A minor point in an otherwise largely accurate description.
Rahvin writes:
Most of the Earth is still molten, and it's far from homogenous.
Hate to disagree, but I suppose there is a first time for everything.
Most of the Earth is solid. The structure from top to bottom is crust, solid mantle, a plastic layer in the mantle around 20-50km down but only a few km thick. 1800 more miles of solid mantle, a liquid outer core and a solid inner core. The mantle makes up the majority of the Earth by both volume and mass and it is solid.
How is this known? Geophysics. Disturbances such as earthquakes generate four types of waves. Rayleigh, Love, Shear and Pressure. For purposes of this discussion, we will only cover what is important, Shear and Pressure waves. Now in physics it is shown that a shear wave will not propagate through a liquid while a pressure wave has no problem. However a liquid can bend a pressure wave, due to its different properties. When there is an earthquake in opposite points on Earth, for instance Sumatra and California, the shear wave leaves a blank spot, hence the outer core. The pressure wave propagates through the mantle, with a bit of diminution at the plastic layer, throughout the mantle. However, due to the interface between the inner and outer core. there is a bit of a bounce in the pressure wave, hence the inner core is deduced from the data.
Hope this helps.
Edited by anglagard, : Corrected error using solid when meant liquid, see next post by Moose. (need to go to bed, not post)
Read not to contradict and confute, not to believe and take for granted, not to find talk and discourse, but to weigh and consider. - Francis Bacon