|If the outer core is melted metal it would have
to be hot enough for the metal to melt of course.
If the inner core is also made of the same
materials as the outer core than why does the
inner core not melt as well? If the inner core is
completely surrounded by melted metal than it
should be breaking down as well shouldn't it?
What causes the inner core to stay solid?
|Question Date: 2003-10-28|
The short answer to that question is: Pressure.
The inner and outer core are made of the same
material, chiefly the metal Iron, with small
amounts of Nickel and some other lighter
However, geologists have discovered
through a variety of experiments that as the
pressure on a solid material increases, its
melting temperature increases as well. In the
earth, pressure increases with depth below the
surface of the planet.
So for example rocks
that are molten at the surface of the earth (lava)
at a certain temperature, would be solid at the
same temperature at a greater depth below the
surface where pressure is greater. In the outer
or liquid core the temperature of the Iron is
great enough and the pressure not quite high
enough for it to exist in a liquid state. As you
increase the depth (and therefore the pressure) to
the inner core, the pressure becomes so great that
the melting point for iron is greater than the
temperature of the core, and the iron is forced
into a solid state.
The short answer: pressure.
temperature of any material (as well as the
boiling temperature of any liquid) changes with
the pressure under which it is put, and that
change reflects the change in density of the
substance in question. Water is unusual in that
the solid form(ice) is actually less dense than
the liquid form, so if you put ice under enough
pressure, it will melt.Most substances, including
the nickel and iron that the core is made out of,
are denser when solid, and so putting the liquid
under enough pressure will cause it to 'freeze'
(turn solid, that is - it will actually get even
hotter by releasing the energy that was keeping it
The outer core is under pressure
of about three thousand kilometers of rock, the
Earth's mantle. The inner core is under that
pressure, as well as another two thousand
kilometers of molten metal (the outercore). That
difference is enough to freeze the innercore. If
you took a piece of the inner core to the surface
of the Earth, it wouldn't be liquid or even gas -
it would be a plasma, since the temperature(8000
degrees C - hotter than the surface of the sun)is
This is a great question.
The melting point of
a solid depends on PRESSURE .
To give you an
example from volcanology, to melt a block of now
frozen basalt requires heating the rock up to
about 1200 deg C at the EARTHS surface (pressure
But if we buried this same
rock under 30 km of rock, where the pressure is
10,000 atmospheres, we would have to heat the rock
up to 1250 deg C to melt it.
The same is true
of iron. The outer core of the Earth is
liquid...but as one dives deeper and deeper within
the core the pressure increases. Finally a
pressure is reached such that the ambient
temperature is LESS than the freezing point...and
so the iron liquid crystallizes to become a
The melting points of MOST substances
increases as pressure goes up. One notable
exception is water.
From about 1 atmosphere
pressure up to several hundred atmosphere pressure
the melting point of ice actually DECREASES as
pressure goes up!.
This is very important to a
hockey player!That is, when someone is on skates,
the metal blade of the skate increases the
pressure on the ice right beneath the blade. This
LOWERS the melting point of the ice immediately
beneath the blade and that makes some liquid
water. This makes the ice SLIPPERY. Then as the
blade passes by, the pressure is released and the
film of water laying on top of the ice beneath
where the blade passed refreezes!
materials do not show this anomalous behavior and
in fact have melting points that INCREASE with
increasing pressure...iron is such an example.
Click Here to return to the search form.
Copyright © 2017 The Regents of the University of California,
All Rights Reserved.