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Earth is the third planet from the Sun and the only body in the Solar System known to harbour life. It is the largest terrestrial object in the Solar System.
| Siderial Period | Perihelion (AU) | Aphelion (AU) | Inclination (degrees) | Axial Tilt (deg) | Axial Period (d, h, m) |
|---|---|---|---|---|---|
| 365.26 d | 0.98 | 1.02 | 0.0 | 23.45 | 0, 23, 56.1 |
| Equatorial Diameter (km) | Oblateness | Mass (Earth = 1) | Density (water = 1) | Albedo (geom.) | No. of Satellites |
| 12, 756 | 0.003 | 1.00 | 5.5 | 0.37 | 1 |
Plate tectonics was first proposed by Alfred Wegener, a German meteorologist, in 1904. However, the idea did not catch on until the 1960's when fresh interpretation of this fundamental mechanism began.
The Earth is a very dynamic body. Plate tectonic processes are driven by the large amounts of internally-produced heat, changing the face of the planet more rapidly than any other body in the Solar System. The shape of oceans and land-masses can change completely over periods of only a few tens of millions of years.
Water plays a vital role not only in the preservation of life, but in plate tectonics as well. As oceanic crust is created along oceanic ridges (such as the Mid-Atlantic Rift or the East Pacific Rise), water is chemically combined with the molten rock materials and locked into it. The rock is consequently much denser than rocks formed on dry land. This causes oceanic plates to subduct beneath continental plates when they collide, which induces melting of the rock, rising of the magma, and eruption of the water-laden rocks onto the surface, releasing large amounts of water back into the atmosphere.
Geologists have been able to study the Earth in great detail. It is the only body in the Solar System that we have been able to study so closely. (Although Man has been to the Moon, it is not yet economically feasible to study it as closely as we can the Earth.) Consequently, a detailed history of the Earth has been built up since the 1960's, to which the geological timescale is central. In the table below, time is measured in millions of years before the present (Ma). (Source: Harland et al., 1989)
| Era | Period | Epoch | Dates from... (Ma) |
|---|---|---|---|
| Cenozoic | Quaternary | Holocene | 0.01 |
| Pleistocene | 1.64 | ||
| Tertiary | Pliocene | 5.20 | |
| Miocene | 23.30 | ||
| Oligocene | 35.40 | ||
| Eocene | 56.50 | ||
| Palaeocene | 65.00 | ||
| Mesozoic | Cretaceous | Gulf | 97.00 |
| Early | 145.60 | ||
| Jurassic | Malm | 157.10 | |
| Dogger | 178.00 | ||
| Lias | 208.00 | ||
| Triassic | Late | 235.00 | |
| Mid | 241.10 | ||
| Scythian | 245.00 | ||
| Palaeozoic | Permian | Zechstein | 256.10 |
| Rotliegendes | 290.00 | ||
| Carboniferous | Pennsylvanian | 322.80 | |
| Mississippian | 362.50 | ||
| Devonian | Late | 377.40 | |
| Middle | 386.00 | ||
| Early | 408.50 | ||
| Silurian | Pridoli | 410.70 | |
| Ludlow | 424.00 | ||
| Wenlock | 430.40 | ||
| Llandovery | 439.00 | ||
| Ordovician | Bala | 463.90 | |
| Dyfed | 476.10 | ||
| Canadian | 510.00 | ||
| Cambrian | Merioneth | 517.20 | |
| St. David's | 536.00 | ||
| Caerfai | 570.00 | ||
| Eon | Era | Period | Dates from... (Ma) |
| Proterozoic 2500 Ma to 570 Ma | Sinian | Vendian | 610.00 |
| Sturtian | 800.00 | ||
| Riphean | Karatau | 1050.00 | |
| Yurmatin | 1350.00 | ||
| Burzyan | 1650.00 | ||
| Animikian | 2200.00 | ||
| Huronian | 2450.00 | ||
| Archean 4000 Ma to 2500 Ma | Randian | 2800.00 | |
| Swazian | 3500.00 | ||
| Isuan | 3800.00 | ||
| Hadean | Early Imbrian | 3850.00 | |
| Nectarian | 3950.00 | ||
| Priscoan 4560 Ma to 4000 Ma | Basin Groups 1-9 | 4150.00 | |
| Cryptic | 4560.00 | ||
Life probably evolved spontaneously on Earth, but many astronomers are convinced that it is extra-terrestrial in origin. Many outer Solar System natural satellites contain large amounts of ice consisting of not only water but also hydrocarbons. Spectral analysis of material in cometary tails and comas has also revealed the presence of hydrogen, carbon, oxygen and nitrogen, which could have reached Earth very easily.