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Origin and Evolution of the Earth

2018-05-29

I. Origin of the Earth

The origin of the Earth is closely related to the origin of the solar system, so in order to study the origin of the Earth, firstly it is necessary to understand three main characteristics of the solar system at present. To sum up, they are:

1. The nine planets in the solar system revolve around the Sun anticlockwise. The Sun rotates in the same direction, which is called the same direction of celestial bodies’ motion in the solar system.

2. The orbital plane of revolution of above planets around the Sun is very close to the same plane and included angle between this plane and rotation equatorial plane of the Sun is less than 6°. This feature is called coplanarity of orbital motion of planets.

3. In addition to the Mercury and Pluto, revolution orbit of other planets around the Sun is very close to a circular orbit. This feature is called near-circular property of orbital motion of planets.

With regard to the origin of the Earth, there has been a long history of study. In ancient times, people discussed the formation of all things, including the Earth. During this period, the “Creationism” on the origin of the universe was gradually formed. One of the most widely widespread versions is the Creationism in the Bible. In the human history, Creationism has occupied a dominant position in a long time.

Since Polish astronomer Copernicus proposed heliocentric theory in 1543, the discussion of the evolution of celestial body has broken through the shackles of religious theology and the truly scientific approach on the origin of the Earth and solar system began. In 1644, R.Descartes put forward the first theory on the origin of solar system in his book named Principiae Philosophiae. He thought that Sun, planets and moons were formed in the different sizes of vortexes which were formed in the cosmic material vortex motion. A century later, G.L.L. de Buffon put forward the second theory in the General and Special Natural History in 1745, that: a massive object is assumed to be a comet and once collided with the Sun so as to make the materials of the Sun split into fragments and scatter into the space, forming the Earth and planets. In fact, because the mass of the comet is generally very small, it is unable to collide with the Sun to make a large amount of materials to form the Earth and planets. In the 200 years after Buffon, people put forward many theories and these theories basically tended to the “Monism” of Descartes, that is, the Sun and planets were condensed from the same original gas cloud; there was a theory of Dualism, that the planetary material is separated from the Sun. in 1755, the famous German classical philosophy founder I. Kant proposed “Nebular Hypothesis”. In 1796, the famous French mathematician and astronomer P. S. Laplace independently proposed another nebular hypothesis on the origin of the solar system in his book named The Cosmical Syesteme. Since the theories of I. Kant and P. S. Laplace are consistent in the basic argument, later generations called the theories of them as “Kant- Laplace Theory”. Throughout the nineteenth century, this theory has always been in a dominant position in astronomy.

By the turn of the century, because Kant- Laplace Theory cannot make satisfactory explanation to more and more observational facts of the solar system, the theory of “Dualism” became popular again. In 1990, American geologist T. C. Chamberlain proposed a theory of the origin of the solar system, called “Planetesimal theory”; in the same year, F. R. Moulton developed this theory and he thought once a plant moves near the Sun to make the front and back of the Sun produce huge tides, thereby throwing a lot of materials. These materials gradually condensed into many solid clumps or particles, called planetesimals. They further polymerized into the planets and moons.

Modern researches show that due to long distance between stars in the universe, the possibility of colliding with each other is extremely small. Therefore, the theory of Moulton cannot be convincing. Due to the common characteristics of all catastrophic theories, the origin of the solar system is attributed to some extremely fortuitous events. Therefore, it is lack of sufficient scientific evidence. The famous Chinese astronomer Mr. Dai Wensai proposed a new theory of the origin of the solar system in 1979 and he thought that the whole solar system was formed by the same primordial nebula. The main components of this nebula were gas and a small amount of solid dust. At first, the primordial nebula had its own rotation and contracted due to the self-gravity at the same time so as to form the nebular disk. The middle part evolved into the Sun and the edge formed nebula and further accreted and evolved into the planet.

In general, there are more than 40 theories about the origin of the solar system. The catastrophic theory rapidly popular in the beginning of the century is the challenge to the Kant- Laplace Theory; the new nebular hypothesis rising in the middle of this century is the more perfect theory explaining the origin of the solar system that was established on the basis of Kant- Laplace Theory. People’s understanding of the origin of the Earth and solar system is also deepened in the tortuous development process.

At this point, we can give the following possible conclusions about the matters and manners for the formation of primitive earth. The main matters for the formation of primitive earth are the above prime matters of the nebular disk and their main compositions are hydrogen and helium, accounting for about 98% of the total mass. In addition, there is solid dust and materials thrown out of the early contraction and evolutionary stages of the Sun. In the process of the formation of the Earth, due to the role of material differentiation, light components are separated with hydrogen and helium and other volatile substances and brought to the outer solar system by the solar radiation pressure and materials from the Sun, therefore, only emphasizing materials or earth materials gradually condense together to form the primitive earth and then it evolves into today’s earth. Mercury, Venus, and Mars, like the Earth, may have a similar formation due to shorter distance to the Sun. They retain more emphasizing materials; Jupiter, Saturn and other planets still keep more light components due to the farther distance to the Sun. For the formation ways of the primitive earth, although there is much speculation, the views of most researchers are consistent with the conclusions of Dai Wensai. That is, after the formation of above nebular disk, due to the role and instability of the gravity, the substances in the nebular disk, including dust horizon, accrete because of the collision to form many primitive asteroids or planetesimals. After gradual evolution, they get together to form the planet. So the Earth is formed in them. According to estimates, the time required for the formation of the Earth is about 10 million to 100 million years. The time required for the formation of the planet nearer the Sun (terrestrial planets) is relatively shorter. The time required for the formation of the planet father away from the Sun is longer and even up to hundreds of millions of years.

As for the primitive earth has high temperature or low temperature, the scientists also have different claims. From the ancient theory on the origin of the Earth, most people have believed that the Earth was a molten body at first. After billions of years of geological evolution history, the Earth has still maintained its heat. The results of modern research are more inclined to the theory on the origin of the Earth in low temperature. Whether the early state of the Earth is high temperature or low temperature, there is still a controversy. However, either the theory on the origin in high temperature or low temperature, the Earth generally experienced a stage from hot to cold. Because the internal Earth has a heat source, the cooling process is extremely slow. Until today, the Earth is still in the cooling process.

II. Evolution of the Earth

Primary profile of the Earth’s surface can be clearly divided into two parts, namely continent and oceanic basin. Continent is the highland on the Earth’s surface and oceanic basin is the relatively low-lying area, filled with a huge amount of seawater. Continents and oceanic basins together form the basic components of geosphere. Therefore, the evolution of geosphere is equivalent to the evolution of continent and oceanic basin. Please see the section of the structure of Earth's spheres for relevant interior structure of the Earth.

Now, most of the geoscientist have confirmed the phenomenon of continental drift and agreed that the structural distribution and changes of ocean and continents on the earth are directly related to the movement of continental drift. The hard lithosphere plate of the earth moves as a unit on the asthenosphere of the earth below; since the relative movement of the lithospheric plate results in continental drift and forms the current distribution of ocean and continents on the earth. The lithosphere of the earth can be divided into oceanic and continental lithosphere. In general, the former is half as thickness as the latter. The thickness of the oceanic lithosphere is very uneven, and the greatest thickness is 80 km.

Most of the large earth plates are composed of continental and oceanic lithospheres. However, the enormous pacific plate is composed of simplex oceanic lithospheres. The continent area on the earth accounts for about 30% of the total area of the earth, among which about 70% of the continents is distributed in the Northern Hemisphere and located near the equator and the mid-latitude region of the Northern Hemispheres. This is likely related to the pole-fleeing movement of the continental blocks caused by earth rotation.

On a global scale, the continental crust islands distributed near the continents are almost all on the east coast of the continent. The eastern edge of individual continent is surrounded by festoon-like island groups constituted by a series of continental crust islands, which forms a significant island arc projecting eastward. The distribution characteristics of the global continental crust islands can be explained by the theory of the common westward movement of lithospheric plates and the extension of seabed margin. For a long time, people have noticed that the some continental tectonics on the surface of the earth can be some continental tectonics can be pieced together, which seems to be a jigsaw puzzle. Especially, the west coast of Africa and the east coast of South America are most obviously identical. This phenomenon can be explained by the direct rupture of the continental lithospheres and the long-term drift of the continental blocks. This is the theory put forward by A. L. Du Toit that the current continents are formed by drift after the rupture of Laurasia in the Northern Hemisphere and Gondwanaland near Antarctica, which we will introduce then.

In 1966, H. W. Menard compiled all the exploring data about ocean depth at that time, carried out the statistics on world ocean depth again and obtained the data that the global average elevation of the continents above the sea level is 0.875 km and the average ocean depth is 372.9 km. There are edge zones of the width of tens of kilometers submerged by sea water between the continents and the oceans. The areas include the continental shelves and the continental slopes. The two accounts for 10.9% of the earth surface together. There are apparent differences between the continental crust and the oceanic crust. The chemical composition of the continental crust is mainly granite, and the rocks beneath the ocean basin are mainly composed of basalt and gabbro. Therefore, the whole crust can be divided into two types, including continental sialic crust and oceanic simatic crust.

For the questions related to the origin of the continents, A. L. Du Toit, geologist and geophysicist, put forward that there was the model of two original continents on the earth in his Our Wandering Continents in 1937. If the model is true, the two original continents are called Lanrasia and Gondwanaland, respectively; this is actually just like piecing the global continents into only an ancient continent which A.L.Wegener et al. proposed before. A. L. Du Toit believes the two original continents were formed near the two poles of the earth, among which Laurasia was in the north and Gondwanaland was in the south. After they were formed, they gradually broke and drifted to the positions of continental masses today.

As early as the end of the 19th century, the geologist E. Suess had realized that the geological structures of the continents in the Southern Hemisphere of the earth are very similar and pieced them into an ancient continent for research. He called it Gondwanaland. This name originates from the name of a standard stratigraphic region in eastern and central India (Gondwana). Gondwanaland includes today’s South America, Africa, Madagascar, Arabian Peninsula, Indian Peninsula, the Island of Sri Lanka, Antarctica, Australia and New Zealand. They were formed in the same geologic age and have the same plant fossils in the terrane, which are called Gondwana rocks. The main evidences that A. L. Du Toit used to prove the existence and drift of Laurasia and Gondwanaland are from geological, paleontological and paleoclimatological aspects. According to the information accumulated in thirty years, it was strongly proved that the theory of Gondwanaland was basically correct.

Laurasia is the combination of Europe, Asia and North America. These landmasses are not dispersed away from each other even now. Laurasia has a very complex history of formation and evolution. It is mainly merged by several ancient continental landmasses, including ancient North American landmass, ancient European landmass, ancient Siberian landmass and ancient Chinese landmass. In the Neopaleozoic (about 300 million years ago), these ancient landmasses gradually closed up and collide with each other and were gradually closed roughly in early to middle Carboniferous to Permian (i.e. 200 million to 270 million years ago). The ancient geological, climatic and biological information shows that Laurasia was in middle and low latitudes from Carboniferous to Permian period. After the Mesozoic (i.e. last 100-200 million years), Laurasia gradually broke apart, which led to the expansion and formation of the North Atlantic. The researches show that the formation and distribution of new global new orogen is the tectonic result of rupture and drift of Laurasia and Gondwanaland. During this process, the laws of the uneven westward movement and the pole-fleeing movement of the continental blocks are very obvious. In general, Laurasia was located in the high latitudes of the Northern Hemisphere and Gondwanaland was located in the Southern Hemisphere near the South Pole; the two continents are separated by the area called Tethys (also known as Tethys geosyncline).

Before A. L. Du Toit (1937) put forward the theory of Laurasia and Gondwanaland, A. L. Wegener put forward the theory that there was only one original continent called Pangaea on the earth early in 1912. A. L. Wegener believes that it was formed in Carboniferous period (about 220 million to 270 million years ago). A. L. Wegener took as a starting point for his description of continental drift. However, according to the people’s current understanding of Pangaea, the Pangaea proposed by A. L. Wegener is never an original continent. Although there are still a lot of people who agree with the opinion of Pangaea, the restored map of the ancient continent made by them is quite different from that made by A.L.Wegener. On the contrary, it is a little close to the theory of distribution of two ancient continents of A. L. Du Toit.

The continental drift and plate movement in the last 200 million years have been proved and widely recognized. However, someone speculates that the plate movement may have started early in 3 billion years ago. The speeds of plate movements in different geologic times are different. There were several times of collision and integration and repeated rupture and separation among the continental blocks. Multiple collisions among the continental blocks have formed a number of fold mountain chains and connected them to form a new continent. New ocean basins have been formed due to sea floor spreading. Therefore, it is very difficult to accurately recover the so-called continental “Drift Prior to Drift” occurred more than 200 million years ago. The earth is more than 4 billion 600 million years old. At present, it is known that the oldest rocks on the earth are 3 billion 700 million years old. They are distributed in a very small area. Thus, from 4 billion 600 million years to 3 billion 700 million years, there is a time interval of about 900 million years with complete lack of geologic information. In addition, the geological records on the earth formed 2.5 billion years ago are also very limited, which brings a lot of difficulties to the research of the early historical situation of the earth. Therefore, we still don’t have a unified theory of the early history of the earth.

Origin and Evolution of the Oceans

Researches on the origin and evolution of the oceans didn’t begin until the beginning of this century. In general, it is previously believed that the oceanic basin is a perpetual form on the surface of the earth, that is, the oceanic basin has fixed location and distribution pattern since it is formed and begins to store water. With the development of geoscience, especially the revolutionary theory of continental drift proposed by A. L. Wegener et al. at the beginning of this century, there is breakthrough in the understanding of the origin and evolution of the oceans in the last 200 million years.

The theory of continental drift wasn’t supported by many people at the beginning because the problem of force source, mechanism causing the continental drift, was not solved at that time. In 1931, Holmes et al. put forward the theory of mantle convection to explain the force source of continental drift. However, this opinion received attention of few people at the time. In the late 19th Century, someone established the theory of global tectonics of earth contraction to explain why there are such large-scale orogenic movements on the earth. However, after 1950s, with the discovery of evidence of huge extension in the rift valleys of the global oceans, the contraction theory is generally abandoned. At the same time, the theory of earth expansion soon became popular. The expansion theory believes that the earth is very small at the beginning and it is half the diameter of the current earth. Due to the large expansion of the earth, the original earth crust ruptured and become the present continents. The ruptured area becomes the modern ocean basins with continuous development. In addition, the so-called continental drift caused by the large expansion of the earth indicates that the continental blocks basically stay in the original places, that is, there is no obvious movement between the continent and between the continent and the mantle. Since the theory of expansion cannot explain how are the structural characteristics of the fold mountain chains widely developed on the continental crust formed, the theory of mantle convection proposed by Holmes et al. was valued again soon. In the early 1960s, with the rapid accumulation of sea floor detection, H. H. Hess and R. S. Dietz firstly developed the scheme of mantle convection into the sea floor spreading theory. In 1962, H. H. Hess published a paper of History of Ocean Basins and put forward a new idea of the origin of the oceans, which is the sea floor spreading theory. H. H. Hess believes that the main structure of the sea floor is the direct effect of mantle convection. The sea floor spreading theory proves that the continents and the ocean floor are passively moving on the convective mantles instead of the situation that the continents are actively moving on the ocean floor proposed in the early continental drift theory. Shortly after the sea floor spreading theory was proposed, some other ocean floor observation results, such as ocean-floor crust structure, geomagnetism, earthquake origin and geothermal flow distribution, provided strong evidences for this theory. In this case, most of the scholars have turned to the researches on sea floor spreading. It has been widely acknowledged that the origin and evolution of the oceans can be explained with the theories of sea floor spreading and plate tectonics. The basis for the theoretical explanation of theories of sea floor spreading and plate tectonics on the origin and evolution of the oceans is mantle convection.

Modern researches have confirmed that the oceans were originally conceived in the interior of the continents and began in the rift valleys of the continental lithospheres. The continents broke apart and separated from each other at the rift valleys, so as to create new ocean basins. A. L. Wegener took the coincidence of the two opposite banks of the South Atlantic as the starting point of the description of continental drift. In fact, by piecing South American and African continents together, not only the terrain contours of the edges are very identical, but also the rock types and geological structures can also be docked. At present, it has been proved that the Atlantic didn’t exist in Permian (250 million years ago). It is estimated that the continental rift valley forming the Atlantic occurred in later Triassic (about 160 million to 190 million years ago). Till the late Jurassic (about 120 million years ago), the Middle Atlantic may have been opened up to 1000 km in width; the opening of the South started approximately in the early Cretaceous (about 110 million years ago), and the initial rift valleys occurred in the Late Jurassic (about 130 million years ago); the opening of the North Atlantic was the latest and started approximately at the beginning of the Tertiary (about 60 to 70 million years ago). Meanwhile, it extended from the North Atlantic Rift valley to the northeast and extended into the area between the Greenland and Europe. The Norwegian Sea opened with it. From 60 million years to 20 million years ago, the subjects of the Norwegian Sea, the Baffin Sea and the North Atlantic extended. However, both the speed and the direction changed. To sum up, the current vast ocean basins are not as vast as it always was. It is the result of long-term movement and evolution of the earth. The development of the oceans from the narrow bays to the vast basins is achieved through the continuous occurred large-scale seafloor spreading process. The driving force of both sea floor spreading and plate movement is mantle convection.

Since the original crust of the earth never stops the large-scale movement of geological structural forms since its formation. Therefore, it is certain that the forms of the oceans and continents on the earth are the result of the large-scale crustal movement over the past billions of years.

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