When the earth's core melted, did the diameter of the earth increase?

  1. Solids tend to expand when heated or when they transition to a liquid. Is this thermal expansion a factor in the earth? If the diameter/volume increases, this would cause the surface area to increase also. This would lead to cracks, rifts or faults in the crust of the earth. Could this be a factor in the creation of the plates that we now experience in plate tectonics? I cannot find any information that mentions this. Also, if this could happen, could the change in volume be calculated via the coefficients of thermal expansion and the change in temperature?
  2. jcsd
  3. SteamKing

    SteamKing 10,972
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    Which do you think happened first: the earth's core became molten or the crust formed? What about the mantle (all the stuff between the core and the crust)?
  4. Not sure. My understanding is that the earth was a solid first as gravity pulled all the dust and debris into a solid earth. As radioactive decay of thorium and uranium occurred, the inside heated up and melted all the material inside the earth. The heavier elements like iron began to be drawn to the center to form the iron core. How the crust was affected, I really don't know. I was thinking the crust may have remained solid and acted as an insulator that trapped the heat in and allowed it to build up.
  5. davenn

    davenn 4,354
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    If you do a little searching on the www, you will find that the inner planets were all molten for a start and cooled down with time. The coalescing of the gasses and other materials that made up the planets involved a lot of heat. That is this material coming together was hot and was producing more heat as it collided.
    The outer surface cooling first forming the crust. The mantle remains "soft" due to the radioactive decay and also convective heat from the outer core.
    I did find one pdf of a paper that explained how there is a molten outer core and solid inner core.
    without going into it in depth (no pun intended) it has a lot to do with the transition of Iron between liquid and solid and the pressures and temperatures within the inner and outer core that allowed this to happen.

    Also consider that the origin of the Moon is believed to have been from the Earth when it was impacted with an object ~ the size of Mars at a time when the Earth was still primarily a large ball of hot molten material.

    Last edited: Jun 13, 2013
  6. davenn

    davenn 4,354
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    Ohhh and to specifically answer your thread title

    Since you have now discovered that the earth formed as a molten orb
    it means that as its cooled, it will have contracted, hence the diameter will have decreased.
    as what happens to most cooling objects

    By how much, I personally dont have the info/knowledge to work that out
    maybe some one else does

  7. Interesting thread, but I think the title sets us off on the wrong foot.

    The title starts off with the premise that "the core melted" -- implying that it existed in a solid state and then melted.

    That is plainly wrong to my understanding. Actually the Earth would have started off without a core. The Fe would have dripped down as liquid early in the Earth's formation. It would have formed initially as a liquid core and has now some 4.5 Ga later only partially solidified in the middle (to form an inner core).
  8. davenn

    davenn 4,354
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    yes this is what I commented on in my previous post :)

    Unfortunately the OP hasnt returned to confirm whether he has been helped in his understanding :(

  9. D H

    Staff: Mentor

    That isn't true. You can still find plenty of web pages that support the old-style view that terrestrial planets became molten only after those planets form. Whether the terrestrial planets were molten from the get go or somehow became hot enough after formation was a long-standing conundrum. The rationale for the latter view is simple: Basic physics dictates that collisions do not provide anywhere close to enough energy to yield temperatures sufficient to melt iron.

    On the other hand, the known planetary heating mechanisms, primarily radioactive decay of long-lived species plus a bit of extra help from gravitational collapse and collisions, would have required several hundreds of millions of years before the proto-Earth became molten. The problem with decay of long-lived species as the heating source is that extrapolating back in time yields a heating rate of that is only 4 to 5 times the rate that observed presently. This in turn yields that several hundreds of millions of years figure. There are a number of problems with this view. One problem is that this several hundred million year interval doesn't jibe with geological evidence of the young Earth. Another problem is to explain where all those iron-nickel meteorites came from.

    For several decades, renegade geologists and planetary physicists have proposed a way out of this conundrum: Heat supplied by radioactive species that are not quite so long-lived. The key suspects were 26Al and 60Fe, both of which have a half life that is on the order of a million years. Decay of these shorter-lived species explains where those iron-nickel meteorites came from. Am asteroid that was massive enough to have undergone differentiation could do so because the heat from those shorter-lived species led the asteroid to melt. It also explains how the proto-Earth melted so fast. The asteroid-sized chunks that collided to form the Earth were molten (and possibly already differentiated) at the times of the collisions.

    There's yet another problem here, however. Geology is a conservative science. Geologists don't like renegades (geology is a conservative science), they are not keen on outsiders poking into their science (e.g., Wegener and his continental drift), and they are not keen on magical explanations (e.g., Wegener's continental drift had no causative mechanism). Invoking heating via 26Al and 60Fe decay involves all three problems rolled into one. It was renegade insiders or outsiders poking into geology who proposed this heating mechanism, and it is magic. The half lives are short enough that neither one exists in the Earth.

    It's only been very recent that evidence has accumulated in favor of this explanation. With evidence, it is of course now science rather than magic.
  10. davenn

    davenn 4,354
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    Hi DH

    do you have some specific links ?

    When this thread was first started, I did some hour or so of googling and some interesting reading before I posted. I didn't see any that didn't support the theory that the Earth wasn't totally molten initially when it formed out of that proto-planetary dust and gas cloud along with the sun and other planets

    am always willing to learn :)

  11. Models clearly show that the Earth's mantle would have completely melted when an object the size of Mars smashed into it early in its history. This impact formed the moon -- or at least that is the leading scientific hypothesis. I agree with D H in that the Earth would not have started off molten. However, I do not know enough to comment on the short lived radio isotopes theory of melting. I suspect that any evidence for it has long since been lost in the geological record, and it may or may not have been an important mechanism in early planetary dynamics.
  12. This is perhaps an unfair generalisation. Geology may have started off as an old boys club but that was some ~300 years ago, which might explain its "stuffy" or "conservative" image to some.

    These days there are plenty of "outsiders" working alongside geologists in all frontiers. I would argue that geology is one of, if not the most, cross-disciplinary sciences there is. Now days that the old boy clubs are all but gone it can be hard to even identify an outsider in the midst.

    Name a science that is keen on "magical explanations".
  13. So the initial heat of the inner planets came from the friction of gas as the planet formed?
  14. D H

    Staff: Mentor


    The terrestrial planets formed initially from micron sized dust particles. These collided, in pairs, to eventually form centimeter sized objects. The next step is a bit murky, but these eventually built up to become meter-sized and eventually kilometer-sized planetesimals. Those planetesimals in turn collided to create ever larger objects, eventually becoming protoplanets, then moon-sized objects, and then a final stage of collisions between these very large objects to form planets.

    It's only that very final stage where collisions provide an immense amount lot of energy, and even that most likely wouldn't be to enough to melt a planet -- assuming the planet wasn't already molten. If the Earth had cooled to the point of a solid crust and mantle, the impact would have left a large portion of the Earth still solid. If the Earth was already close to solidus, that proto-Moon / proto-Earth collision would have completely melted a good chunk of the Earth.

    The subject of this thread is the Earth's core. When did it form? How did it form? The basics how the Earth's core formed is called planetary differentiation.

    One answer is found in iron-nickel meteorites. Where did they come from? One early model was that a Mars-sized planet in the asteroid belt was blown asunder by one of those large collisions. That doesn't work; dynamic simulations show that perturbations from Jupiter would have kept the region between Mars and Jupiter in such turmoil that a Mars-sized object couldn't have formed. Another sign is that Vesta, which some now think of as being a protoplanet rather than an asteroid, has an iron core. An obvious solution is that planetary differentiation occurs with objects much smaller than the Moon. An obvious problem with this is insufficient heat.

    And that's where shorter-lived radioactive species come into play. They solve the heat problem.


    C. B. Agnor et al. (1999) On the Character and Consequences of Large Impacts in the Late Stage of Terrestrial Planet Formation, Icarus 142, pp 219–237

    J. Blum and Gerhard Wurm (2008), The Growth Mechanisms of Macroscopic Bodies in Protoplanetary Disks, Annual Review of Astronomy and Astrophysics, 46, pp 21-56

    E. Gaidos et al. (2009)26Al and the formation of the solar system from a molecular cloud contaminated by Wolf-Rayet winds[/i], ApJ 696 1854

    P. J. Hevey and I. S. Sanders (2006), A model for planetesimal meltdown by 26Al and its implications for meteorite parent bodies, Meteoritics & Planetary Science, 41:1, pp 95–106

    I. D. Hutcheon and R. Hutchison, Evidence from the Semarkona ordinary chondrite for 26A1 heating of small planets, Nature, 337, pp 238-241

    N. Moskovitz and E. Gaidos (2011), Differentiation of Planetesimals and the Thermal Consequences of Melt Migration, Meteoritics & Planetary Science, 46:6, pp 903–918

    S. Mostefaoui et al. (2005), 60Fe: A Heat Source for Planetary Differentiation from a Nearby Supernova Explosion, ApJ 625 271

    V. S. Solomatov (2000), Fluid Dynamics of a Terrestrial Magma Ocean, in Origin of the Earth and Moon, University of Arizona Press

    A. Shukolyukov and G. W. Lugmair (1993), Live Iron-60 in the Early Solar System, Science, 259:5098 1138-1142, pp 1138-1142
  15. For a good review of "early earth differentiation" see:

    Walter, M., & Tronnes, R. (2004). Early Earth differentiation. Earth And Planetary Science Letters, 225(3-4), 253–269. doi:10.1016/j.epsl.2004.07.008

    DH will be happy that this paper puts quite a bit of emphasis on the importance of 26Al heat in core formation. Although doubt about the amount of 26Al available might still require an important role for impact heating among planetesimals.
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