Saturday, 30 May 2015

The history of Earth concerns the development of the planet Earth from its formation to the present day.Nearly all branches of natural science have contributed to the understanding of the main events of the Earth's past. The age of Earth is approximately one-third of theage of the universe. An immense amount of biologicaland geological change has occurred in that time span.
Earth formed around 4.54 billion (4.54×109) years ago by accretion from the solar nebula. Volcanic outgassingprobably created the primordial atmosphere, but it contained almost no oxygen and would have been toxic to humans and most modern life. 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 the 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 forlife on Earth are graphite found to be biogenic in 3.7-billion-year-old metasedimentary rocksdiscovered in Western Greenland and microbial mat fossils found in 3.48-billion-year-oldsandstone discovered in Western Australia. Photosynthetic life appeared around 2 billion years ago, enriching the atmosphere with oxygen. Life remained mostly small and microscopicuntil about 580 million years ago, when complex multicellular life arose. During the Cambrianperiod it experienced a rapid diversification into most major phyla. Although more than 99 percent of all species that ever lived on the planet are estimated to be extinct, there are currently 10–14 million species of life on the Earth.
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 they harbor. The biosphere, in turn, has had a significant effect on the atmosphere and other abiotic conditions on the planet, such as the formation of the ozone layer, the proliferation of oxygen, and the creation of soil.
Composition by volume   
78.08% nitrogen (N2) (dry air)
20.95% oxygen (O2)
0.930% argon
0.039% carbon dioxide
~ 1% water vapour (climate-variable)
Physical characteristics
Mean radius 6371.0 km
Equatorial radius 6378.1 km
Polar radius 6356.8 km
Flattening 0.0033528
1/298.257222101 (ETRS89)
·         40075.017 km (equatorial) 
·         40007.86 km (meridional) 
·         510072000 km2
·         (148940000 km2 (29.2%) land
·         361132000 km2 (70.8%) water)
Volume 1.08321×1012 km
5.97219×1024 kg
(3.0×10-6 solar mass)
Mean density
5.514 g/cm3
9.807 m/s2 (1 g)



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