Gradual cooling of the earth

A minor point in an otherwise largely accurate description. 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)

You got me, a definite error on my part. Oh well, make mistakes, even dumb ones, but fess up quick.As to sources you could try the wiki on Structure of the Earth, my original source is from memory of Physics and Geology by Jacobs, Russell and Wilson, the textbook for general geophysics at New Mexico Tech back in 1980. Don't think the physics or structure has changed much since.I think some confusion may occur because the mantle is referred to as viscous in the wiki article, however that is not synonymous with liquid and does not act that way according to physics.{ABE} Actually managed to get a D in the class, Schlue had high standards and graded on a strict Bell Curve, took it too early, was in Calculus 3 at the same time, competition already took electrical physics and burned Maxwell into the ground. Lowest grade carried until a few years later when managed to flunk structural engineering at UNM (how embarrassing, missed last day to drop, please don't ask me to design a bridge). But I feel I remember more about that one than many I made an A in later.{/ABE}Edited by anglagard, : Just too chatty, past bedtime