The world, the earth

 The world is the third planet from the Sun,the largest in the Solar system`s four terrestrial planets and the only astronomical object known to accommodate life. The life on earth arose at least 4.5 billion years ago by accretion from the solar Nebula.Volcanic out gassing probably created the primordial atmosphere, but it contained almost no oxygen and would have been toxic to humans and most modern life. 

The age of the Earth is approximately one-third of the age of the Universe. Earth or the world is currently home to 10-14 million species of life, including over 7.3 billion humans who depends upon its biosphere and minerals. Scholars estimate that over 99 percent of all species that ever  lived on the planet are extinct. Earth was formed around four and a half billion years ago. Within its first billion years life appeared in its oceans and began to affect its atmosphere and surface, promoting the proliferation of aerobic as well as anaerobic organisms and causing  the formation of the atmosphere`s ozone layer. This layer and the geomagnetic field blocked the most life-threatening parts of the Sun`s radiation , so life was able to land as well as in water.Much of the Earth was molten because of frequent collisions with other bodies which led to extreme volcanism. One very large collision is thought to have been responsible for tilting to Earth at an angle and forming the Moon.Over time, the planet cooled and formed a solid crust , allowing liquid water to exist on the surface.The first life forms appeared between 3.8 and 3.5 billion years ago.The earliest evidences for life on Earth are graphite found to be  3.7 billion year old ,metasedimentary rocks discovered in Western Greenland and microbial mat fossils found in 3.48 billion year, old sandstone discovered in Western Australia. Since the, the combination of Earth`s distance from the Sun ,its physical properties have allowed life to thrive and evolve. This Earth`s lithosphere is divided into several rigid tectonic plates that migrate across the surface over periods of many millions of years.

 Seventy-one percent of the Earth`s surface is covered with water, with the remainder consisting of continents and islands that together have many lakes and other sources of water that contribute to the hydrosphere.Earth`s poles are mostly covered with ice that includes ice of the Antarctic and the Sea ice of the polar ice.Earth gravitational interacts with other objects in the space, especially with the Sun and the Moon. During one orbit around the Sun, Earth rotates about its own axis 366.26 times, creating 365.26 solar days or one sidereal year. The Moon is Earth`s only natural satellite. It began orbiting Earth about 4.53 billion years ago.The Moon`S gravitational interaction with Earth stimulates oceans tides, stabilizes the axial tilt and gradually slows the planet`s rotation. Photosynthetic life appeared around 2 billion years ago, enriching the atmosphere with oxygen. Life remained mostly small and microscopic until about 580 million year ago, when complex multicellullar life arose. During the Cambrian period it experienced a rapid diversification into most major phyla.Geological change has been constantly occurring on Earth since the time of its formation and biological change since the first appearance of life. Species continuously evolve, taking on new forms,splitting into daughter species, or going extinct in response to an ever changing planet. The process of plate tectonics has played a major role in the shaping of Earth`s oceans and continents as well as the life harbor.Plate tectonics is a scientific theory that describes the large-scale motion of Earth`s lithosphere. On Earth there are seven  major plates :

• North American plate
• Pacific plate
• South American plate
• African plate
•  Eurasian plate
•  Australian plate
• Antarctic plate 

              and many minor plates.Where plates meet, their relative motion determines the type of boundary; convergent,divergent,or transform. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occuralong these plates boundaries. The lateral relative movement of the plates typically varies from zero to 100 mm annually. Tectonic plates are composed of oceanic lithosphere and thicker continental lithosphere, each topped by its own kind of crust. Along convergent boundaries, carries plates into the mantle; the material lost is roughly balanced by the formation of new(oceanic) crust along divergent margins by seafloor spreading. In this way, the total surface of the globe remains the same. The prediction of plate tectonics is also referred to as the conveyor belt principle. Earlier theories propose gradual shrinking or gradual expansion of the globe. Tectonic plates are able to move because the Earth`s lithosphere has greater strength than the underlying asthenosphere. Lateral density variations in the mantle result in convection. Plate movement is thought to be driven by a combination of the motion of the seafloor away from the spreading ridge and drag,with downward suction, at the subduction  zones. The initial crust, formed when Earth`s surface first solidified, totally disappeared from a combination of this fast Hadean plate tectonics and the intense impacts of the Late Heavy bombardment.However it is thought that it was basaltic in composition, like today`s oceanic crust, because little crustal differentiation had yet taken place.The first larger pieces of continental crust,which is a product of differentiation of lighter elements during partial melting in the lower crust,appeared at the end of the Hadean, about 4.0 Ga. What is left of these first small continents are called cratons . 

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The oldest rocks on Earth are found in the North American craton of Canada. They are tonalities from about 4.0 Ga. They show traces of metamorphism  by high temperature, but also sedimentary grains that have been rounded by erosion during transport by water,showing rivers and seas existed then. Cratons consist primarily of two alternating types of terranes , the first so-called greenstone belts and the second type is a complex of felsic magmatic rocks. Lets talk about Pangaea and the way that the continents were separated.

Pangaea was a super continent that existed during the late Paleozoic and earthly Mesozois eras.It formed approximately 300 million years ago and then began to break apart after about 100 million years. Unlike the present Earth,much of the land mass was in the southern hemisphere. Pangaea was the first reconstructed super continent and it was surrounded by a super ocean,known as Panthalassa. There were three major phases in the break-up of Pangaea. The first phase began in the Early Middle Jurassic, when Pangaea began to rift from the Tethys Ocean in the east to the Pacific in the west, ultimately giving rise to the super continents Laurasia and Gondwana. The rifling that took place between North America and Africa produced multiple failed rifts. One rift resulted in a new ocean, the North Atlantic ocean. The Atlantic Ocean did not open uniformly,rifting began in the north-central Atlantic.The south Atlantic did not open until the Cretaceous when Laurasia started to rotate clockwise and moved northward with north America to the north,and Eurasia to the south. The clockwise motion of Laurasia led to the closing of the Tethys ocean. Meanwhile, on the other side of Africa and along the adjacent margins of east Africa, Antarctica and Madagascar, new rifts were forming that would not only lead to the formation of the southwestern Indian Ocean but also open up in the Cretaceous.                 The second major phase in the break-up of Pangaea began in the early Cretaceous when the minor super continent of Gondwana separated into multiple continents(Africa,South America,India,Antarctica and Australia).  About 200 Ma , the continent of Cimmeria,collided with Eurasia.However,a subduction zone was forming,as soon as Cimmeria collided. This subduction zone is called Tethvan Trench.This trench might have subducted what is called the Tenyan mid-ocean ridge, a ridge responsible for the Tethys Ocean`s expansion,it probably caused Africa, India and Australia to move northward. In the early Cretaceous Atlantica, today`s South America and Africa,finally separated from eastern Gondwana(Antarctica,India,Australia)causing the opening of a "South Indian Ocean". In the Middle Cretaceous ,Gondwana fragmented to open up the South Atlantic Ocean as South America started to move westward away from Africa. The south Atlantic did not develop uniformly,it rifted from south to north.  

Also, at the same time, Madagascar and India began to separate from Antarctica and moved northward,opening up the Indian Ocean.Madagascar and India separated from each other 100-90 Ma in the Late Cretaceous. India continued to move northward toward Eurasia at 15 centimeters a year,closing the Tethys Ocean,while Madagascar stopped  and became locked to the African Plate.New Zealand,New Caledonia and the rest of Zealandia began to separate from Australia,moving eastward toward the Pacific and opening the Coral Sea and Tasman Sea. 

The third major and final phase of the break-up of Pangaea occurred in the early Cenozoic(Paleocene to Oligocene). Laurasia split when North America(also called Laurentia) broke free from Eurasia,opening the Norwegian Sea about 60-55 Ma. The Atlantic and Indian Oceans continued to expand, closing the Tethys Ocean. Meanwhile,Australia split from Antarctica and moved rapidly northward,just as India had done more than 40 million years before.It is currently on a collision course with eastern Asia. Both Australia and India are currently moving northeast at 5-6 centimeters in a year. Antarctica has been near or at the South Pole since the formation of Pangaea about 280 Ma. India started to collide with Asia beginning about 35 Ma, forming the Himalayan orogeny , and also finally closing the Tethys Seaway; this collision continues today. The African Plate started to change directions, from west to northwest toward Europe,and South America began to move in a northward direction,separating it from Antarctica and allowing complete oceanic circulation around Antarctica for the first time.This motion, together with decreasing atmospheric carbon dioxide concentrations,caused a rapid cooling of Antarctica and allowed glacier to form. This glaciation eventually coalesced into the kilometers-thick ice sheets seen today.Other major events took place during the Cenozoic, including the opening of the Gulf of California, the uplift of the Alps, and the opening of the Sea of Japan.The break-up of Pangaea continues today in the Red Sea Rift and East African Rift. 

There are a lot of mystery about the creation of the Earth and the life on it..