Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How come the ocean hasn't been absorbed into the ground?

  1. Dec 31, 2011 #1
    I've tried researching this question and I couldn't find a satisfying answer. How come the water on Earth hasn't been absorbed by the ground over billions of years? The Earth's water evaporate but get's replenished when it comes back down as precipitation. Is there a similar process that occurs underground? Is it buoyancy issue? If the water does slowly get absorbed does it come back up somehow?
     
  2. jcsd
  3. Dec 31, 2011 #2
    Many reasons, but a big one would relate to permeability. There are layers of clay, rock, etc, which prevent the water from going down beyond a certain depth in certain locations.

    Water which does percolate down tends to flow along underground rivers and things like that, and back to visible bodies of water.
     
  4. Dec 31, 2011 #3
    Don't cracks and fissures form regularly that would allow water to seep down to deeper levels? From what I understand there can be lots of moisture even deep underground so evidently water can get there somehow.
     
  5. Dec 31, 2011 #4
    Perhaps there is too much of it?

    Any given body of ground can only hold just so much water before it becomes saturated. The 'water table' (or phreatic surface if you want the posh term) is an underground surface below which the rocks are saturated.
     
  6. Dec 31, 2011 #5

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    In a sense it is. Wet sediments are deposited on the ocean floor and water does seep into the oceanic crust. However, this oceanic crust is fairly short lived, 200 million years or less. The oceanic crust arises at mid-ocean ridges and subsides back into the Earth at subduction zones. Those wet sea-floor sediments and saturated oceanic crust lose their water as the material subsides into the Earth. That migrating water comes back to the surface in the form of steam. The volcanoes near those subduction zones can sometimes be quite explosive because of the large amounts of steam they produce.


    Edit
    See Ranero et al., Bending-related faulting and mantle serpentinization at the Middle America trench, Nature 425, 367-373 (25 September 2003)
    http://dx.doi.org/10.1038/nature01961 [Broken]
     
    Last edited by a moderator: May 5, 2017
  7. Jan 30, 2012 #6
    Remember the water body is larger than the land itself if this could happend the whole world will be in a sticky mess of water
     
  8. Jan 30, 2012 #7

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    The other way around. The original post by Flatland essentially asked why the Earth has oceans at all: Why isn't the Earth all dry land? After all, rainfall does soak into the ground and even into the bedrock to form aquifers. So what stops all of the surface water from soaking into the ground? Why do we have oceans?

    The answer is that this does happen. The ocean's waters do indeed seep into the mud and crustal material that underlies the ocean. What the OP missed is that the ocean crust is young and is constantly being recycled. Volcanos at the subduction zones spew out immense amounts of water expelled from saturated ocean crust as it dives into the Earth due to subduction. The Earth is more or less in a steady state between water lost due to seepage and water added due to volcanos. It is plate tectonics that maintains this steady state.
     
  9. Mar 13, 2012 #8
    Almost all crustal rocks have some amount of porosity (i.e. the volume fraction of the entire rock that is not composed of solid minerals, like holes in swiss cheese) either through the natural packing configuration of grains or fractures on crystalline rock. All the porosity in these rocks is completely filled with water (except for the rare occurrences of hydrocarbons, CO2, etc), from the surface to large depths where the water is absorbed by the surrounding minerals through chemical reactions forming new hydrous minerals precipitated in the place of the porosity. So there is a depth at which no more porosity exists and water cannot keep going down.

    Then, as others explained, through crustal recycling, the hydrous minerals are exposed to incredibly high temperatures when they make contact with the mantle. This generates a melt that includes water in the form of bubbles. In fact, the presence of water drops the melting point of the entire rock allowing this to happen at relatively shallow depths. Then that melt makes its way up through more crustal rock, taking in all the water in its path, and releasing everything at the surface in the form of a volcano. Also, water surrounding the path of the melt may also be heated and its pressure raised such that it ejects at the surface through fractures in what we call geysers.
     
  10. Mar 16, 2012 #9

    chemisttree

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    It does penetrate the crust. It's penetration defines the Mohorovicic discontinuity.
     
  11. Apr 1, 2012 #10

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Surely the main reason why most of the water stays on the surface is that it is a lot less dense than most other materials. Any large voids underground may get water in them but they will eventually fill up with small rocks / sand /mud which will displace the water upwards.
    The natural place for water is on or near the surface because it just floats up there.
     
  12. Apr 2, 2012 #11
    A common misconception. "Underground rivers" are rare and only occur in karstic systems, basically limestones and chalk. Groundwater typically flows through pore space and the occasional fracture in the rock.


    I think the important point with water in subduction zones is that water greatly reduces the melting temperature of rock. Therefore wherever and whenever there is water in the system there is much more likely to be melt. Melt (full of hydrous volatiles) travels upward s back to the surface, and indeed water is released by volcanoes. Although careful, the majority of steam (water) is likely inherited from ground around the volcano, not necessarily the rising magma. Incidentally there is believed to be a vast amount of water in the Earth's mantle.

    Umm. what? I don't think so.
     
  13. Apr 5, 2012 #12
    I second billiard's surprise at this comment. Please provide appropriate citations to justify this seemingly bizzare claim.

    The Moho, taken as the boundary between crust and mantle, is defined by a large change in seismic velocity. This is traditioanlly ascribed, broadly, to a change from mafic to ultramafic rocks with corresponding mineralogical differences. I am unaware of any research that attributes these fundamental differences to water content. I would be fascinated to learn that such was the case and to understand how this functions as the primary mechanism defining the discontinuity.
     
  14. Apr 11, 2012 #13

    chemisttree

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Be fascinated.
     
  15. Apr 11, 2012 #14

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    As no one seems to have taken up my simple argument based on density, would someone 'who knows' tell me what sort of proportion of the Earth's water content is actually contained in the Oceans, compared with the amount that's underground? The actual (estimated) quantities involved would be nice to know if we want a good picture of what's actually going on. I realise that the total volume of the Oceans is really small, compared with the whole volume of the Crust.
     
  16. Apr 11, 2012 #15
    I assume you are referring specifically to the line in reference to the Moho.

    '...but it may be an alteration front corresponding locally to the depth of circulation of seawater down fractures and faults into the Earth's interior.'

    This MAY be true, but note it is talking about LOCAL phenomena within a MODEL of ocean crust. This in no way is equivalent to the inference that water cannot penetrate into the mantle anywhere at all.

    Indeed on the very same page as you linked to we find in relation to the development of textures in serpentinized peridotites a sentence which states:

    'Progressive serpentinization and hydrothermal alteration from greenschist to zeolite facies were spatially and temporally related to the development of multiple generations of macroscopic veins and were associate with the penetration of seawater-dominated fluids into the upper mantle rocks.'

    It seems that water can and does penetrate into the mantle, and believe me, there is a lot of work on the Earth's deep water cycle to back that statement up.
     
  17. Apr 13, 2012 #16

    chemisttree

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Right. I don't believe I inferred that at all.

    Of course on the same page it states that where extensive faulting occurs that serpentinization continues into the upper mantle and that the boundary of serpentine and unaltered rock defines the MOHO... within the upper reaches of the mantle!
     
  18. Apr 13, 2012 #17

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Forgive me but is this concentration on the qualitative rather than the quantitative typical of Geological discussion? I should have thought the the quantities involved would be much more important to you guys than this discussion would sugest. I, personally, would welcome some enlightenment as to how much actual data exists about the quantity of water you are discussing.
    Are the Oceans the equivalent to the condensation on the ceiling of a swimming baths or the contents of the pool itself?
     
  19. Apr 13, 2012 #18
    So you're not saying that the Moho (crust mantle boundary) is the boundary to which water can penetrate? And hence the inference that water cannot penetrate into the mantle?

    OK, that was my reading of your statement below...

    In which case I have absolutely no idea what this statement is supposed to mean.
     
  20. Apr 13, 2012 #19

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Will you guys stop squabbling please and try to answer the OP?
    An answer could be of the form "There is very little water underground" or "There is much more water underground than in all of the Oceans" - or something in between.
    That information would be useful to Flatland, I'm sure, and to me also.
     
  21. Apr 13, 2012 #20
    No it's not typical, people speculate quantitatively, however the 2 sigma uncertainty is likely to be huge and I don't happen to know the numbers off head. The data about water content has to be inferred from geophysical measurements, of things like the electrical conductivity of the mantle, and the seismic properties. It's hard enough to make good geophysical measurements in the first place, because there is always a lot going on in the signal and the Earth is not a conducive beast. For example if you want the signal from the upper mantle you need to remove the crust signal because your instruments are always on (or above) the crust and so any signal from the mantle necessarily has to pass through the crust. Then these measurements are very indirect and need to be put in context using mineralogical data. It's very hard to convert knowledge of seismic wavespeed and electric conductivity into a number for water content. Partly because there are other things that can have a similar effect to water on your data, and partly because most of those things have not even been worked out in terms of the mineral physics.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook