# Why did the Earth cool down?

In physics text books, it has only stated that Earth cooled down and so, water vapour condense and other life form develop and et cetera. But, why did Earth cooled down in the first place? What caused it to cool down?

In addition, when the Earth cooled down, did the water vapour condense to possible 'raw' magma/lava and then, creating the tectonic plates?

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I think, the answer to the first question shouldn't be anything but electromagnetic radiation. –  Ali Aug 12 '13 at 23:21
A hot earth radiates heat into space. If I catch the direction of the second part of your question correctly, it actually may be more the more interesting part: What was the history of The Early Earth and Plate Tectonics –  Mark Rovetta Aug 13 '13 at 2:13
By what reaction do you expect water vapor to transform to magma or lava ..? If it cools, it condenses to water and can get frozen to ice, but these other stats still consist of H2O ... The last sentence in this question makes no sense. –  Dilaton Aug 13 '13 at 10:40
Sorry, I actually meant that if, based on my memory of the text book, the young earth was 'covered' with volcanoes, or just molten rocks,when the substances that the volcanoes erupted I.e. the water vapour in this case, condense, did the water vapour caused the tectonic plates to form by cooling in great temperature change (water vapour condense on lava)? Or just possible slow cooling along the Earth and the condensation was way after the formation of the tectonic plates? –  Shaoyen Aug 14 '13 at 2:41

Space does have a little bit of heat, since it's filled with the cosmic microwave background, which is thermal radiation at around $3\,\mathrm{K}\approx -270^{\circ}\mathrm{C}$. You are right that this is extremely cold, however. –  Nathaniel Aug 13 '13 at 8:15