Why is the Earth's Core so hot?

[steve_bham]

I know its not technically astro related, but with the recent volcano activity got me wondering why is the earth's core so hot?

Does anyone know?

Steve

[rwg]

Steve,

some of it is leftover heat from when the planet formed, but mostly it's down to energy released by radioactive decay of things like uranium 238 (and other long lived radio-isotopes).

There's an awful lot of 'stuff' down there, so even though the rate that heat is produced per cubic meter is pretty small, it all adds up to a lot of heat which has to slowly diffuse its way out to the surface.

Robin

[Paxo]

Yep, Robins right, mainly down to radioactive decay. Let's not forget also, the lack of a molten iron core has been the demise of the atmosphere on planets such as Mars due to bombardment and subsequent stripping by the solar wind which makes the subject very much astro related, so let us all be grateful we still have the molten core.

Steve..

[Astro_Baby]

I thought gravity played a part as well with the pull on Earth from the sun, the moon and other bodies compressing and stretching the planet.

[brianb]

I thought gravity played a part as well with the pull on Earth from the sun, the moon and other bodies compressing and stretching the planet.

Well it does ... but the tidal forces on the Earth are much smaller than those inflicted by Jupiter on Io ... in any case tidal forces act mostly on the crust & mantle, & the core is the hottest ... since long half life radioactive isotopes are mostly rather heavy, they will have largely settled out into the core - in some ways the "miracle" is that there is as much uranium, thorium etc. accessible to surface dwellers as there actually is.

The Earth contains quite sufficient U238 to explain the current core temperature. A rather deeper question is, why was there so much U238 in the dust cloud that collapsed to form the solar system? My understanding of the atomic physics associated with supernovae is that the proportion of U238 (and other heavy nuclei) made in type I & II supernovae should be a couple of orders of magnitude less than the observed occurrence, but there appears to be no other viable source for the stuff.

[skye at night]

The Earth is still (even after 4.5 bn years) releasing energy dating back to its accretion from dust, debris and gas as gravitational energy and also from the decay of radioactive elements. The core is probably solid (Fe) due to the density and despite the enormous (4500-5000 deg ) temperature...

The Iron/nickel? core combined with the overal mass of the planet and its rotation are 'probably' responsible for gravity...

The Moon influences our gravity to some extent and affects tides for example...

Volcanoes are not directly linked to the core, but are a result of the heat transfer through from core through mantle to crust via convection cells (massive ones) and crust flaws like 'hot spots' (thinner active point on ocean floors mainly) plate boundaries and rifts (MOR and Great African Rift Valley for example).

Iceland sits on the Atlantic Mid-Ocean -Ridge so is basically an Island splitting apart and growing through volcanic activity...

If you ever want to see a 'really' active volcano... go to Stromboli Island in Italy and get a local fishing boat or other to take you out there...

Erupts every 5 minutes...

Steve

[DarkerSky]

Radioactivity plays its part. But a principal source of heat today is the heat still emanating from the liquid iron core layer that surrounds the solid iron core at the centre of Earth.

Radioactivity in early earth caused the Earth to heat up to above the melting point of iron. Internally, Earth was a homogenous body at that point with iron spread throughout from core to surface.

Iron is the heaviest component of Earth, so when it melted it sank, under gravity to the core. This was the beginning of chemical zonation (differentiation), the process by which is thought to have raised Earth's temp by some 2000c, melting most of the planet.

4 billion years later and a large part of that iron core is still molten - still radiating heat that keeps what is above it in the Earth's vast mantle melted (although less so close to the crust).

Convection currents of molten/ semi-molten rock in the mantle it is thought drive plate tectonics at the surface, creatinig collision and spreading zones.

Iceland is on a spreading zone where the American and European plates are moving apart. This makes the surface weak along the line between the two (the north atlantic ridge) enabling magma to spew up from the molten mantle below.

[steve_bham]

is the very centre of our core not solid highly compressed iron?

[DarkerSky]

solid highly compressed iron it is...surrounded by a much larger liquid iron core at a depth of 2900 to 4980kms

[brianb]

solid highly compressed iron it is...surrounded by a much larger liquid iron core at a depth of 2900 to 4980kms

The iron is far from pure! Molten metals do not differentiate very easily, just as the salt in sea water does not seperate out & sink to the bottom of the ocean even though NaCl is denser than H20.

[vlebo]

I thought gravity played a part as well with the pull on Earth from the sun, the moon and other bodies compressing and stretching the planet.

Thats what I thought plus the earths rotation causing friction within the core.

[Earl]

As soon as anyone says Molten Core i hve nightmares of WoW session wiping on Ragnaros !

[Colan]

lol, a fellow WOW head :-)

[Joe105]

Nobody knows.

Hypotheses have been built up about it.Every so often a new one comes out.Some hypotheses sound quite convincing, others sound like they've come out of Dream Time. The answer ,however ,remains the same. We don;t really know. The hypotheses are built on assumptions.but thats all they are. Assumptions. No proof.

We don't really know what the Earth looks like over 5 miles down.

Most of our knowledge of things astronomical are the same.Hypotheses built on certain assumptions which are continuously changing.

Perhaps that is what makes astronomy so absolutely fascinating. There's so much more to discover.Nothing is set in stone and any one of us can observe and speculate and, who knows what future discoveries any one of us will make.

Thats whats good about being an amateur astronomer.We are free to make our own discoveries We are not bound by the rigid,'sacred' laws of establishment science.

[brianb]

We don't really know what the Earth looks like over 5 miles down.

Yes we do. The fact that we can't handle the material directly doesn't stop us measuring its properties by various techniques - seismology is the most powerful but by no means the only one. And we can handle material which was at one time hundreds of miles down, we see it in rocks which have been brought to the surface, we know they've been subjected to the sort of pressures & temperatures that can only occur deep in the planet's mantle by the minerology.

[astromerlin]

Nobody knows.

Hypotheses have been built up about it.Every so often a new one comes out.Some hypotheses sound quite convincing, others sound like they've come out of Dream Time.

The answer is simple! The difference between the core's rotation and the earth's crust rotation creates a dynamo effect which creates the earth's magnetic field this also causes heating of the planet. The core is solid due the the great pressures down there and so it cannot turn into a liquid, further out from the core the pressures are less and at some point there is a highly viscous boundary layer before you get to the liquid core.

[acey]

In the 19th century Lord Kelvin calculated the age of the earth on the assumption that it had started as a molten body and been cooling ever since. He got a figure of about 100 million years. In the 1890s John Perry made the model more realistic by allowing for convection, but that still only gave a figure of a little over 2 billion years. Then when radioactivity was discovered it was realised that this provided an extra source of heat, and the reason why our planet still has a molten core, 4.5 billion years after first forming.

[ollypenrice]

'Most of our knowledge of things astronomical are the same.Hypotheses built on certain assumptions which are continuously changing.'

I think you are maybe overstating the case here, Joe? The role of hypothesis is preliminary. The hypothesis must make testable predictions, which must be validated by rigorous observation, before anyone would consider giving them the title of 'theory.' There are indeed assumptions but these are identified in advance and I'm not aware of too many that are changing. (Cosmology features quite a few, yes, like the speed of light remaining constant over time, the Universe being homogenous and isotropic, but in astronomy proper there are not so many, are there?) Jocelyn Bell said recently, though, that we spend insufficient time thinking about the way these initial hypotheses are created and too much time on understanding what follows. She has a point, I think.

What amazes me is that so often quite independent observations give similar results or produce slightly different results allowing new hypotheses to be produced. (Eg the redshift galactic distances are not quite the same as the Tully-Fisher galactic distances but the differences are systematic and suggest something new, the presence of 'great attractors.' But I certainly don't think our theories are set in stone. I just disagree on a question of degree, I think, to phrase it rather oddly!!

Olly

PS Acey makes a good point above but Kelvin's calculation was known at the time to have created a 'problem' because there was a conflict with the geologists. Such conflicts are relished by scientists because they are the starting points for new research. The refusal of general relativity to give an inch is, therefore, a problem for physics rather than a solution.

It was the same for Maxwell's equations, which were so nice that Einstein preferred to throw absolute time and absolute space aside in order to save them.

Great, isn't it!