 63 moons, millions of smaller from the terrestrial planets. The solar winds bodies like asteroids and comets and huge were not all that intense to cause similar quantity of dust-grains and gases. removal of gases from the Jovian planets. Out of the eight planets, mercury, venus, (iii) The terrestrial planets are smaller and earth and mars are called as the inner planets their lower gravity could not hold the as they lie between the sun and the belt of escaping gases. asteroids the other four planets are called the outer planets. Alternatively, the first four are called The Moon Terrestrial, meaning earth-like as they are made up of rock and metals, and have relatively high The moon is the only natural satellite of the densities. The rest four are called Jovian or Gas earth. Like the origin of the earth, there have Giant planets. Jovian means jupiter-like. Most been attempts to explain how the moon was of them are much larger than the terrestrial formed. In 1838, Sir George Darwin suggested planets and have thick atmosphere, mostly of that initially, the earth and the moon formed a helium and hydrogen. All the planets were formed single rapidly rotating body. The whole mass The Solar System Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Distance* 0.387 0.723 1.000 1.524 5.203 9.539 19.182 30.058 Density@ 5.44 5.245 5.517 3.945 1.33 0.70 1.17 1.66 Radius# 0.383 0.949 1.000 0.533 11.19 9.460 4.11 3.88 Satellites 0 0 1 2 16 about 18 about 17 8 * Distance from the sun in astronomical unit i.e. average mean distance of the earth is 149,598,000 km = 1 @ Density in gm/cm3 # Radius: Equatorial radius 6378.137 km = 1 THE ORIGIN AND EVOLUTION OF THE EARTH 17 became a dumb-bell-shaped body and started getting separated depending on their eventually it broke. It was also suggested that densities. This allowed heavier materials (like the material forming the moon was separated iron) to sink towards the centre of the earth from what we have at present the depression and the lighter ones to move towards the occupied by the Pacific Ocean. surface. With passage of time it cooled further However, the present scientists do not and solidified and condensed into a smaller size. accept either of the explanations. It is now This later led to the development of the outer generally believed that the formation of moon, surface in the form of a crust. During the as a satellite of the earth, is an outcome of ‘giant formation of the moon, due to the giant impact, impact’ or what is described as “the big splat”. the earth was further heated up. It is through A body of the size of one to three times that of the process of differentiation that the earth mars collided into the earth sometime shortly forming material got separated into different after the earth was formed. It blasted a large layers. Starting from the surface to the central part of the earth into space. This portion of parts, we have layers like the crust, mantle, blasted material then continued to orbit the outer core and inner core. From the crust to the earth and eventually formed into the present core, the density of the material increases. We moon about 4.44 billion years ago. shall discuss in detail the properties of each of this layer in the next chapter. EVOLUTION OF THE EARTH Do you know that the planet earth initially was Evolution of Atmosphere and Hydrosphere a barren, rocky and hot object with a thin The present composition of earth’s atmosphere atmosphere of hydrogen and helium. This is is chiefly contributed by nitrogen and oxygen. far from the present day picture of the earth. You will be dealing with the composition and Hence, there must have been some events– structure of the earth’s atmosphere in Chapter 8. processes, which may have caused this change There are three stages in the evolution of from rocky, barren and hot earth to a beautiful the present atmosphere. The first stage is planet with ample amount of water and marked by the loss of primordial atmosphere. conducive atmosphere favouring the existence In the second stage, the hot interior of the earth of life. In the following section, you will find contributed to the evolution of the atmosphere. out how the period, between the 4,600 million Finally, the composition of the atmosphere was years and the present, led to the evolution of modified by the living world through the life on the surface of the planet. process of photosynthesis. The earth has a layered structure. From The early atmosphere, with hydrogen and the outermost end of the atmosphere to the helium, is supposed to have been stripped off centre of the earth, the material that exists is as a result of the solar winds. This happened not uniform. The atmospheric matter has the not only in case of the earth, but also in all the least density. From the surface to deeper terrestrial planets, which were supposed to depths, the earth’s interior has different zones have lost their primordial atmosphere through and each of these contains materials with different characteristics. the impact of solar winds. During the cooling of the earth, gases and How was the layered structure of the water vapour were released from the interior earth developed? solid earth. This started the evolution of the present atmosphere. The early atmosphere largely contained water vapour, nitrogen, Evolution of Lithosphere carbon dioxide, methane, ammonia and very The earth was mostly in a volatile state during little of free oxygen. The process through which its primordial stage. Due to gradual increase the gases were outpoured from the interior is in density the temperature inside has called degassing. Continuous volcanic increased. As a result the material inside eruptions contributed water vapour and gases 18 FUNDAMENTALS OF PHYSICAL GEOGRAPHY Geological Time Scale Eons Era Period Epoch Age/ Years Life/ Major Events Before Present Quaternary Holocene 0 - 10,000 Modern Man Pleistocene 10,000 - 2 million Homo Sapiens Cainozoic Tertiary Pliocene 2 - 5 million Early Human Ancestor (From 65 Miocene 5 - 24 million Ape: Flowering Plants million years and Trees to the Oligocene 24 - 37 million Anthropoid Ape present Eocene 37 - 58 Million Rabbits and Hare times) Palaeocene 57 - 65 Million Small Mammals : Rats – Mice Mesozoic Cretaceous 65 - 144 Million Extinction of Dinosaurs 65 - 245 Jurassic 144 - 208 Million Age of Dinosaurs Million Mammals Triassic 208 - 245 Million Frogs and turtles Permian 245 - 286 Million Reptile dominate-replace amphibians Carboniferous 286 - 360 Million First Reptiles: Palaeozoic Vertebrates: Coal beds 245 - 570 Devonian 360 - 408 Million Amphibians Million Silurian 408 - 438 Million First trace of life on land: Plants Ordovician 438 - 505 Million First Fish Cambrian 505 - 570 Million No terrestrial Life : Marine Invertebrate Proterozoic 570 - 2,500 Million Soft-bodied arthropods Archean 2,500 - 3,800 Million Blue green Algae: Pre- Unicellular bacteria Cambrian Hadean 3,800 - 4,800 Million Oceans and Continents 570 Million - 4,800 form – Ocean and Million Atmosphere are rich in Carbon dioxide Origin of 5,000 Million Origin of the sun Stars 5,000 - 13,700 Supernova Million 12,000 Million Origin of the universe Big Bang 13,700 Million to the atmosphere. As the earth cooled, the that the oceans are as old as 4,000 million water vapour released started getting years. Sometime around 3,800 million years condensed. The carbon dioxide in the ago, life began to evolve. However, around 2,500-3,000 million years before the present, atmosphere got dissolved in rainwater and the the process of photosynthesis got evolved. Life temperature further decreased causing more was confined to the oceans for a long time. condensation and more rains. The rainwater Oceans began to have the contribution of falling onto the surface got collected in the oxygen through the process of photosynthesis. depressions to give rise to oceans. The earth’s Eventually, oceans were saturated with oxygen, oceans were formed within 500 million years and 2,000 million years ago, oxygen began to from the formation of the earth. This tells us flood the atmosphere. THE ORIGIN AND EVOLUTION OF THE EARTH 19 Origin of Life inanimate matter into living substance. The record of life that existed on this planet in The last phase in the evolution of the earth different periods is found in rocks in the form relates to the origin and evolution of life. It is of fossils. The microscopic structures closely undoubtedly clear that initially the earth or related to the present form of blue algae have even the atmosphere of the earth was not been found in geological formations much older conducive for the development of life. Modern than some 3,000 million years. It can be scientists refer to the origin of life as a kind of assumed that life began to evolve sometime chemical reaction, which first generated 3,800 million years ago. The summary of complex organic molecules and assembled evolution of life from unicellular bacteria to the them. This assemblage was such that they modern man is given in the Geological Time could duplicate themselves converting Scale on page 18. EXERCISES 1. Multiple choice questions. (i) Which one of the following figures represents the age of the earth? (a) 4.6 million years (c) 4.6 billion years (b) 13.7 billion years (d) 13.7 trillion years (ii) Which one of the following has the longest duration? (a) Eons (c) Era (b) Period (d) Epoch (iii) Which one of the following is not related to the formation or modification of the present atmosphere? (a) Solar winds (c) Degassing (b) Differentiation (d) Photosynthesis (iv) Which one of the following represents the inner planets? (a) Planets between the sun and the earth (b) Planets between the sun and the belt of asteroids (c) Planets in gaseous state (d) Planets without satellite(s) (v) Life on the earth appeared around how many years before the present? (a) 13.7 billion (c) 4.6 billion (b) 3.8 million (d) 3.8 billion 2. Answer the following questions in about 30 words. (i) Why are the terrestrial planets rocky? (ii) What is the basic difference in the arguments related to the origin of the earth given by : (a) Kant and Laplace (b) Chamberlain and Moulton 20 FUNDAMENTALS OF PHYSICAL GEOGRAPHY (iii) What is meant by the process of differentiation? (iv) What was the nature of the earth surface initially? (v) What were the gases which initially formed the earth’s atmosphere? 3. Answer the following questions in about 150 words. (i) Write an explanatory note on the ‘Big Bang Theory’. (ii) List the stages in the evolution of the earth and explain each stage in brief. Project Work Collect information about the project “Stardust” (website: www.sci.edu/public.html and www.nasm.edu) along the following lines. (i) Which is the agency that has launched this project? (ii) Why are scientists interested in collecting Stardust? (iii) Where from the Stardust is being collected?