science question

[HelPick2]

How did earth core remain very hot at core for billions of years?

[Zep]

Sssssshhh! It's a secret! The earth is hollow, and we're living inside it! :D

[HelPick2]

Originally posted by Zep
Sssssshhh! It's a secret! The earth is hollow, and we're living inside it! :D
the earth is not hollow. this can be seen from the amount of gravity we experience and volcano.

[exarch]

Originally posted by Ixabert
How did earth core remain very hot at core for billions of years?Pressure.

[HelPick2]

Originally posted by exarch
Pressure.
that does not explain anything to me. i need more detailed explanation please. Always try to explain what you say. Thank you.

[Zep]

Originally posted by Ixabert
the earth is not hollow. this can be seen from the amount of gravity we experience and volcano. Sssssh! That's just Hollywood special effects! Don't spoil the surprise for the other folks!

[HelPick2]

okay, someone who knows the answer to my question please answer my question.

[drkitten]

Originally posted by Ixabert
okay, someone who knows the answer to my question please answer my question.

Whenever fluids are under pressure, they get warmer. You can see this yourself in a bicycle tire if you want to experiment; just pump a lot of air into the tire and feel how warm it is. Space (vacuum) is actually a pretty good insulator, as is the upper surfaces of the earth, so the amount of heat that leaks away from the pressurized core is relatively small. There's enough radioactive material in the earth (uranium and such) to compensate for most of the heat leakage.

[richardm]

Originally posted by Ixabert
How did earth core remain very hot at core for billions of years?

Is there a particular reason you're asking this, BTW? Just curiosity, or has someone been suggesting a Young Earth to you?

[HelPick2]

Originally posted by drkitten
Whenever fluids are under pressure, they get warmer. You can see this yourself in a bicycle tire if you want to experiment; just pump a lot of air into the tire and feel how warm it is. Space (vacuum) is actually a pretty good insulator, as is the upper surfaces of the earth, so the amount of heat that leaks away from the pressurized core is relatively small. There's enough radioactive material in the earth (uranium and such) to compensate for most of the heat leakage.
Thank you very much for the answer. now it makes sense to me. Before, i was just thinking a few hours ago, how it doesn't make sense for the heat to stay there for millions of years; now things are starting to make a lot more sense to me, and I now have to study this further.
Is there a particular reason you're asking this, BTW? Just curiosity, or has someone been suggesting a Young Earth to you?
The reason? curiosity. I do not know much about the world, and have a lot of questions about the world.

[Jeff Corey]

The earth's core contains radioactive elements, like uranium and thorium. As they decay, heat is generated.

[Sloe_Bohemian]

Correct me if I'm wrong, but the genesis of the process is simply gravity. All molecules produce a small level of gravitational pull... so that in a real sense... as I hold my hand above a hammer on the table... there is a gravitational pull between my hand and the hammer. But the great mass of the earth (spherical just because of this very reason) and the musculature of my arm prevent each of these objects from moving towards each other. Well, that and the fact that the mass involved between my hand and a hammer is so minute, that it has little, to no, effect.

With the gravity field created by the mass of material which collected into our planet, the material in the center is crushed under the "weight" of the upper material. This pressure and the subsequent heat has melted solid rock (magma). And if I remember right, the pressure gets so great that liquid material is actually pressed back into solid matter despite the great temperatures... at the deepest levels in the core.

Or am I way off on all of this?

[davefoc]

ixabert, welcome to the forum.

It seems like your question has been answered to a degree.

The heat in the interior of the earth comes from the originial heat when the earth first accreted plus heat from radioactive decay.

See a this link from a Wikipedia article for a little information on this:

http://en.wikipedia.org/wiki/Earth

Estimates have been made of the total heat flow out of the Earth. The source of this heat is principally the vestigial heat from the initial compression and the radioactive heating.

I found a source that estimates the heat flow out of the earth as 3-4 x 10^13 watts.

http://mines.unr.edu/geology/Home/faculty_pages/Geol333/heat%20flowslides.pdf

I tried to find a source that would provide an estimate of the total heat of the earth so I could get a feel for how long it would last if there wasn't any extra heat added from radioactive decay. I didn't find one so I made a rough estimate myself.

Using 3.5 x 10^13 watts for Earth's outward heatflow yields about a heat loss of 1.1 x 10^21 Joules per year.

Using 5.9 x 10^24 Kg for the mass of the earth.
Using .84 x 10^3 Joules per Kg per degree as the specific heat of Bassault
Source: http://www.engineeringtoolbox.com/24_154.html
Assuming the entire earth was made of Bassault
Using 5720 degrees C as the core temperature of the earth
Assuming the average surface temperature of the Earth was about 20 degrees C

I came up with a rough estimate of 5.9 x 10^30 Joules for the total heat of the earth above its surface temperature.

This suggests that there was plenty of heat from the initial formation of the earth to keep the core hot for billions of years.

My calculations were rough and I would like to find a source where somebody has done them more precisely so that it was possible to get a better idea of how much of the heat is coming from the vestigial heat and how much is coming from radioactive decay. I think though there is a lot of rough guessing that needs to be done with this calculation, even by somebody that knows what they are doing so that a good estimate of the ratio of earth's heatflow coming from radioactive decay and from vestigial heat may not exist.

[CBL4]

I would to add a historical note for skeptics who may be unaware of it.

Lord Kelvin (William Thomson) argued that the earth could not be old enough for Darwin's theory of evolution because the earth would have gone cold in a million years or so. This was a particularly vexing argument for Darwin and other evolutionists. His arguments were more or less sound according to the physics of the day.

Of course, once radioactivity was discovered, his argument fell apart. This does not prevent creationists from recycling his completely disproven arguments.

Bio of Kelvin (http://octopus.phy.bg.ac.yu/web_projects/giants/kelvin.html)

CBL

[pupdog]

How much heat is added by friction of liquid outer core (flow creating our magnetic field) and movement of material in the mantle?

[davefoc]

pupdog, I think the answer to your question is that source of energy for making the liquid core move is the vestigial heat and the heat from radioactive decay. So for the most part the friction from the movement in the liquid core doesn't add more heat to the interior of the earth. (see addition below)

But, the rotating earth and the moons gravity probably affects the motion of the liquid core and thereby adds some additional heat energy from friction heating.

I've heard about this kind of heating for a Jupiter moon that orbits close to Jupiter, but there must be at least some heat generation because of this in the earth. I've never seen it discussed and I'm not sure I could calculate even a ballpark figure for howmuch energy is generated within the earth because of it.

Maybe there's something on the web about this.

edited to add:
What I was trying to say there was that while of course friction from the movement of the liquid core produces heat, the energy to drive the motion came from the heat of the earth and when the motion is slowed by friction the earth doesn't have a net energy gain. All that happens is kinetic energy is transferred back to heat energy.

But where the kinetic energy is created by tidal forces there is a net energy increase in the earth's interior.

[Dragon]

I was going to mention Kelvin but CBL4 beat me to it - I'll add this link though, from Talk Origins (http://www.talkorigins.org/faqs/geohist.html)(excellent site!).
The use of Kelvin's argument for a (relatively) young Earth by creationists is probably why the first couple of replies to your OP were a tad facetious or terse.

[Dancing David]

Originally posted by Jeff Corey
The earth's core contains radioactive elements, like uranium and thorium. As they decay, heat is generated.


Bingo! In fact Lord Kelvin determined the age of the earth to be forty thousand years soley from the idea of heat generated by gravitational collapse. He firgured that all the heat would disipate withing forty thousand years, so that must be the oldest the earth could be.

The confounding age of the earth was not accounted for until the effect that Mr. Corey states. There would also be a very small amount of heat generated by the tidal bending of the earth by the moon.

[davefoc]

I looked for a better calculation of the total heat stored in the earth's interior.

I found many references to Lord Kelvins calculation of the earth's age as 20-40 million years. Unfortunately I couldn't find any place where they actually showed the calculation so I couldn't determine why my estimate of the age of the earth based on this method is so much larger than Kelvin's.

I rechecked my calculation and couldn't see any obvious arithmetic errors. If I had assumed the earth was made entirely of iron the estimate of the total earth heat capacity would be reduced by a factor of 2 (specific heat of iron=.45 and specific heat of bassault=.84 kJ/kG*degC).

[Matabiri]

Originally posted by drkitten
Whenever fluids are under pressure, they get warmer. You can see this yourself in a bicycle tire if you want to experiment; just pump a lot of air into the tire and feel how warm it is.

To be pedantic, whenever fluids are pressurised they get warmer. The energy used to compress them gets dissipated as heat. Merely being under pressure is not enough to make something hot...

[tracer]

Originally posted by davefoc
I found many references to Lord Kelvins calculation of the earth's age as 20-40 million years.
Although conventional wisdom has it that Kelvin was off because there were radioactive materials at the center of the Earth heating it, the story I've heard was that it was radioactive materials IN THE EARTH'S CRUST which were giving it a high temperature NEAR THE SURFACE, and this is what threw Kelvin's calculations off.