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Thread: Dune stability

  1. #1 Dune stability 
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    I walked around among some sand dunes in the Kalahari desert this past weekend (located at about 29 degrees 20' South and 18 Degrees 58' East) and wondered about the following.

    The dunes I saw had some sparse vegetation on them (unlike the dunes a few hundred kilometers further North-East that are completely bare).

    My questions are :
    1) Am I correct that such dunes will not move at all because of being "fixed" in place by the vegetation, or is there still very gradual movement that will only be measurable over long periods of time?
    2) How did these dunes form? Are they evidence of a previous period when this area was even more arid and almost no vegetation at all was present, or could they have formed under current climate conditions (which I doubt?)

    Thanks!


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  3. #2  
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    I believe that towards the end of the last ice age wind speed in that region and in the Sahara was higher, direction was more consistent and windy days were more common. That led to the formation of very large dunes that could not readily form in today's conditions. These were mobile. With the reduction in wind strength and consistency these dunes tend to become fixed, a process that is assisted by the growth of vegetation.

    This is based on dimly remembered geomorphology readings several decades ago, so I stand ready to be corrected by someone with more current knowledge.


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  4. #3  
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    Thanks JG. Your explanation makes sense.

    The only thing that still bothers me a bit is that if you're correct it means that the dunes I was walking on must have stayed almost unchanged for about 10 000 years! They aren't very big to begin with (maybe 12 to 15m tall from base to summit) and one would expect them to have weathered away over such a long time. They are still quite sharply defined (maybe check them out on google earth?), so it doesn't seem possible that they might have been much bigger 10 000 years ago and that only a bit of them are remaining today. Or is there a way in which the current climate keeps re-enforcing their shape without actually moving them? For that to work the wind must blow mainly in one direction I presume, which I'm not sure if its true?
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  5. #4  
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    For that to work the wind must blow mainly in one direction I presume, which I'm not sure if its true?
    If we're talking large scale, long term wind movements as against day to day weather, yes. It is quite possible for the air movement to be mainly in one direction.

    Of course, there are other matters. I'd not be surprised if plants in such an environment had some capacity to "walk" or at least rock backwards and forwards and gradually move along with the surface they're rooted in. If banana plants can do it in an ordinary banana plantation, other plants in mobile soils (? for want of a better word) in sand dunes might let themselves be moved along in a similar way. But not at a pace we'd normally recognise as 'walking'.
    "Courage is what it takes to stand up and speak; courage is also what it takes to sit down and listen." Winston Churchill
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  6. #5  
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    if memory serves, John was pretty accurate,
    kalahari gets 5 inches or so of rain annually,(?) and dunes were considered to be fairly stable there for thousands of years.

    a little rain over a long time still erodes
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  7. #6  
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    Quote Originally Posted by adelady View Post
    Of course, there are other matters. I'd not be surprised if plants in such an environment had some capacity to "walk" or at least rock backwards and forwards and gradually move along with the surface they're rooted in. If banana plants can do it in an ordinary banana plantation, other plants in mobile soils (? for want of a better word) in sand dunes might let themselves be moved along in a similar way. But not at a pace we'd normally recognise as 'walking'.
    So, Adelady, you're guessing that the dunes might still be mobile after all and that the plants are just moving along with them?
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  8. #7  
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    Quote Originally Posted by sculptor View Post
    if memory serves, John was pretty accurate,
    kalahari gets 5 inches or so of rain annually,(?) and dunes were considered to be fairly stable there for thousands of years.

    a little rain over a long time still erodes
    Sculptor, your post seems to be a bit self contradictory to me. You say on the one hand that you think John was right in stating that the dunes have probably been stable for thousands of years, but then you also say that there should have been erosion over a long time (which doesn't make sense when looking at how clearly defined they still are today).

    I agree that there is certainly still erosion happening today (the rainfall in the area I visited is only about three inches per annum however), so if the dunes are not moving, there must definitely be some re-enforcing mechanism that keeps their shape as is.
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  9. #8  
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    Guessing would be right. I have vague, foggy recollections of seeing or reading something about dunes and winds and everything, but it's not clear enough to think of a searching approach.

    Still you'd think judicious use of google earth might give some evidence one way or another. What was the landscape 5 years ago, what is it now, that sort of thing.
    "Courage is what it takes to stand up and speak; courage is also what it takes to sit down and listen." Winston Churchill
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  10. #9  
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    Quote Originally Posted by adelady View Post
    Guessing would be right. I have vague, foggy recollections of seeing or reading something about dunes and winds and everything, but it's not clear enough to think of a searching approach.

    Still you'd think judicious use of google earth might give some evidence one way or another. What was the landscape 5 years ago, what is it now, that sort of thing.
    I don't know if I might be somewhat "google earth illiterate", but I can only seem to go back about 10 years. During that period there doesn't seem to be any change at all in the dunes, but then again it is such a short time that it doesn't mean much.
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  11. #10  
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    erosion
    It seems as if the dunes you traversed are old and stable, but as per your observations rather short
    ergo
    erosion, over thousands of years, slowly watering the dunes and washing off anything dusty or loose
    so, stable on a short timescale(yay vegitation and the small blessing of moisture),
    but getting less height over their lifetime. It would be interesting to see the effects down valley(if not down stream).

    Old mountain ranges are "stable" but they get less tall as time and erosion effect them
    Last edited by sculptor; November 30th, 2012 at 07:06 PM. Reason: typo
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  12. #11  
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    Dunes are a phenomena of wind blown sand. The wind born sand both forms and sustains them. Vegetation will stabilize the dune but as long as teh wind moves sand the dune will continue to change shape a little.
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  13. #12  
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    Quote Originally Posted by Sealeaf View Post
    Dunes are a phenomena of wind blown sand. The wind born sand both forms and sustains them. Vegetation will stabilize the dune but as long as teh wind moves sand the dune will continue to change shape a little.
    Interesting. It sounds like you're speaking with some authority?

    Keeping your assertion that the existence of the dunes are thanks to continual wind action in mind, is it possible that the dunes as they are now (and have been for local human history) could not have formed under current conditions, but were formed under stronger and more consistent wind conditions long ago, and today are basically just maintained with very little movement or change in shape?

    Thus, are they evidence of different climatic conditions long ago, or could they form under today's conditions?
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