The concept of our planet's origin and the occurrence of life upon it took human minds many centuries to comprehend. Today, it remains a subject of hot scientific discussion and debate, again giving rise to new hypotheses. Early theories of evolution were abstract and were unsupported by strict scientific facts and methods. Now, however, scientists of different disciplines have accumulated large volumes of observed information, and they have succeeded in reconstructing the main stages of the formation of the Oceans and continents, and the occurrence of life on Planet Earth.
 
 

Our home, the Earth, is one of the smaller planets revolving around the Sun. The Sun is one of countless billions of stars existing in Universe. Millions of galaxies are so far from us, that the light from some of these far solar systems began its way to us more than 4.5 billion years ago. Our near-Sun planetary system is probably not unique in the Universe, but the direct proof of existence of other such systems are, at this time, absent. Only infinitesimally small, periodic movements, noticed by an astrophysicist observing one of the stars nearest to us, give weak, indirect indications that there may be other solar systems similar to ours.

How and when were our solar system and Planet Earth formed? The first written thoughts of Man about the origins of the Universe, solar system and Planet Earth appeared during the 3rd and 2nd millennia BC. Later, observations and opinions of philosophers and scholars of Ancient Greece and Ancient Rome were written and have been translated. For many centuries, however, the teachings of divine creation of the World dominated thought, but alongside them were hypotheses, with striking similarities with modern ideas. The scientist, Heraclitus, of ancient Greece wrote 500 years BC: "The World, uniform as a whole, was not created by Gods or people, and was, is, and will eternally burn, but yet, inflammable and normally fading". The Roman philosopher, Tit Lucretius Car, in the poem "On the Nature of Things," proclaimed the idea that the Universe is infinite and in it there is a set of worlds, similar to ours. Though many theories have been introduced over many centuries, only since the middle of the 20th century has this complicated problem been investigated, on the basis of new scientific data.

By studying the structure of meteorites, scientists have determined their age - about 4.6 billion years old. In making a determination of an absolute age of most ancient rocks of the Earth, an analysis of the distribution of isotopes of the same chemical element over time permits us to make a reasonably accurate age-determination of our planet: 4.5 - 4.7 billion years, as is the age of the Moon and the other planets. These and many other data have gradually resulted in the acceptance of modern cosmogonic hypotheses, a huge role in the development of which was played by the Soviet scientists O. Yu. Shmidt, V.G. Fesenkov, A.P. Vinogradov and others.

Of the modern cosmogonic theories, the most significant is that of the "Big Bang," is being advanced. According to this theory, the Universe existed in a kind of small volume of super-concentrated substance - "a primitive atom", some 20 billion years ago. As a result of its explosion, a gas-dust cloud was formed, and further condensation within the cloud resulted in formation of the Sun and proto-planets, about 15 billion years after the "Big Bang" - about 5 billion years ago. Another theory, that of pulsation of the Universe has also been presented, noting that the Universe has periodically reached a physical maximum size, after which it compressed into itself to the size of "primitive atom", which in turn, exploded, forming a new Universe.
 
 

Structure of the Sun Nucleus Internal layer External layer Chromospere Corona Interplanetary space

The Sun is the central body of our solar system, a scorching-hot plasma ball weighing 2 x 10³⁰ kg and with a radius of 696 thousand km. Hydrogen makes up about 90% of its weight, helium almost 10% and other elements, less than 0.1%. The source of solar energy is from continuous nuclear reactions, converting hydrogen into helium. The structure of the Sun includes a nucleus, internal and external layers and a solar "atmosphere," in which there is the chromosphere and solar corona. The bottom layer of solar atmosphere is named the photosphere.

The solar nucleus has a temperature of about 14 million degrees, Celsius. An interaction between the internal and external layers of the Sun causes a transfer of energy from the nucleus to the solar atmosphere to occur. The temperature of the solar corona is about 1 million degrees, Celsius. Almost all of the electromagnetic radiation from the Sun emanates from the photosphere. Solar activity in the photosphere manifests itself as sun-spots and as solar flares, which rise to great altitudes above the surface of the Sun.

The solar atmosphere is very dynamic: in it, chromospheric flares and solar prominences are observed, as is the continual death and rebirth of the solar corona, which emits substances into interplanetary space (solar wind).

The Sun is main source of energy for all manner of processes set in motion on our planet. It provides all of the natural light and heat, without which, life on the Earth (as we know it) is impossible.

The planets of our solar system are divided into two groupings: inner and outer. The inner four are relatively small planets, named Mercury, Venus, Earth and Mars. At and near the orbit of Mars is an extensive belt in which there are thousands of small, planets-like-objects called asteroids. Beyond this belt and further from the Sun are the outer planets - four giants: Jupiter, Saturn, Uranus, Neptune, and finally, a very small planet, Pluto, situated on the periphery of our solar system. The physical characteristics of Pluto are qualitatively different from the characteristics of it's nearest neighbouring planets - giants, and consequently, it cannot be physically related to that part of that grouping. Only its great distance from the Sun places Pluto in the outer planets grouping.

The inner planets have not only much in common, but also essential distinctions: Mercury is the planet that is the heaviest and nearest to the Sun. It has NO atmosphere. Mars has a rarefied atmosphere. Very similar to the Earth in size and weight, Venus has a dense and very hot atmosphere (temperature of a surface of Venus about 400°C), which consists basically of carbon dioxide (CO₂), and excludes the existence of a type of life thus-far found on earth.

The planets in our solar system are accompanied by satellites. The Earth has 1 satellite. - the Moon; Mars - 2; Jupiter - 12; Saturn - 10 (not including its rings); Uranus - 5; and Neptune - 6,.

Planet Earth is a unique planet of our solar system: it has oceans, it is only planet on which we know life exists, and with the evolution of present-day Man, a high level of physical development. However, life is a natural stage in the of development of matter, and therefore Earth cannot be considered as unique, that is, the only inhabited planet in the Universe.
 
 

At the early stages of formation, the Earth appeared as a cold space body, containing all of the chemical elements known in Nature. The atmosphere and hydrosphere did not yet exist; the surface of the planet was completely lifeless. But gradually, due to gravitational forces, energy released by the breakdown of radioactive elements and lunar tides deep inside the core of the Earth began to heat up. When temperatures near the core of the Earth reached that level where the melting of iron oxides and other compounds could occur, the active processes for the formation of a nucleus and the main environment of the planet began to happen

The general process of formation of the Earth's environment, according to a hypothesis of the Academician A.P. Vinogradov, was through zoned melting in the mantle, situated around a nucleus. Thus the dense, and heaviest sank toward the centre, increasing the size of the nucleus, and less dense and lighter elements rose to the surface, forming lithosphere, the top-most part of which is the Earth's crust. These processes caused the onset of great volcanic activity over all areas on the surface of the Earth and produced great and extensive outpourings of basaltic lava, releasing gases and water vapour. Gravity forces kept the gases and water vapour in the near-earth proximity, and these formed a primitive, proto-atmosphere, but deprived of oxygen.

By radiating heat into Space, the Earth's surface gradually cooled. The water vapours (gas) condensed, and became liquid water. Active elements and compounds, discharged from still more volcanic activity, interacted with the water, forming acids and salts.

Deep tectonic processes and the heterogeneous nature of the earth's crust are not uniform. Therefore, raised areas with thickened but chemically lighter crust have developed into land masses, and sunken areas with heavier crust have become the sea floor, covered by water. The availability of an atmosphere, a planetary water-cycle, and seasonal and daily changes of temperature above the surface of the sea promoted the development of processes of weathering, erosion and mass-wasting. The products of mass-wasting and erosion accumulated at the bottoms of basins and reservoirs, forming the layers (strata) of sedimentary rocks. A continuous cycle of formation and destruction of parts of the earth's surface occurred, causing a constant change in the morphology of the mountains, plains and seas.

Redistribution of mantle materials within the Earth and their rise to the surface and into the Earth's caused the development of the atmosphere and hydrosphere. The duration of these processes was all throughout the geological history of the planet is still going on today. The weight of the water on the surface and in the oceans gradually increased, having formed a World Ocean, and the modern salt structure of the seas was established many hundreds of millions of years ago. In the atmosphere, with occurrence and evolution of vegetable (plant) organisms, oxygen was released by the normal processes of plant growth and living. The oxygen accumulated, lowering the ratio of carbon dioxide to oxygen. The Earth's crust was divided into continental and oceanic areas, sharply distinguished by their structure and thickness. The crust under the oceans has a high density, with an average thickness of 5-6 km; continental (subaerial and coastal) density is lower and the average thickness of the land crust is between 30-40 km. Although the thicknesses are very different, the balance is maintained because of the densities of the crust, keeping the planet rotating smoothly on its axis.

There are various points of view regarding the origin and development of continents and oceans, some of which sometimes contradict each another:

According to one of these points of view, the bottom of the oceans represents primary basalt from the earth's crust, and the continents were formed later as a result of the accumulation of very great thicknesses of sedimentary rocks which were deposited in great, shallow ocean basins, which, because of the great weight, became compressed into folds, forming folded mountain systems.

Another hypothesis assumes that modern ocean basins (or their parts) occurred in areas where earlier, huge continents partially collapsed, compressing these materials into the basins and transforming them from "continental" into "oceanic" deposits, during a cycle called "oceanisation."

There is another hypothesis which shows that the Earth has expanded over long periods, during which time, the area of the bottom of the oceans between the continents began moving apart. The distance between these continents increased, and the water, earlier only the lower parts of continents, completely flowed into the ocean basins.

The hypothesis of horizontal movements of lithospheric plates is the most widely accepted today. According to this hypothesis, the uppermost part of the Earth - the lithosphere - is joined by a number of adjacent, rigid plates, under which the effects of convection currents in the mantle cause these plates to move relative to one-another.

Where the plates diverge due to spreading, there is an upwelling of mantle material into the crust in the rifted area. As this hot mantle material (magma) rises, it cools and crystallises. As more magma upwells into the rift zone, it pushes the crystallised material upward and away from the rift zones, causing mid-ocean ridges to form. Moreover, the iron minerals in the magma take the exact properties of the Earth's magnetic field at the time that these minerals crystallise, and are sometimes used in geophysical measurements. You will learn more about this process later.

Where these plates converge upon each other, one of the plates is pushed downward beneath the other plate. This is called "subduction" and occurs at all convergent plate boundaries. As the one subducting plate is being pushed down, the other plate rides up and over the downward moving plate. Very deep trenches are formed in these subduction zones (for example, the Kurile Islands Trench), and just behind them, island arcs (such as the Kurile Islands and Japan), or mountain systems (such as the Andes and Rocky Mountains in the western hemisphere.

Along the edges and borders of plate divergence and convergence, because of the active tectonic processes beneath them, high seismicity and intensive volcanic activity occurs. The movement of the plates have resulted in continental drift,, closing and opening of oceans, but the water of the ocean remained, flowing from one depression to another.

What is the practical value of the study of these geological processes? The knowledge of the history of the formation of the Earth permits us to understand the formation of useful minerals which have come to us from deep within the core of the Earth. By tracing the different continents and oceans, geological structures and borders of lithosphere plates, scientists can reconstruct their previously adjoining areas, and predict the formation of petroleum and other exploitable minerals and ores. They can also try to predict disastrous earthquakes and other natural hazardous phenomena.
 
 

1.Direction of plate movement Intensity of plate movement: 2.At spreading 3.At subduction

1 Core 2 Earth's crust  3 Mid-ocean ridge  4 Convection currents  5 Island-arc  6 Trench  The arrows show the directions of movement of the lithosphere plates moved by convection currents in the mantle.

According to modern scientific thought, life-forms on Earth first formed more than 3 billion years ago, after a long evolution from inorganic to organic compounds and from them, Life on Earth.

In 1871 the English naturalist, Charles Darwin, wrote, "in a small warm pond, where there were all kinds of ammonium salts and phosphorous acid, where there was light and heat and stores of electric energy", chemical compounds formed the conditions necessary for the development of life on Earth.

During the first billion years, the cooling and condensation of materials proceeded on Earth, the result of which was the formation of the Ocean, land and an atmosphere without oxygen, consisting mainly of methane and ammonia. Synthesised organic compounds, derived from inorganic compounds in the waters of the young (proto-) ocean, were subjected to electrical discharges and ultra-violet rays of the Sun. These processes occurred basically in shallow waters and at the shoreline. The opportunity for synthesis of organic compounds, including amino-acids, from a gaseous mixture of methane, ammonium, hydrogen and water vapour by subjecting it to electrical charges has now been proven experimentally in the laboratory.

With the passage of time, the organic compounds saturated the ocean more and more, forming a "primordial soup" - an environment, favourable for the development of Life.

From this chaos of reactions occurring in the waters of the primitive Ocean, a multimolecular system was formed: detached condensation - coacervated droplets, which had simple external and internal structures and could interact with the external environment. Coacervates were gradually transformed into more complex systems - protobionts, capable of self-preservation and increasing weight by taking in nutrients from the external environment. These nutrients consisted of protein, amino-acids and other organic compounds. Further evolution of protobionts resulted in the occurrence of more complex molecules - nucleotides - prototypes of nucleic acids: RNA and DNA (ribonucleic acid and deoxyribonucleic acid), which, during of the following billion years acquired very important properties - the ability to repeat the characteristics of its ancestors, that is, to transmit the hereditary information to descendants. Nucleic acids formed the basis for the formation of primitive structures. The essential element of its structure became an environment consisting of protein. This environment enabled the important function of an exchange of substances to occur between a cell and sea water. Such cells have been called prokaryotes, that is nucleus-free. Toward the end of the second billion of years of Earth history, a nucleus developed within the body of a cell, separated from other its other parts by an environment. One of the main elements of a cell nucleus became a DNA molecule. This cell was called a eukaryote, that is, containing a nucleus. The nucleus controlled the development of a cell, which received properties of solar energy, transforming it into organic compounds (carbohydrates), using atmospheric oxygen (photosynthesis). Gradually the waters of the ocean and the atmosphere became oxygen-saturated. An ozone layer formed in the top layers of the atmosphere, protecting the living cells from destructive ultra-violet radiation from the Sun. Thus began the formation of multi-cell systems. Originally these were only very primitive micro-organisms, which further developed and then divided into the fundamental components of the Plant and Animal Kingdoms.
 
 

Almost all early Earth history from the beginning of the formation of the Earth's crust can be traced in its mountains. All rocks are subdivided into three main types: sedimentary, igneous and metamorphic. The sedimentary rocks contain the information about evolution of organic life, igneous - about eruptions of magma and metamorphic - about temperatures and pressure which changed the nature of rocks in the first two categories.

Geologists are able to define a sequence of formation of those or other by studying the remains of extinct organisms in sedimentary rocks. The main rule, established in the 18th century by Dutch scientist N. Steno, remains today for most cases: the deeper into the Earth that a layer exists, the older it is. There are a few exceptions to this rule, but these are not common, and occur only in very deformed areas

To determine the relative ages of rocks, not only mineralised remains of animals are used, but also those of plants (spores, pollen, leaf and bark imprints in sediments). The most characteristic mineral organisms for a certain geological time have received the name "guide fossils". Knowing when in a sequence of change one representative fossil of the animal and plant kingdom existed, it is possible to differentiate younger sedimentary layers (strata) from older ones. This method permits us to define the relative age of rocks. This age-dating method, however is only true for sedimentary rocks, since igneous and metamorphic rocks do not contain former life-forms.

The discovery of radioactivity near the start of the 20th century has allowed us to establish an almost absolute age for rocks - the duration of time, from the time of formation before now. This method is based on a determination of the rate of decay of radioactive isotopes of some elements - lead, strontium, helium and others. It is now agreed that these rates of decay for the radio-isotopes of these elements have remained constant throughout the geological history of the Earth.

The geological history of our planet is divided into five large stages, called eras: Archean, Proterozoic, Palaeozoic, Mesozoic and Cenozoic. The last three eras are divided into periods, which are, in turn, further partitioned into epochs. The names of the eras are logical. For example, the Archean era means the era of the oldest life; Mesozoic - middle life; Cenozoic - new life. In this way, the changes that occurred during the main geological events were recorded, as well as the occurrence and development of life on Earth. In the Archean and Proterozoic eras, organic life was very primitive and developed only in the oceans. Therefore, both of these eras are joined in a time frame known as the Cryptozoic eon, meaning "latent life. " The subsequent three eras are grouped into the Phanerozoic eon, meaning "obvious life".

Methods of the determination of relative and absolute ages of rocks and organic remains (fossils) contained within them supplement one another and are widely used to refine the geochronologic scale, which reflects the sequence of intervals of time, during which all rocks were formed.
 
 

The organic kingdom of the Archean contained only micro-organisms, unicellular and not possessing nuclei. Among them were diverse bacteria and primitive algae. They lived not only in top layers of the water, but also in places in the shallow ocean.

During the Proterozoic the first multicellular organisms evolved. Among the plants there were blue-green algae, possessing a more complex internal structure, and in the cell structure, a nucleus. Blue-green algae excreted limy substances, formed carbonate structures when they accumulated. These were called stromatolites.

Two large groups of organisms gradually evolved. One had ability to synthesise organic substances from carbon dioxide and water under effect of solar rays (photosynthesis). The other consumed organic materials. A division of a the Plant and Animal Kingdoms had occurred

In the late Proterozoic (about 1 billion years ago), the oceans were inhabited by invertebrate animals: sponges, primitive echinoderms, medusa (jellyfish), coelenterates (corals), worms, foraminifera, radiolarians and a numerous group - archaeocytes - which later became totally extinct. The best-preserved fossils of the animals of this time period were discovered in Southern Australia and on the coast of the White Sea.

Academician V.I. Vernadskiy investigated the role of the organic kingdom in life of the Earth and came to the conclusion that living substances played an active part in all geological processes on the surface of the Earth and in the formation of atmosphere.
 
 

The Palaeozoic era (570-230 million years ago), is divided into six periods: Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian. In the extensive oceans of this era the plant and animals kingdom developed and evolved. Many marine animals already had calcareous or silicic shells and skeletons. In the early Palaeozoic time, the first vertebrate animals evolved. By the middle of the Palaeozoic era, extensive parts of the land-mass had become colonised by plants and various species of fishes developed in the oceans. By the end of the Palaeozoic the first animals--vertebrates, began to leave the sea. These were amphibians. At the time-boundary between the Palaeozoic and Mesozoic, the single continent (land-mass) was split into two "super-continents" - Laurasia in the Northern Hemisphere and Gondwana in the Southern Hemisphere.
 
 

The Mesozoic era (230-66 million years ago) covers three periods: Triassic, Jurassic and Cretaceous. By the end of the Cretaceous, both Gondwana and Laurasia had already begun to split up and the "plates" began to drift apart, on what are modern continental boundaries.

In the Mesozoic era, molluscs prevailed in the oceans, and bony fishes developed. Highly advanced reptiles gradually developed, among the largest of which were dinosaurs, achieving lengths of 30 m.

By the end of the Mesozoic, flowers, higher-order plants, and primitive birds and mammals began to the land. The warm, damp climate favoured the growth of tropical woods from the Equator to very high latitudes in both the Northern and the Southern hemispheres.
 
 

The Cenozoic era (beginning 66 millions years ago and extending to the present time) covers three periods: Palaeozoic, Neogene and Anthropogene. During this era , the Atlantic and Indian Oceans expanded considerably, thus reducing the volume and area of the Pacific Ocean. The continents acquired their present-day outlines. The air temperature cooled gradually and the result was the development of mixed, deciduous forests and coniferous woods. In high latitudes, there was a zone of tundra. Mammals became the prevailing life-forms on the land. However, the most outstanding event came at the end of the Cenozoic era: the emergence of Man, the first living thing that has had an active effect on the future of life of the Earth.
 
 

Information provided by HDNO:  http://www.oceansatlas.com/unatlas/about/ContactInfoForHDNO.html