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Earth is the third planet from the sun and the fifth largest

 Surface

    The Earth's surface is very young because 500 000 000 years ago erosion and tectonic processes destroyed and recreated most of the Earth's surface. So the early history of the Earth was erased and the current surface is only about 4.5/4.6 billion years old. The oldest rocks are about 4 billion years old and the oldest fossils are less than 3.9 billion years old. 71% of the Earth's surface is covered in water, and Earth is the only planet that can sustain liquid water at its surface. The heat capacity of the oceans keeps the Earth's temperature stable, and water is also responsible for the erosion and weathering of the continents.

 Interior

    The composition of the Earth is divided into layers each with distinct chemical and seismic properties. There are seven layers: crust, upper mantle, transition region, lower mantle, D'' layer, outer core and inner core. The crust varies in thickness, being thinner under the oceans and thicker under the continents. The inner core and crust are solid and the outer core and mantle are plastic/semi-fluid. The main mass of the Earth is in the mantle and then the core and the part of the Earth that humans inhabit is only a tiny fraction of the earths mass. 

    The core is composed of iron and nickel and is hotter than the surface of the sun (7500K). The lower mantle is comprised of silicon, magnesium and oxygen with a little iron, calcium and aluminium; the upper mantle is comprised of iron and magnesium silicates with a little calcium and aluminium and the crust is made of quartz (silicon dioxide). Therefore the Earth's overall composition is: iron(34.6%), oxygen(29.5%), silicon(15.2%), magnesium(12.7%), nickel(2.4%) and sulphur(1.9%).

     The crust of the Earth is divided into solid plates which float on the hot mantle below and are governed by plate tectonics. This consists of two processes:1)spreading and 2)subduction. Spreading consists of two plates moving away from each other and a new crust being created from the underlying magna, whereas subduction consists of two plates colliding where one plate dives under the other and is destroyed. There are eight major plates and twenty or more minor ones with earthquakes occurring at plate boundaries.

 Atmosphere

    The atmosphere of the earth is 77% nitrogen, 21% oxygen, with traces of argon, carbon dioxide and water. When the earth was first formed there were large amounts of carbon dioxide but this has been incorporated into carbonate rocks, dissolved in oceans and consumed by plants over time. Nowadays plate tectonics and biological processes maintain a continual flow of carbon dioxide in the atmosphere. The tiny amount of carbon dioxide in the atmosphere is essential to the maintenance of the earths surface temperature via the greenhouse effect which raises the surface temperature by 35 degrees. Without this the oceans would freeze and life would end.

Free oxygen in the atmosphere is remarkable from a chemical point of view. Oxygen is reactive gas and would usually combine with other elements, but in the atmosphere it is produced and maintained by biological processes. Therefore, without life, there would be no free oxygen.

 Temperature

    The average temperature on Earth is 15 degrees centigrade.

 Magnetism

    The Earth has a modest magnetic field produced by electric currents in the outer core. The interaction of solar wind, Earth's magnetic field and Earth's upper atmosphere cause auroras and produce Van Allen radiation belts, a pair of doughnut shaped rings of ionized gas trapped in orbit around the earth.

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