What are earthquakes useful for?
Well, despite the death and destruction they
cause on the earth's surface, they have helped
us figure out that the Earth has a core.
Using measurements from all round the world,
scientists could tell that something was stopping
the shockwaves produced by a big earthquake
from travelling straight through the earth
from one side to the other.
So although we have never been more than 11
kilometres below the earth's surface, the
measurements showed that the inside of the
earth was divided up into different spheres.
The crust, the bit we live on, is about seventy
kilometers deep at its thickest.
Then there is a very thick layer called the
mantle.
This is under high pressure and is very hot
so the rock flows like a very sticky liquid.
The mantle is up to two thousand nine hundred
kilometres thick.
Then there is the core -- actually the inner
core, which is solid and then, between the
inner core and the mantle, the outer core.
The inner core is a ball that's a bit smaller
than the moon.
It is made of solid iron and nickel and it
is VERY HOT!
Scientists estimate it is as hot as the surface
of the Sun -- maybe a bit hotter.
Probably the whole core used to be liquid
once, but the inner core is the part of the
core that has cooled down to a solid over
the billions of years since the Earth was
formed.
That would mean the inner core must be slowly
expanding as the outer core cools down, too.
The outer core is not sold like the inner
core, but liquid.
It is about two thousand two hundred kilometres
thick and lies between the mantle and the
inner core.
It is also extremely hot -- over six thousand
degrees centigrade near the inner core.
As the earth spins, the inner core and the
outer core move at different speeds because
one is solid and one is more liquid, and this
results in eddies forming within the outer
core.
This produces a massive magnetic field that
surrounds the Earth and protects it.
If it wasn't for this magnetic field, the
earth's atmosphere would get stripped away
by the solar wind.
Scientists suspect that this is what happened
to Mars -- its core cooled down, stopped generating
a magnetic field and the planet gradually
lost its atmosphere.
The crust is a very thin layer and is made
up of sections called plates.
These plates are very slowly moving because
of interactions between the mantle and the
crust.
Beneath the crust the top layer of the mantle
is also solid and together the crust and this
sold layer are known as the lithosphere, from
the Greek for 'rocky' -- lithos.
The next layer of the mantle is called the
asthenosphere, from the Greek for 'weak' -- aesthenes.
The asthenosphere is liquid and flows, rather
than being solid and rigid like the lithosphere.
There are enormous convection cells in the
asthenosphere which slowly circle round and
round.
The rigid plates sitting on top of the asthenosphere
move along with it, too.
When these convection cells mean plates are
forced against each other, earthquakes result
from the huge pressures involved.
