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How do the continents float?

  1. Feb 18, 2015 #1
    Given that both the continents and the oceans are "floating" on magma, and the continents are much heavier, why doesn't the pressure created by the continents force the ocean floor upward?
     
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  3. Feb 18, 2015 #2

    Orodruin

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    What makes you think the continents are much heavier than the inner parts of the Earth?
     
  4. Feb 18, 2015 #3
    No, heavier than the oceans.
     
  5. Feb 18, 2015 #4
    What makes you think the continents and the oceans are floating on magma?

    The oceanic lithosphere is denser than the continental lithosphere, hence it is lower.
     
  6. Feb 18, 2015 #5

    Bystander

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    It does. Continental plates are thicker than oceanic plates; oceanic plates are denser than continental plates.
     
  7. Feb 18, 2015 #6
    Thank you; can you provide more details?
     
  8. Feb 18, 2015 #7

    Bystander

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    Can you constrain your question --- there are libraries full of information on the topic.
     
  9. Feb 18, 2015 #8

    D H

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    Continental crust is considerably less dense than the upper mantle. Oceanic crust is only slightly less dense than the upper mantle. In fact, at subduction zones, the older, cooler oceanic crust is more dense than asthenosphere rock. That's why it subducts.

    As an aside, neither one is floating on magma. They are instead floating on the somewhat ductile but still solid upper mantle. There is no ocean of magma underneath us. Magma is instead localized. It's found at divergent plate boundaries (mid-ocean ridges) and convergent plate boundaries (subduction zones), and hot spots.
     
  10. Feb 18, 2015 #9
    So the overall crust mass per square mile, including the water of the oceans, is approximately constant over the earth's surface?
    How does this occur? what causes continental crust to be thick and light?
     
  11. Feb 18, 2015 #10

    D H

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    It doesn't. You're hung up on the oceans. Forget the oceans.

    By way of analogy, suppose you toss some thick chunks of cork and thin chunks of oak in a swimming pool. Because the chunks of cork are thicker and less dense than the oak, the tops of the chunks of cork will be higher than the tops of the chunks of oak.

    Continental crust is thicker and less dense than is oceanic crust. Those chunks of cork represent continental crust and the chunks of oak, oceanic crust.

    Water causes the crust to be less dense. The rock carried down into the Earth at a subduction zone is heavily saturated with water. This water is squeezed out down inside the Earth. Most of it escapes as steam in a volcano, but some of it changes the chemistry of the rock. The result is granitic rock. The lava emerging along the rim of fire is mostly granitic while the lava emerging at the mid-ocean ridges is basaltic.
     
    Last edited by a moderator: Feb 22, 2015
  12. Feb 18, 2015 #11
    Is the lower boundary of the crust at the same height for continental & oceanic crust?
    Does the lightness of continental crust also explain why it's thicker?
    Thanks alot, I'm learning alot here!
     
  13. Feb 18, 2015 #12

    D H

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    No. Let's go back to my thick cork vs thin oak analogy. While the top of the cork is higher than the top of the oak, the bottom of the cork is lower than the bottom of the oak. The same applies to the continents and oceans.

    Plate tectonics is perhaps a better explanation. The continents get pushed and pulled around in a cycle that forms and then breaks apart supercontinents. Convergent collisions between two continental crust plates results in mountain building; the Himalayas and the Alps, for example. This makes continental crust thick and old. Mountains are where the crust is thickest. Convergent collisions between an oceanic crust plate and some other plate results in subduction, with the subducting plate being oceanic crust. This keeps oceanic crust young and thin.
     
  14. Feb 19, 2015 #13
    Ok. So the difference in thickness is due to the differing responses to collision, and that difference is due to density, which it turn depends on water content at the time the rocks formed. Is that correct?
    If the lower boundaries are different, then shouldn't some of the continental crust soften from the temperature and become part of the asthenosphere, or maybe some asthenosphere rock solidify onto the oceanic crust?
    If I'm enjoying this too much, let me know and I'll head back to Wikipedia!
     
  15. Feb 22, 2015 #14

    D H

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    A key concept that you are perhaps missing is that the rock that form oceanic crust and continental crust are a bit different from one another. That said, geologists have found oceanic crust thousands of meters above sea level. These are "ophiolites". Google that term for more info. In fact, a lot more info. Ophiolites are one of the keys that matured Wegener's concept of continental drift into plate tectonics.

    The rock that form the crust and mantle are even more different from one another than are the rock that form the oceanic and continental crust. One might even naively think that because uranium has such a high atomic number that there would be a lot of uranium in the Earth's core. Some people with PhDs (but in the wrong field) have naively suggested exactly that, and have even conjectured that there might be enough uranium in the Earth's core to create a natural nuclear reactor at the very center of the Earth that is responsible for the heating that creates the Earth's magnetic field. This is not the case. Compared to abundances found in meteors and in the Sun, uranium is somewhat depleted in the Earth's core, highly depleted in the mantle, and overly present in the crust.
     
  16. Feb 22, 2015 #15
    This is incorrect.
    1. The principal consequence of the water is to lower the melting point of minerals within the subducting slab and adjacent mantle. i.e. the physics of the rock are changed, rather than the chemistry.
    2. This generates andesitic magma, not granitic magma. Granite is much more silicious (and less dense) than andesite.
    3. Most granite is generated through metamorphism and selective melting of sedimentary and igneous rocks within the crust.
     
  17. Feb 22, 2015 #16

    D H

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    What I wrote is not incorrect. Granite most definitely is less dense than basalt, and granite would not occur without water.

    I did not want to get into the details of selective melting with a novice.
     
  18. Feb 22, 2015 #17

    davenn

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    I almost made the same comment that Ophiolite did ... thinking that was just not right :wink:
    But who was lowly me to question a mentor ? :smile:

    That is true, But we don't get eruptive granitic volcanoes, granites are an intrusion formed rock, not eruptive as you suggested
    The continental volcanics are as Ophiolite said, andesites .... dacites etc


    Dave
     
  19. Feb 22, 2015 #18
    I did not dispute the lower density of granite and I specifically noted the role of water in lowering the melting temperature of rocks. However the other points you made were incorrect. I fully acknowledge the importance of simplification (my own response was grossly oversimplified) in answers, but the simplification should not include actual errors.
     
  20. Feb 24, 2015 #19
    What you are interested in is

    "isostacy" and
    http://en.wikipedia.org/wiki/Isostacy

    "gravity anomalies."
    http://en.wikipedia.org/wiki/Gravity_anomaly

    The usual analogue is blocks of wood floating on water. The upper surfaces of thinner and denser blocks of wood, say ebony, are closer to "sea level" than the upper surfaces of thicker and less dense blocks (say balsa). High topographic regions, such as large mountain ranges, extend deeper into the mantle than low topographic regions, namely ocean basins.
     
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