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Molten Earth

  1. Dec 21, 2007 #1
    Hi,

    I'm not good enough to contribute answers yet, but I hope to entertain you with questions.

    Practically all the earth formation stuff I've read talks about the earth being molton. That always seems strange to me. If I think of a space body without an atmosphere, I generally think of it being damm cold. If I think of a proto-earth gathering up rocks and dust thru aggregation, then I think of this space dust being freezing cold grains of sand.

    I guess what I'm missing is a clearer picture of the dust cloud that was probably the origin of our planet. Was this dust cloud super hot? Where there molten bits of sand and iron floating around in space raining down on proto-earth? I dunno. I just think a huge dusty "asteroid" looking space body is easier to imagine than this fire and brimstone stuff. Sure, when the planet gets big enough, then the silicates etc will turn molten under the pressure from above. But maybe, there was always a dust crust on earth as we vacuumed up everything in our near orbit.

    Thanks for any help.

    conan
     
  2. jcsd
  3. Dec 21, 2007 #2

    russ_watters

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    The dust could doesn't have to be hot - the act of collapsing converts potential energy to kinetic energy.
     
  4. Dec 21, 2007 #3

    DaveC426913

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    I'm pretty sure that, as dust and gas coalesces into a proto-solar system, its temperature goes up substantially. Likewise, gas clouds sprinkled around the galaxy do this when put under pressure from ,say, a nearby exploding star.

    THe dust is only freezing once a system is gratly thinned out, like ours is now. Our SS is virtually vacuum compared to what it was like while forming.
     
  5. Jan 4, 2008 #4
    Thanks for the answer Dave. If I can think of a contradictory example, then it's Saturn's rings and moons. This is part of the solar system that is not vacuumed out and contains moons and chunks of debris circling amongst the fine particles and these objects are anything but hot. Perhaps the density is not as high as it was in the days of planet formation, but there's a decent amount of stuff there.
     
  6. Jan 4, 2008 #5
    If the earth was indeed molten it may not have lasted that long; considering that the oldest known mineral is nearly 4.5 billion years old, versus the Earth 4.56 billion years.
     
  7. Jan 4, 2008 #6

    mgb_phys

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    As Russ and Dave said, the gravitiational energy as the stuff collapses is enough to melt the proto Earth, another example would be the sun - the much greater mass made this a lot hotter.
    The crust would have cooled very quickly since it is exposed to the cold of space and is a poor conductor so doesn't receive much of the heat of the molten core.
    The 4.5 BYr rock is interesting - when I learnt this stuff it was thought that the Earth had completely remelted after 500 MYr due to radioactive decay products. That melting must not have been quite so complete.

    Saturn's rings don't coellesce into the planet for the same reason that all the planets don't fall into the sun - angular momentum. The planets formed from an gas/dust cloud that had enough particles moving in different random directions to allow collisions to dump enough angular momentum to allow them to collapse.
     
  8. Jan 4, 2008 #7

    chemisttree

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    Google 'Iron Catastrophy".
     
  9. Jan 4, 2008 #8

    mgb_phys

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    Never understood why it is called that - it was a GOOD thing not a catastrophe !
     
  10. Jan 7, 2008 #9
    Hi mgb,

    thanks for the answer, but perhaps I need to know more about gravitational collapse, at least as far as rocky planets go. There are still things that I can imagine, and stuff that I cant. I can imagine some asteroid or comet type rock ploughing thru zillions of smaller objects, growing like a giant dustball. I can imagine that all turning into a lopsided mess, then getting rounder etc. However, I can only imagine the collapse as gradual, a continual shifting of material towards the centre, making the planet rounder. I can't imagine a sudden cataclysmic collapse, where a couple of rocks shift and the whole dustball suddenly collapses in on itself resulting in a burning fury. I can imagine that after a certain amount of time, the weight and pressure from above causes a molten core on the inside, but the outside, that would just keep attracting dust.

    Perhaps the gas cloud from which we all originated was super hot at the time. Maybe all the dust and crap that got stuck on the proto-earth was boiling hot to start with. I find that easier to believe than a gravitational collapse as a cause of earth being molten.

    Regarding Saturn and its ring. The argument was about whether it will coalesce or not, it was an example of a relatively large body going thru a non-vacuumed part of space and not being boiling hot.

    regards

    conan
     
  11. Jan 7, 2008 #10

    mgb_phys

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    The intial collapse was gradual (it took 0.5billion years) and the minerals were melted by the heat and pressure into a solid ball. Heat then generated by radioactive elements melted the core which, because the outer mantle and crust are good insulators is still molten.
    Actually the core isn't really a liquid - it flows like ice in a glacier - imagine a very thick treacle, or a soft metal washer squashing when you tighten it.

    Parts of the crust were also remelted by the impacts of asteroids - in the early solar system there were alot of them, one impact was big enough to break off the moon, but these aren't a significant source of the energy to create a planet.
     
  12. Jan 9, 2008 #11
    Hi mgb,

    Your description seems to fit my picture of the early earth well. A dustball accumulating crap until it starts very slowly to gravitaitionally collapse in on itself. My problem with the all the classic pictures of early earth was that is viewed as a molten hell. You describe that the minerals were melted by heat and pressure. However the surface of the of the proto-earth super dustball would've not been subjected to pressure. This layer, however thin, would've been cold. Certainly a gradual collapse over 0.5 billion years isnt going to create a whole lot of heat on the surface thru friction.

    Unless of course there were other factors that I'm missing out on. For example, the gas cloud that we came from was superhot. Or perhaps there was a ton of atmospheric gas, much, much more than now, pressing down causing enough pressure to melt the minerals. But in the occasional reading that I've done, none of this is mentioned. It's all just, "the earth was a molten burning hell that got so hot that the iron melted and sunk to the middle".

    I have no problem believing that after proto-earth reached a certain size the middle of the earth became a molten treacle and has remained so ever since. I have no problem with lots of asteroids banging to the early earth either, seems logical. And that they hit with such an impact to mix up the 'pudding', no problem either. My problem is imagining that the earths crust was molten, as is often pictured and described. My specific problem is that this early earth had to cool down before offering conditions suitable for life as we know it. It doesnt seem logical that the entire early dustball suddenly went totally molten and lost any crusty bits on the outside.

    Perhaps I should've clarifed that a bit more.

    So what does everyone think/suppose? Was the earth landscape as seen from surface a molten brimstone hell, or was it similar to most solar bodies now, an uneven potmarked but firm landscape (that was possibly quite cold)?
     
  13. Jan 9, 2008 #12

    DaveC426913

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    The Solar System of yesteryear is not like today. That's the big mistake.

    It was full of hot gas and dust, which had been heated as it contracted around the sun. (Even today, as we look at nebulae where new stars are forming, we can see how the gas is superheated by coalescence).

    ProtoEarth was not cold because it was never left alone. It was incessantly pelted by dust, pebbles, rocks, boulders, comets, meteors and protoplanetoids with such frequency in the very dense early solar system that its was constantly being returned to a molten state. Look at the Moon to see vast areas (called Mare, which means "sea") where giant meteors punched holes in it and turned large areas of its surface to molten craters. Magma welled up and filled in the craters, creating these vast, very smooth seas which then solidified. Earth's surface looked like the Moon's surface before weather erosion wiped it clean.
     
  14. Jan 11, 2008 #13

    chemisttree

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    It also contained much higher concentrations of highly radioactive elements which heated the protoplanet dustball sufficient to melt the iron which fell into the interior of the planet. The release of this potential energy and its conversion into heat was sufficient to finish melting the entire mass of the earth. The total energy input into the protoplanet included heat from the sun, meteoric collisions, radioactive decay and finally collapse of the iron into the core. The final straw was the iron melting and migrating to the core. This is what is referred to as the Iron Catastrophy.
     
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