Fundamentals of Physical Geography - Unit 2 (PDF)

12 FUNDAMENTALS OF PHYSICAL GEOGRAPHY UNIT II THE EARTH This unit deals with Origin and evolution of the earth; Interior of the earth; Weg...

12 FUNDAMENTALS OF PHYSICAL GEOGRAPHY 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 2024-25 CHAPTER THE ORIGIN AND EVOLUTION OF THE EARTH formed through the process of accretion. D o you remember the nursery rhyme “…Twinkle, twinkle little star…”? However, scientists in later period took up the problems of origin of universe rather than Starry nights have always attracted us since that of just the earth or the planets. the childhood. You may also have thought of these stars and had numerous questions in MODERN THEORIES your mind. Questions such as how many stars Origin of the Universe are there in the sky? How did they come into The most popular argument regarding the origin existence? Can one reach the end of the sky? of the universe is the Big Bang Theory. It is also May be many more such questions are still called expanding universe hypothesis. Edwin there in your mind. In this chapter, you will Hubble, in 1920, provided evidence that the learn how these “twinkling little stars” were universe is expanding. As time passes, galaxies formed. With that you will eventually also read move further and further apart. You can the story of origin and evolution of the earth. experiment and find what does the expanding universe mean. Take a balloon and mark some EARLY THEORIES points on it to represent the galaxies. Now, if you start inflating the balloon, the points marked Origin of the Earth on the balloon will appear to be moving away A large number of hypotheses were put forth from each other as the balloon expands. by different philosophers and scientists Similarly, the distance between the galaxies is regarding the origin of the earth. One of the also found to be increasing and thereby, the earlier and popular arguments was by German universe is considered to be expanding. philosopher Immanuel Kant. Mathematician However, you will find that besides the increase Laplace revised it in 1796. It is known as in the distances between the points on the Nebular Hypothesis. The hypothesis considered balloon, the points themselves are expanding. that the planets were formed out of a cloud of This is not in accordance with the fact. Scientists material associated with a youthful sun, which believe that though the space between the was slowly rotating. In 1950, Otto Schmidt in galaxies is increasing, observations do not Russia and Carl Weizascar in Germany support the expansion of galaxies. So, the somewhat revised the ‘nebular hypothesis’, balloon example is only partially correct. though differing in details. They considered that The Big Bang Theory considers the the sun was surrounded by solar nebula following stages in the development of the containing mostly the hydrogen and helium universe. along with what may be termed as dust. The (i) In the beginning, all matter forming the friction and collision of particles led to formation universe existed in one place in the form of a disk-shaped cloud and the planets were of a “tiny ball” (singular atom) with an 2024-25 14 FUNDAMENTALS OF PHYSICAL GEOGRAPHY at any point of time. However, with greater 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 Singularity of hydrogen gas in the form of a very Figure 2.1 : The Big Bang large cloud called nebula. Eventually, growing nebula develops localised clumps of gas. These clumps continue to grow unimaginably small volume, infinite into even denser gaseous bodies, giving rise to temperature and infinite density. formation of stars. The formation of stars is (ii) At the Big Bang the “tiny ball” believed to have taken place some 5-6 billion exploded violently. This led to a huge years ago. expansion. It is now generally accepted that the event of big bang took place A light year is a measure of distance and 13.7 billion years before the present. not of time. Light travels at a speed of The expansion continues even to the 300,000 km/second. Considering this, present day. As it grew, some energy the distances the light will travel in one year is taken to be one light year. This was converted into matter. There was equals to 9.461×10 12 km. The mean particularly rapid expansion within distance between the sun and the earth fractions of a second after the bang. is 149,598,000 km. In terms of light Thereafter, the expansion has slowed years, it is 8.311minutes. down. Within first three minutes from the Big Bang event, the first atom Formation of Planets began to form. (iii) Within 300,000 years from the Big The following are considered to be the stages Bang, temperature dropped to 4,500 K in the development of planets : (Kelvin) and gave rise to atomic matter. (i) The stars are localised lumps of gas The universe became transparent. within a nebula. The gravitational force The expansion of universe means increase within the lumps leads to the formation in space between the galaxies. An alternative of a core to the gas cloud and a huge to this was Hoyle’s concept of steady state. It rotating disc of gas and dust develops considered the universe to be roughly the same around the gas core. 2024-25 THE ORIGIN AND EVOLUTION OF THE EARTH 15 (ii) In the next stage, the gas cloud starts and the lighter ones to move towards the getting condensed and the matter surface. With passage of time it cooled further around the core develops into small- and solidified and condensed into a smaller size. rounded objects. These small-rounded This later led to the development of the outer objects by the process of cohesion develop surface in the form of a crust. During the into what is called planetesimals. formation of the moon, due to the giant impact, Larger bodies start forming by collision, the earth was further heated up. It is through and gravitational attraction causes the the process of differentiation that the earth material to stick together. Planetesimals forming material got separated into different are a large number of smaller bodies. layers. Starting from the surface to the central (iii) In the final stage, these large number of parts, we have layers like the crust, mantle, small planetesimals accrete to form a outer core and inner core. From the crust to the fewer large bodies in the form of planets. core, the density of the material increases. We shall discuss in detail the properties of each of EVOLUTION OF THE EARTH this layer in the next chapter. Do you know that the planet earth initially was a barren, rocky and hot object with a thin Evolution of Atmosphere and Hydrosphere atmosphere of hydrogen and helium. This is The present composition of earth’s atmosphere far from the present day picture of the earth. is chiefly contributed by nitrogen and oxygen. Hence, there must have been some events– You will be dealing with the composition and processes, which may have caused this change structure of the earth’s atmosphere in Chapter 8. from rocky, barren and hot earth to a beautiful There are three stages in the evolution of planet with ample amount of water and the present atmosphere. The first stage is conducive atmosphere favouring the existence marked by the loss of primordial atmosphere. of life. In the following section, you will find In the second stage, the hot interior of the earth out how the period, between the 4,600 million contributed to the evolution of the atmosphere. years and the present, led to the evolution of Finally, the composition of the atmosphere was life on the surface of the planet. modified by the living world through the The earth has a layered structure. From process of photosynthesis. the outermost end of the atmosphere to the The early atmosphere, with hydrogen and centre of the earth, the material that exists is helium, is supposed to have been stripped off not uniform. The atmospheric matter has the as a result of the solar winds. This happened least density. From the surface to deeper not only in case of the earth, but also in all the depths, the earth’s interior has different zones terrestrial planets, which were supposed to and each of these contains materials with have lost their primordial atmosphere through 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 Evolution of Lithosphere largely contained water vapour, nitrogen, The earth was mostly in a volatile state during carbon dioxide, methane, ammonia and very its primordial stage. Due to gradual increase little of free oxygen. The process through which in density the temperature inside has the gases were outpoured from the interior is increased. As a result the material inside called degassing. Continuous volcanic started getting separated depending on their eruptions contributed water vapour and gases densities. This allowed heavier materials (like to the atmosphere. As the earth cooled, the iron) to sink towards the centre of the earth water vapour released started getting 2024-25 16 FUNDAMENTALS OF PHYSICAL GEOGRAPHY condensed. The carbon dioxide in the Origin of Life atmosphere got dissolved in rainwater and the temperature further decreased The last phase in the evolution of the earth relates to the origin and evolution of life. It is undoubtedly causing more condensation and more clear that initially the earth or even the atmosphere rains. The rainwater falling onto the of the earth was not conducive for the development surface got collected in the depressions of life. Modern scientists refer to the origin of life to give rise to oceans. The earth’s oceans as a kind of chemical reaction, which first were formed within 500 million years from generated complex organic molecules and the formation of the earth. This tells us assembled them. This assemblage was such that that the oceans are as old as 4,000 million they could duplicate themselves converting years. Sometime around 3,800 million inanimate matter into living substance. The record years ago, life began to evolve. However, of life that existed on this planet in different periods around 2,500-3,000 million years before is found in rocks in the form of fossils. The the present, the process of photosynthesis microscopic structures closely related to the got evolved. Life was confined to the present form of blue algae have been found in oceans for a long time. Oceans began to geological formations much older than some have the contribution of oxygen through 3,000 million years. It can be assumed that life the process of photosynthesis. Eventually, began to evolve sometime 3,800 million years ago. oceans were saturated with oxygen, and The summary of evolution of life from unicellular 2,000 million years ago, oxygen began to bacteria to the modern man is given in the flood the atmosphere. Geological Time 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 is not related to the formation or modification of the present atmosphere? (a) Solar winds (c) Degassing (b) Differentiation (d) Photosynthesis (iii) 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) What is meant by the process of differentiation? (ii) What was the nature of the earth surface initially? (iii) What were the gases which initially formed the earth’s atmosphere? 2024-25 THE ORIGIN AND EVOLUTION OF THE EARTH 17 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? 2024-25