Ask A Scientist© Environmental Earth Science Archive

 
 >   name        jeremy
 >   status      student
 >   age         11

 >   Question -  What are the purposes of each layer of the earth?
 > Crust,mantle, outer core and inner core.

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    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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