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Name: Jeremy
Status: Student
Age: 11
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Question:
What are the purposes of each layer of the earth? Crust,mantle, outer core and inner core.



Replies:
What makes you think they have a purpose? I know it seems like most of what's in the world has a reason in the sense that things ``wouldn't work'' if they were otherwise. I mean, if a chicken's egg were as big as ostrich egg, the chicken couldn't lay it. If the air weren't here, we couldn't breathe, and so forth.
But this is an illusion. It happens because you with your imagination can imagine one part of the world changed without everything else changing too. So you can imagine what it would be like if there were no air on the planet, but there were still beings living here (us) who needed air. It seems marvelous, when you think about it this way, that there really is air here, and that it is just the kind we like to breathe, too, as opposed to the air on Jupiter or Venus, which would be poison to us.

But reality isn't like your imagination. It's much less flexible. (So you see that imagination is a really wonderful thing!) Everything in reality must change together to keep everything consistent and working together. The laws of physics really don't allow any other possibility. So it isn't really possible for air-breathing creatures to ever come to exist on a planet that has no air, or poisonous air. If there were no air on the Earth, we would not be here to be inconvenienced by that fact. Similarly, the chicken's egg is the exact right size because the chicken wouldn't exist to be bugged by a bigger or smaller egg.

Very little in the Universe has a purpose, because a purpose implies a choice, a possibility that things could be otherwise. And that is very rarely the case. Most everything about the Universe is the way it is because there is no other choice, nothing else is consistent with the basic laws of physics.

But this is far from what you want to know. Let me try to be more useful!!

The layers in the Earth arise for the same reason that the foam in your mug of root beer divides from the liquid. Substances of quite different densities (weight per fluid ounce) tend to separate when gravity is operating on them. Why? Well, imagine light stuff (feathers) mixed up with heavy stuff (lead). A bit of the light stuff is on the bottom. A little jiggle comes along, a passing truck vibrates the pile or something, and a piece of the lead falls down and boots the feathers out of place, and they pop up to the top of the pile. It could happen the other way, too, a bit of feathers dropping down and booting out a piece of lead, but -- here's the catch -- it's much less likely. You'd need a big chunk of feathers to weigh enough to boot out a piece of lead, and the lead would have to be rattling around in a hole that was big enough to accomodate the big chunk of feathers. Whereas, you only need a small piece of lead to displace any amount feathers, and the lead will *always* fit into the hole the feathers leave behind, if the feathers do not weigh more than the lead, which must be the case.

After a while, the process leads to all feathers on top, all lead below. All you really need is gravity (to give weight to the feathers and lead) and some random jiggling from the environment to keep things moving.

All right, the Earth consists of three separate types of materials: heavy metals like nickel and iron, and medium-weight rock (metal oxides) like basalts, and lightweight rocks like granite. Left to itself long enough, these three types of material will sort themselves out, with the heaviest -- the metals -- sinking down to the bottom to form the core, the medium-weight in the middle, and the light stuff at the top, forming the crust. Seems odd to consider something like granite as being light froth bubbling up to the top of the root beer mug, but that's how it is. On the scale of materials rock is not very heavy. It weights about 5 grams per cubic centimeter on average, I think, while metals tend to weight about two to three times as much. Try it, sometimes: hold equal size pieces of iron and rock in your hand, and see which seems heavier. Better yet, put them both on a balance and find out for sure!

Dr Grayce



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