MadSci Network: Earth Sciences

Re: If the core of the planet is so hot, then why doesn't the earth turn into

Date: Tue Dec 11 18:44:32 2001
Posted By: David Scarboro, Faculty, Earth Sciences, The Open University
Area of science: Earth Sciences
ID: 1006464659.Es

Dear Jennifer,

The Earth is very hot inside, as you rightly say, and the temperature 
rises the deeper you go.  In the outer core of the Earth, which is mostly 
made of liquid iron and nickel, the temperature is estimated to be about 
8,000° C.  Hot as this is, however, it does not come close to the 
temperatures reached inside stars.  The temperature at the core of the Sun 
is estimated to be about 15,000,000° C.

A point in your question that needs to be defined is what you mean by 
a “fireball”.  Fire implies combustion, which is the chemical combination 
of oxygen with another material, as happens when wood burns.  In this 
sense the Sun cannot be described as a fireball because its heat is a 
result not of combustion but of nuclear fusion in its core, in which 
hydrogen atoms are fused to form helium atoms.  The Earth is not like the 
Sun because it is not made primarily of hydrogen and its heat comes not 
from nuclear fusion but from nuclear fission, in which radioactive 
elements inside the Earth, such as uranium and thorium, decay, releasing 
heat in the process.  You will notice that nuclear fission in the Earth is 
not combustion any more than nuclear fusion inside the Sun is, for oxygen 
is not involved.

Therefore, one answer to your question is that no planet or star can 
accurately be described as a fireball because they do not burn via 
combustion, no matter how hot they become.

Another way to answer your question is to see how the materials of which 
the Earth is made behave when exposed to high temperatures.  I think that 
you might, therefore, rephrase your question like this:  “If the Earth is 
so hot inside, why doesn’t it melt?”  The answer is that the behaviour of 
any material, including rocks inside the Earth, is controlled not only by 
temperature but also by pressure.  The Earth is a rocky planet.  It is 
composed primarily of minerals rich in silica making up the crust and 
mantle, and primarily of an alloy of iron and nickel in the core.  The 
combined conditions of temperature and pressure in the outer core are such 
that the iron/nickel alloy is liquid.  In the inner core, however, even 
though the temperature is actually higher than in the outer core, the 
pressure is also greater, and the combined temperature/pressure conditions 
are such that the iron/nickel alloy is solid!

At shallower depths, in the mantle especially, conditions of temperature 
and pressure are not able to melt the rocks except in some regions where 
magmas form.  Overall, the mantle is hot enough and under such great 
pressure that the rocks behave like warm plastic, and are able to flow 
slowly.  This allows the mantle to convect very slowly, like water boiling 
in a saucepan, and it is this motion in the mantle that drives the 
movement of the crustal plates above (plate tectonics).

So, this answer to your question is that the conditions of temperature and 
pressure inside the Earth’s mantle are, in some limited regions, right for 
the rock to melt, and that conditions in the outer core are right for the 
iron/nickel alloy there to be liquid, but that in most of the mantle and 
in the inner core conditions do not permit melting.

A final point, however, is that if you could travel back in time to the 
very early history of the Earth more than 4.5 billion years ago, 
scientists think that the Earth went through a phase when it consisted of 
what is called a “magma ocean”.  In this phase the entire Earth was molten 
because the heat released by the process of accretion under gravity of the 
materials that formed the Earth was so great that the temperatures 
throughout the Earth were hot enough to melt everything!  In this phase of 
its history, I would think any passing aliens might have thought the Earth 
looked like a fireball!  Fortunately, enough time has elapsed since the 
Earth was a magma ocean for it to cool down and (mostly) solidify.

I hope this answers your question.

Best wishes,

David Scarboro

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