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Thread: c\land masses growing steadily

  1. #1 c\land masses growing steadily 
    Forum Radioactive Isotope sculptor's Avatar
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    Have landmasses been growing steadily for over 3 billion years?


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    Undefined question.

    Given that material has been falling onto the Earth for billions of years, it's safe to say that the Earth has gained Mass in the past 3 billions years.
    If more rocky material that watery comets- then yes landmasses have grown by a small amount.


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    not what i had in mind
    lemme rephrase, thusly:
    Has continantal crust been growing at a relatively steady rate, or in great spurts interspersed with relative stability.

    for spurts: the southern rockies grew out of a shallow sea in less than 1 million years.
    very little continental crust is older than 3 billion years, with most less than 1 billion years old, and quite a lot less than 100my old.

    I cannot think of instances of continental crust being subducted, so the land nasses are growing, and have been for at least 3.5 billion years.
    There was far less land when algea colonized fresh water lakes, whose clade then colonized the land.

    If spurts, then does the far-field bolide impact scenario come into play?
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    I figured that wasn't what you had in mind.
    How old the crust is at a point of measurement is nonsense. You mean to say, the crust was exposed to the surface for that amount of time. It is older than 3 billion years, obviously, unless the Earth was magically created 3 billion years ago...

    It Moves Around.

    Unless you can show a source for NEW crustal material, you're only asking if that stuff that moves around is "growing" in mass because it moved around. That's goofy.
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  6. #5  
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    The concept of land masses growing is really common knowledge, They are growing at the expense of surface area od oceans and shallow seas.
    That ain't really in question.
    The Earth is around 4.5 billion years old. At present 75 per cent of the continental crust – the crust we live on – is less than 1 billion years old, and only around 7 per cent of it is made up of rocks of Archaean age (that is older than 2.5 billion years).
    Bristol University | News from the University | Geodynamics of continental growth

    You might also see:
    The Size, Composition, And Surface Features of the Planets Orbiting the Sun ... - Google Books

    What is brought into question, is the bolide effect on plate tectonics, and timing of (likely) spurts of growth of the contental crust.
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    Quote Originally Posted by sculptor View Post
    The concept of land masses growing is really common knowledge, They are growing at the expense of surface area od oceans and shallow seas.
    That ain't really in question.
    The Earth is around 4.5 billion years old. At present 75 per cent of the continental crust – the crust we live on – is less than 1 billion years old, and only around 7 per cent of it is made up of rocks of Archaean age (that is older than 2.5 billion years).
    Bristol University | News from the University | Geodynamics of continental growth
    That quote does NOT support the claim that Landmass is constantly growing- only that it has shifted and turned over.

    Show evidence and causes of landmass growth and water coverage loss.
    Show the numbers, not elusive suggestions.
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  8. #7  
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    Jeez neverfly, you obviously don't know the basics, so any reasonable discussion of the far-field effect just ain't gonna happen
    thanx anyway
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    If I don't know the basics- Educate me.
    I'm skeptical of your claims and not without reason. Given the anti-anthropogenic global warming arguments you often give, I can easily see you claiming that the ice caps aren't melting because the landmasses are growing or something.

    And I doubt your claims, anyway. New Matter, aside from that listed in post number two, would be needed to explain significant growth.
    Your 'age of the exposed rocks arguments' are a red herring, that age is based on the change of the rock state, not it's actual time spent as part of the Earth. It does not demonstrate landmass growth- it only demonstrates change.

    And telling me I obviously don't understand the basics for your VAGUE assertion is equally baseless considering that the basics include induction, subduction, plate tectonics and shift and NONE of that shows significant LandMass Growth- only movement.

    So get off your high horse, stop making unsupported assertions and stop being vague and them whimpering when people don't kow tow and take your little word for it. If it's so basic, why did you start a thread asking about whether or not it is valid? Contradict yourself much, there?
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  10. #9  
    Universal Mind John Galt's Avatar
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    Sculptor is correct to a degree, though his question could have been better phrased.

    The Earth's crust consists of continental crust and oceanic crust. The former is thick, light and constitutes the vast majority of land. The latter is thin, denser and underlies the oceans.

    Oceanic crust suffers two fates. The bulk of it is subducted at oceanic trenches, but some is obducted onto the continental mass.

    The continental crust also increases by volcanic activity, as in the case of the andesitic lavas arising from partial melting of the subducting Nazca plate.

    What is not wholly clear is the rate at which these processes have added continental crust throughout Earht history. The rate is almost certainly not constant, since the amount of subduction has varied of geologic time, but how variable it has been and whether there are long term trends present is debated.

    Sculptor is presenting basic information that is not subject to any significant dispute. If you wish to challenge it then onus is on you to provide relevant citations, not him.


    And yes, based on your ignorance of these elementary points, you don't understand the basics.
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    Quote Originally Posted by John Galt View Post
    What is not wholly clear is the rate at which these processes have added continental crust throughout Earth history. The rate is almost certainly not constant, since the amount of subduction has varied of geologic time, but how variable it has been and whether there are long term trends present is debated.
    Accurate- let's look below...

    Quote Originally Posted by John Galt View Post
    Sculptor is presenting basic information that is not subject to any significant dispute. If you wish to challenge it then onus is on you to provide relevant citations, not him.
    Wrong- You claiming that the two of you are citing facts does not make them facts. Neither of you have provided support for your assertions.

    His fallacies are not my onus to prove. It is not on me to "prove" that his vague and rather disjointed assertions are in error. It is on you and him, now, to provide support for your assertions and looking below- It looks like you'll be having difficulty in presenting a solid case that is Not Up For Dispute and Basic as you two fallaciously claim it to be.
    Quote Originally Posted by John Galt View Post
    And yes, based on your ignorance of these elementary points, you don't understand the basics.
    Let's examine that:
    [B]Many theories of crustal growth are controversial, including rates of crustal growth and recycling, whether the lower crust is recycled differently than the upper crust and over how much of Earth history plate tectonics has operated and so could be the dominant mode of continental crust formation and destruction. [8]
    It is a matter of debate whether the amount of continental crust has been increasing, decreasing, or remaining constant over geological time. One model indicates that at prior to 3.7 Ga ago continental crust constituted less than 10% of the present amount.[9] By 3.0 Ga ago the amount was about 25%, and following a period of rapid crustal evolution it was about 60% of the current amount by 2.6 Ga ago.[10] The growth of continental crust appears to have occurred in spurts of increased activity corresponding to five episodes of increased production through geologic time.[11]
    Supercontinent cycle - Wikipedia, the free encyclopedia
    Continental crust - Wikipedia, the free encyclopedia

    Basic, elementary points and not subject to dispute, eh?
    Wrong.

    It is very much in dispute and not by you or me but by Geologists that do this for a living.

    Edit:
    The title of the thread is "Growing steadily." Not, "occasional spurts of growth and uncertainty as to whether any actual lasting growth over the course of time has actually occurred."
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  12. #11  
    has lost interest seagypsy's Avatar
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    Quote Originally Posted by sculptor View Post
    Jeez neverfly, you obviously don't know the basics, so any reasonable discussion of the far-field effect just ain't gonna happen
    thanx anyway
    If you had to ask the question in the OP, it would seem YOU don't know the basics, so how would you be able to tell if NF knows the basics or not. Do you simply want a particular answer and since he didn't give it, you instantly assume he doesn't know the basics?

    Subduction of continental plates does take place. Just because you are not personally aware of it that does not mean it doesn't happen.

    Edit: my error. I misread and thought sculpture said no subduction happened at all. I missed that he meant subduction of continental plates. I read it but it didn't process in my head right off the bat.
    Last edited by seagypsy; June 18th, 2013 at 12:07 AM.
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    Quote Originally Posted by seagypsy View Post
    Subduction of continental plates does take place. Just because you are not personally aware of it that does not mean it doesn't happen.
    It happens, but less frequently than subduction of dense oceanic crusts. This is the links I posted, as well.
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  14. #13  
    has lost interest seagypsy's Avatar
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    There are also rifts.
    For instance...

    East African Rift - Wikipedia, the free encyclopedia
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  15. #14  
    Universal Mind John Galt's Avatar
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    Neverfly, we don't indulge people who ask for citations proving Newton's laws of motion. I don't indulge people who challenge well established geological consensus. You really don't want to turn this into a battle, or do you? If so you are out of luck. Go play somewhere else.


    Now sculptor, I would like to know why, once again, you ask a vague question when you actually have a well marked out agenda. That may not be intended to be deceitful, but it sure as hell looks that way. I am quite happy to discuss with you, not debate, the uncertainties over the amount and timing of continental growth. However, if you are wanting to pursue discussion of some minority view on what might trigger increased rates of subduction and you already have data sitting ready to bring forward, then shame on you for your dishonesty. So, are we going to have a sensible and honest discussion, or are you going to play games? Neverfly had excluded himself from the discussion. Are you going to do the same? I hope not.
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    Effects of Erosion. Another example I can give is on the American Atlantic coast where a loss of mass from glacial melting reduced pressure on the plate in the north, causing the southern end of the plate to drop down lower (Like a teeter-totter), resulting in land becoming submerged.

    If I'm not mistaken, that was the cause of the fate of Holiday Island, which is now utterly gone and totally beneath the waves.
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    Quote Originally Posted by John Galt View Post
    Neverfly, we don't indulge people who ask for citations proving Newton's laws of motion. I don't indulge people who challenge well established geological consensus. You really don't want to turn this into a battle, or do you? If so you are out of luck. Go play somewhere else.
    I already provided a citation showing that the assertions were in error.
    I have shown and demonstrated it and so if you think a battle was fought- I just won it.
    Get over yourself.
    Yes, we DO ask people to provide citations if we are skeptical- Period. This is science, not the biased frontier where preferred ideas get exclusive rights.
    If I am skeptical of a claim- no matter what the claim is- I AM entitled to ask for citations and the onus is on them to provide them.
    According to many sources, a steady growth is NOT the geological consensus- not at all.

    And not just old Wikipedia either; USGS and CalTech, as well:
    Phanerozoic addition rates to the continental crust and crustal growth - Reymer - 2010 - Tectonics - Wiley Online Library
    Archean greenstone belt magmatism and the continental growth?mantle evolution connection: constraints from Th?U?Nb?LREE systematics of the 2.7 Ga Wawa subprovince, Superior Province, Canada
    JSTOR: An Error Occurred Setting Your User Cookie

    Even though the onus is not on me, I have provided a lot of support for my assertions and you have given only vague insults and derision.

    Enjoy your battle.

    Quote Originally Posted by John Galt View Post
    Now sculptor, I would like to know why, once again, you ask a vague question when you actually have a well marked out agenda. That may not be intended to be deceitful, but it sure as hell looks that way.
    Oh, NOW You admit this after giving me a hard time for being accurate on a scientific topic?
    Really?
    Nice.
    Quote Originally Posted by John Galt View Post
    However, if you are wanting to pursue discussion of some minority view
    Basic, Elementary points that are not up for dispute, right?
    Quote Originally Posted by John Galt View Post
    Neverfly had excluded himself from the discussion.
    Hardly.
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  18. #17  
    has lost interest seagypsy's Avatar
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    Quote Originally Posted by John Galt View Post
    Neverfly, we don't indulge people who ask for citations proving Newton's laws of motion. I don't indulge people who challenge well established geological consensus. You really don't want to turn this into a battle, or do you? If so you are out of luck. Go play somewhere else.
    I don't know who "we" is referring to but you should. If you aren't willing to at least offer a link to where someone less knowledgeable on the subject can at least learn the basics about the subject in question, you are basically alienating people from science based on their current lack of knowledge. But how can people learn if people aren't willing to teach or at least guide those who ask for guidance?
    Neverfly likes this.
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    Agreed. That line was the most unscientific, "I can't be bothered" statement I've seen on here. There is not just an option of supporting assertions, but an obligation to do so, no matter how many people might agree (Argumentum ad populum) with your assertion. Empirical evidence is required, not assertions of popularity, baseness, or 'my word is better than yours.'
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  20. #19  
    WYSIWYG Moderator marnixR's Avatar
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    Quote Originally Posted by sculptor View Post
    not what i had in mind
    lemme rephrase, thusly:
    Has continantal crust been growing at a relatively steady rate, or in great spurts interspersed with relative stability.

    for spurts: the southern rockies grew out of a shallow sea in less than 1 million years.
    very little continental crust is older than 3 billion years, with most less than 1 billion years old, and quite a lot less than 100my old.

    I cannot think of instances of continental crust being subducted, so the land nasses are growing, and have been for at least 3.5 billion years.
    There was far less land when algea colonized fresh water lakes, whose clade then colonized the land.

    If spurts, then does the far-field bolide impact scenario come into play?
    my understanding of the question is whether continental crust is growing at any noticeable rate, and the answer there i think must be "we don't know for sure"
    some continental debris gets dragged in during subduction but the thinking is that this then probably erupts in volcanic activity
    some oceanic crust gets obducted, and over time this could lead to a small growing trend since i don't know of any substantial mechanism to destroy continental crust apart from the mechanism i've described earlier

    it is clear that in the first billion years continents grew substantially (the so-called precambrian cratons) and it is also clear that this sort of growth has slowed down substantially since, but whether continents nowadays are still growing at any noticeable rate i don't

    my gut feel tells me that there's probably still a small amount of growth but the overall effect is minor, even over geologic time
    (this is not backed up by any reading of the literature on my part, but more out of a feeling that the initial split between continental and oceanic crust amounts to some sort of fractionation, and i would have thought that the separation has, after the inital growth spurt, reached some sort of dynamic equilibrium)
    sculptor and Neverfly like this.
    "Reality is that which, when you stop believing in it, doesn't go away." (Philip K. Dick)
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    Back on track---

    In order for Steady Significant growth (meaning not so little growth that it's pointless anyway) for the last 3 billion years, we would need to see one of these:
    -A mechanism to replace moved material with emptiness and have it hold up. This means that if continental crust had steady growth for three billion years and that material did not have an extraterrestrial source, where the material came from is important. Although certain regions of continental crust show growth, due to plate tectonics, this is often offset by other forces at work and why we don't see much more land area than we actually do. What we actually see is the dense oceanic plates sub-ducting but recycled material emerging from deep rifts. What was beneath the ocean has been pushed up in certain areas, exposing fossilized ancient sea creatures.
    However, continental crust has been pushed into the sea, as well. We would need a mechanism that shows that lack of balance; that crust material can be deposited in one location without creating gaps in locations. We do not see these gaps with no crustal material.
    -Added material from elsewhere than the Earth- we covered this, already.

    ETA:
    Quote Originally Posted by marnixR View Post
    my gut feel tells me that there's probably still a small amount of growth but the overall effect is minor, even over geologic time
    (this is not backed up by any reading of the literature on my part, but more out of a feeling that the initial split between continental and oceanic crust amounts to some sort of fractionation, and i would have thought that the separation has, after the inital growth spurt, reached some sort of dynamic equilibrium)
    http://en.wikipedia.org/wiki/Isostasy

    I think this may weigh in the topic a bit...
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    has lost interest seagypsy's Avatar
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    Quote Originally Posted by marnixR View Post
    Quote Originally Posted by sculptor View Post
    not what i had in mind
    lemme rephrase, thusly:
    Has continantal crust been growing at a relatively steady rate, or in great spurts interspersed with relative stability.

    for spurts: the southern rockies grew out of a shallow sea in less than 1 million years.
    very little continental crust is older than 3 billion years, with most less than 1 billion years old, and quite a lot less than 100my old.

    I cannot think of instances of continental crust being subducted, so the land nasses are growing, and have been for at least 3.5 billion years.
    There was far less land when algea colonized fresh water lakes, whose clade then colonized the land.

    If spurts, then does the far-field bolide impact scenario come into play?
    my understanding of the question is whether continental crust is growing at any noticeable rate, and the answer there i think must be "we don't know for sure"
    some continental debris gets dragged in during subduction but the thinking is that this then probably erupts in volcanic activity
    some oceanic crust gets obducted, and over time this could lead to a small growing trend since i don't know of any substantial mechanism to destroy continental crust apart from the mechanism i've described earlier

    it is clear that in the first billion years continents grew substantially (the so-called precambrian cratons) and it is also clear that this sort of growth has slowed down substantially since, but whether continents nowadays are still growing at any noticeable rate i don't

    my gut feel tells me that there's probably still a small amount of growth but the overall effect is minor, even over geologic time
    (this is not backed up by any reading of the literature on my part, but more out of a feeling that the initial split between continental and oceanic crust amounts to some sort of fractionation, and i would have thought that the separation has, after the inital growth spurt, reached some sort of dynamic equilibrium)
    I think it was the "Why did hte dinosaurs die out" thread that got me looking up geological time frames. It's interesting that you can, if you look really close at google map satellite view, see the shapes of the existing techtonic plates as well as major cratons and some domes. I was even able to locate many meteorite impact sites in africa. That's how I found out about the East Africa rift. I was also scanning the Sahara desert, I had no idea there were so many volcanoes there.

    But as far as land growth is concerned. on one coast there may appear to be a few centimeters a year of growth but it likely compares to the few inches to several feet of coast line that is eroded away on the opposite coast due to hurricanes , river mouths widening and so on.
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    Quote Originally Posted by Neverfly View Post
    Back on track---

    In order for Steady Significant growth (meaning not so little growth that it's pointless anyway) for the last 3 billion years, we would need to see one of these:
    -A mechanism to replace moved material with emptiness and have it hold up. This means that if continental crust had steady growth for three billion years and that material did not have an extraterrestrial source, where the material came from is important. Although certain regions of continental crust show growth, due to plate tectonics, this is often offset by other forces at work and why we don't see much more land area than we actually do. What we actually see is the dense oceanic plates sub-ducting but recycled material emerging from deep rifts. What was beneath the ocean has been pushed up in certain areas, exposing fossilized ancient sea creatures.
    However, continental crust has been pushed into the sea, as well. We would need a mechanism that shows that lack of balance; that crust material can be deposited in one location without creating gaps in locations. We do not see these gaps with no crustal material.
    -Added material from elsewhere than the Earth- we covered this, already.

    ETA:
    Quote Originally Posted by marnixR View Post
    my gut feel tells me that there's probably still a small amount of growth but the overall effect is minor, even over geologic time
    (this is not backed up by any reading of the literature on my part, but more out of a feeling that the initial split between continental and oceanic crust amounts to some sort of fractionation, and i would have thought that the separation has, after the inital growth spurt, reached some sort of dynamic equilibrium)
    Isostasy - Wikipedia, the free encyclopedia

    I think this may weigh in the topic a bit...
    haha very funny!

    Yes, I had been following the discussion and had been about to chip in with erosion and isostasy. I suppose some eroded deposits end up on the deep ocean floor and eventually get subducted along with other marine sediments and seawater. Some then gets regurgitated by magma rising to the surface behind the subduction zones, and the rest disappears into the mantle rocks. But I imagine quite a large proportion (maybe even most) of eroded deposits never get this far: they get get laid down in lower areas of continental crust that are eventually uplifted and/or folded, i.e. continental crust -> continental crust and no net loss of continental crust occurs. Similarly, while isostasy might allow more continental crust to sink deeper into the earth, so far as I know it remains attached to the continental block in question and as erosion occurs on the surface, it comes back up again. So not a process for net loss of continental crust back into the mantle.

    As for the processes giving rise to net gain in continental crust, the only one we seem to have is volcanism, mainly that behind the subduction zones (mid-ocean ridge volcanism mostly produces oceanic crust, I understand). So there does not seem to be an obvious mechanism for net gain of continental crust, qualitatively speaking, though obviously there could be as a result of the long term mass balance of all these processes.

    Presumably this question could be rephrased in geochemical terms as whether or not the amount of sial has grown. I presume this is where marnixr's idea of fractionation comes in. Higher Al content mineral species may gradually accumulate in the upper part of the crust, through selective melting and rising towards the surface due to lower density, as part of volcanic activity.

    Interesting subject.
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  24. #23  
    Forum Radioactive Isotope sculptor's Avatar
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    Quote Originally Posted by John Galt View Post
    ...
    Now sculptor, I would like to know why, once again, you ask a vague question when you actually have a well marked out agenda. That may not be intended to be deceitful, but it sure as hell looks that way. I am quite happy to discuss with you, not debate, the uncertainties over the amount and timing of continental growth. However, if you are wanting to pursue discussion of some minority view on what might trigger increased rates of subduction and you already have data sitting ready to bring forward, then shame on you for your dishonesty. So, are we going to have a sensible and honest discussion, or are you going to play games? Neverfly had excluded himself from the discussion. Are you going to do the same? I hope not.
    John: The key word in the op was steadily. There is an ongoing debate in the community as to weather the old hypothesis of steady growth of contenantal crust holds, or whether significant bolide impacts A) started plate tectoninc, and the growth of contental crust, and have led to contental growth spurts, or B) are somewhat isolated from earth-bound causal factors.

    Often, after reading several articles on a subject, and wanting a different perspective, I tend to leave the introductory question vague enough to not poison the responses with my perspective and attendant prejudices.
    Often, this seems to come off as "asking an rhetorical question". That is not my intent.

    I am a confessed lifelong autodidact, and I have discovered that my approach ofttimes leads to the developement of a skewed perspective, which I hope to correct and clarify through the sharing of other's knowledge and perspective.

    Beginning with the opposition hypothesis to "snowball earth" which claimed an angle to the ecliptic greater than 54 degrees, and looking for a triggering mechanism powerful enought to change axial tilt without significantly altering earth's orbit, led me to looking into bolide impacts.

    This current curiousity kicked into gear for me when I read a small intro by dr./prof. Hansen who postulated that bolide impacts kick-started plate tectonics. She may have a really valid point--or not. Further reading led to the far-field postulations, which brought to mind an earlier claim that bolide impacts could lead to massive vulcanism on the opposite side of the earth(that claim was for the triggering mechanism for the siberian traps. ... (I wish I could remember that woman's name)

    All the while, I was reminded of a pool shark I knew in the army who could transfer "english" from the cue ball to his target ball causing it to arc into his chosen pocket.
    Does that seem strange?
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    Forum Radioactive Isotope sculptor's Avatar
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    marnixR and echemist

    If you have $1, and you get another $1, that is a 100% growth rate. Much later, if you have $1000, and gain another $1, that is a .1% growth rate, though the actual gain is the same as when you only had a buck.

    as/re growth rates: You might find: Earth's Continental Crust Growth Rate has Decreased | SciTech Daily of interest. Though, I do not agree with Dr Storey's conclusions.

    seagypsy: The african rift zone/valley is actually currently increasing the size of that contenant. It may eventually open up another ocean, or it may be much like the ancient rift of the mississippi valley, which seperated the lower end of the apalacian mountains(isolating some in Missouri), while opening the interior of north america to a shallow sea, but was then pushed back together creating a larger contenant for it's efforts. It did however leave an active rift(the new madrid fault zone) that occasionally becomes obvious with massive midwestern earthquakes. One of the associated faults, the plum creek fault, underlies the area wherein lies my property. Which, Is why we bought above the dam on the iowa river.
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  26. #25  
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    Quote Originally Posted by sculptor View Post
    marnixR and echemist

    If you have $1, and you get another $1, that is a 100% growth rate. Much later, if you have $1000, and gain another $1, that is a .1% growth rate, though the actual gain is the same as when you only had a buck.

    as/re growth rates: You might find: Earth's Continental Crust Growth Rate has Decreased | SciTech Daily of interest. Though, I do not agree with Dr Storey's conclusions.

    seagypsy: The african rift zone/valley is actually currently increasing the size of that contenant. It may eventually open up another ocean, or it may be much like the ancient rift of the mississippi valley, which seperated the lower end of the apalacian mountains(isolating some in Missouri), while opening the interior of north america to a shallow sea, but was then pushed back together creating a larger contenant for it's efforts. It did however leave an active rift(the new madrid fault zone) that occasionally becomes obvious with massive midwestern earthquakes. One of the associated faults, the plum creek fault, underlies the area wherein lies my property. Which, Is why we bought above the dam on the iowa river.
    Thanks for the reference. Although I stress that, as a mere chemist, I am speaking strictly ex ano on this subject, it does strike me as not unreasonable to imagine that the various elements mixed together in the primordial Earth, as it coalesced from cosmic dust, would fractionate by density over time to give rise to the Fe/Ni-rich core , the mantle made of denser silicate compounds and the crust made of lighter ones. If that happened, one might expect the process to proceed rapidly at first and less slowly as the constituents moved towards an equilibrium distribution. Further, this fractionation by density would be driven by the minimisation of gravitational potential energy and consequently one would expect a heat release to occur, manifested via the movement against viscous or frictional drag of the rocks. I have no idea whether this would be a significant source of internal heat in the Earth. As I understand it, the main source of internal heat in the Earth is attributed to the decay of radioisotopes, but I wonder if this effect could be a contributor.
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    Quote Originally Posted by sculptor View Post
    John: The key word in the op was steadily. There is an ongoing debate in the community as to weather the old hypothesis of steady growth of contenantal crust holds, or whether significant bolide impacts A) started plate tectoninc, and the growth of contental crust, and have led to contental growth spurts, or B) are somewhat isolated from earth-bound causal factors.
    I think you are completely misrepresenting the debate. Contrary to what some people think, no one in "the community" seriously denies that continents have grown over time. I do not recall seeing a single paper, certainly in the last twenty five years, that would make such a claim.

    What is also pretty well agreed is that this growth is not steady. Again, I am unaware of any papers in the twenty five years that would challenge this. Of course there may be some, but they would represent a minority view.

    It also seems fairly clear that growth has occured in pulses, probably in periods lasting 100my or so, but spearated by something of the order of a billion years.

    What is debated is then twofold:
    1) What initiated these pulses?
    2) Have the more recent pulses actually resulted in significant net continental mass growth, or has erosion and recycling of continental crust offset the gain through obduction, accretion, andesite vulcanicity and other mechanisms.

    The notion of bolide impact as a trigger is not, as far as I can determine, a proposal that been of more than passing interest. You appear to have presented a false dichotomy - though your peculiar spelling and grammar make it difficult to be sure . It is not a case of steady state growth or bolide impact induced growth spurts. Rather we need to ask the two questions I have asked above.

    exchemist and marnix: there is a wealth of research material relating to regional geology in back arc basins, ophiolite zones, andesitic provinces, etc; studies of mantle geochemistry evolution provide insights; zircon studies are hugely informative and are pointing us towards confidence in certain conclusions. I'm trying to put together a summary of the key points, but it won't happen overnight. Some of your observations are spot on and some seem a little flaky. Watch this space.
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    Quote Originally Posted by sculptor View Post
    The concept of land masses growing is really common knowledge, They are growing at the expense of surface area od oceans and shallow seas.
    That ain't really in question.
    The Earth is around 4.5 billion years old. At present 75 per cent of the continental crust the crust we live on is less than 1 billion years old, and only around 7 per cent of it is made up of rocks of Archaean age (that is older than 2.5 billion years).
    Bristol University | News from the University | Geodynamics of continental growth

    You might also see:
    The Size, Composition, And Surface Features of the Planets Orbiting the Sun ... - Google Books

    What is brought into question, is the bolide effect on plate tectonics, and timing of (likely) spurts of growth of the contental crust.

    I've always a bit of a problem with the idea that there ever was a "Pangea". I'm not doubting Pangea existed. Nor that it encompassed all the existing continents.

    My problem is the idea that there were no other land masses. How would that work? How could we have all the land on one side of the Earth and none at all on the other side, without balance issues?

    It seems more plausible that there were other land masses at the same time as Pangea, but they vanished utterly. Perhaps they were subducted. Perhaps ice sheets formed on the top of them that pushed them down. Perhaps they found some other way to sink.

    If there's nothing left of them now, then naturally we will have a hard time proving they ever existed, but it stands to reason. I guess that's all I'm saying here.
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    Quote Originally Posted by kojax View Post
    I've always a bit of a problem with the idea that there ever was a "Pangea". I'm not doubting Pangea existed. Nor that it encompassed all the existing continents.

    My problem is the idea that there were no other land masses.,
    That is illogical. If Pangea encompassed all existing continents what were these landmasses? Apart from miniscule oceanic islands and marginally larger, but still insignificant island arcs, land is continental in character. so if you agree that Pangea encompassed all continents, then there was no other significant land.

    And by the way, the Earth doesn't really care whether or not you have a problem with how it does things.

    Quote Originally Posted by kojax View Post
    How could we have all the land on one side of the Earth and none at all on the other side, without balance issues?
    .
    We pretty much have all the land on onse side of the Earth at present, or haven't you noticed the Pacific Ocean. We don't seem to have a balance problem. You might want to compute the problem that a skinny little crust, less than 1% of the Earth's diameter will cause. You don't seem to have heard of isostasy.

    Quote Originally Posted by kojax View Post
    It seems more plausible that there were other land masses at the same time as Pangea, but they vanished utterly..
    Why do you find that more plausible? It seems it's because you overestimate the significance of a minor difference in crustal mass disposition.

    Quote Originally Posted by kojax View Post
    Perhaps they were subducted. Perhaps ice sheets formed on the top of them that pushed them down. Perhaps they found some other way to sink.
    .
    In general continental crust is reluctant to subduct and oceanic crust does not produce significant land. Ice sheets would not pushe them down to the extent they ceased to exist and once the ice melted they would rebound.
    Did they find another way to sink? Like Lemuria and Atlantis and other mythical land masses? Yeah, sure.

    Quote Originally Posted by kojax View Post
    If there's nothing left of them now, then naturally we will have a hard time proving they ever existed, but it stands to reason. I guess that's all I'm saying here.
    No, it does not stand to reason. You have offered no evidence to support this suggestion, only your own misunderstanding,
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    Pangea isn't the only super-continent to be believed to have existed. There are also a few continents that are now underwater (Kerguelen Plateau, Sundaland, Zealandia).

    So it does seem that over extremely long periods of time, land rises and sinks, with sinkage undoing or canceling out any growth, steady or otherwise, along the way.

    http://en.wikipedia.org/wiki/List_of_supercontinents

    There is also the Bering land bridge, that unless they have changed views on it since I learned about it, that is mostly underwater now existing as just a string of islands, but was believed to have been a walkable solid land path from China to North America at one time.
    Last edited by seagypsy; June 18th, 2013 at 03:53 PM. Reason: forgot to put in link for my source
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    deleted.... double post due to forum loading slow and doing that leave page thing again.
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    Quote Originally Posted by John Galt View Post
    I think you are completely misrepresenting the debate.
    Gee, ya think?
    Quote Originally Posted by John Galt View Post
    Contrary to what some people think, no one in "the community" seriously denies that continents have grown over time. I do not recall seeing a single paper, certainly in the last twenty five years, that would make such a claim.
    Who are these "some people?"

    I posted 3 peer reviewed papers and two wiki articles that don't agree with you. They say that it's under serious debate as to whether net growth has occurred over time. You still have presented No Support for your assertions.
    None.
    Zip.
    Zilch.

    The wording is tricky; While it's quite agreeable to say that the continents have grown over time, that does not mean that growth has been steady- it does not mean no loss has occurred and therefor, does not satisfy the question as to whether or not Net Growth has truly occurred.

    Quote Originally Posted by John Galt View Post
    What is debated is then twofold:
    1) What initiated these pulses?
    2) Have the more recent pulses actually resulted in significant net continental mass growth, or has erosion and recycling of continental crust offset the gain through obduction, accretion, andesite vulcanicity and other mechanisms.
    Oh- Look what you just said!

    Wording is tricky here, eh? So what we have is:
    John Galt claims that growth has occurred- Agreeable.
    John Galt claims there is no evidence of Steady growth- Agreeable.
    John Galt claims there is no strong support that there is any significant net growth over time- agreeable.

    All agreeable because I Said all that and John Galt disputed it, said the growth of the topic was Basic, elementary and Beyond Dispute.

    Very tricky wording, indeed. Let's get Dirty.
    Is Extraterrestrial Impact a significant factor? From what we can tell, continental growth isn't any more significant. Existing material is what is getting pushed up in mountainous regions. Volcanic activity is as slow a process as erosion is and any material pushed up to the surface came from right beneath it, causing a slight settling of the mass lower into the mantel over time. Again, we can see this in Yellowstone.
    Yes, there is some growth but no one has given any figures, yet. Significant Growth raises interesting questions and I've yet to see "significant" defined in any meaningful way.
    So what is the dirty part?
    Considering how "scientific" our friend is above about extraterrestrials and Aquatic Apes... I'm not going to just take his word for it when he makes declarations that he claims he does not have to support.

    Especially when he can't read what was actually written and tries to make it appear that others have said things they haven't because he cannot handle getting embarrassed.

    Now, he'll probably send me another Nasty PM because I posted this- except he probably won't now that I said that here But I don't care.
    It's a science topic, not an ego topic and the disputed figures need to be examined. This is not Faith.
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    Quote Originally Posted by Neverfly View Post
    [Wording is tricky here, eh? So what we have is:
    John Galt claims that growth has occurred- Agreeable.
    John Galt claims there is no evidence of Steady growth- Agreeable.
    John Galt claims there is no strong support that there is any significant net growth over time- agreeable.
    Liar.

    There has been significant growth over time. It is uncertain when that significant growth stopped. You deliberately failed to embolden the portion of my sentence that makes my meaning clear. That is unacceptable behaviour. Stop it.
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    Quote Originally Posted by John Galt View Post
    Quote Originally Posted by Neverfly View Post
    [Wording is tricky here, eh? So what we have is:
    John Galt claims that growth has occurred- Agreeable.
    John Galt claims there is no evidence of Steady growth- Agreeable.
    John Galt claims there is no strong support that there is any significant net growth over time- agreeable.
    Liar.

    There has been significant growth over time. It is uncertain when that significant growth stopped. You deliberately failed to embolden the portion of my sentence that makes my meaning clear. That is unacceptable behaviour. Stop it.
    You're words aren't very clear. And you have both lost me. But first you say there has been significant growth over time. Ok That I understand. But then you say "It is uncertain when that significant growth stopped.".... so does that mean that currently there is no significant growth? Because your statement almost directly says that growth stopped, but it is uncertain when it stopped. Is that what you meant?
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    Quote Originally Posted by John Galt View Post
    Liar.

    There has been significant growth over time. It is uncertain when that significant growth stopped. You deliberately failed to embolden the portion of my sentence that makes my meaning clear. That is unacceptable behaviour. Stop it.
    Deliberately, eh? No... I did not see any statement from you that Significant Net Growth has taken place nor have I see any evidence of this assertion.
    I would not have said you said that if I thought you hadn't. I took your line here:
    2) Have the more recent pulses actually resulted in significant net continental mass growth, or has erosion and recycling of continental crust offset the gain through obduction, accretion, andesite vulcanicity and other mechanisms.

    To demonstrate, through your questioning, that it is Unknown Whether Significant Net Growth has accumulated. If I'm in error, it is unintentional. I don't care who is correct- I want to see the correct information. I have seen you provide no support nor evidence for any assertion so far, so I can only go by what you say.
    Either way, what counts as "Significant" needs to be defined.

    Let's try this again:
    John Galt says that Continental Growth has occurred over time- I agree.
    Sculptor asks if it has been steady- I say it has not-- John Galt can clarify- I believe he agreed it has not been steady.
    Debate On Topic: Has Significant Net Growth Occurred?
    Significant Net Growth is continental growth that has accumulated over 3.5 billion years and resulted in a significant change in size of continental crust vs. oceanic crust. While I Agree that recent growth has occurred, I don't agree that significant net growth has as other factors wear down continental crust and the material needs to come from somewhere... If it came from oceanic crust, we should see oceanic crust shrinkage.

    The links I posted said this is far from certain.
    But wikipedia is not always accurate.

    Is there any evidence to show otherwise?
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    Quote Originally Posted by Neverfly View Post
    [
    Significant Net Growth is continental growth that has accumulated over 3.5 billion years and resulted in a significant change in size of continental crust vs. oceanic crust. While I Agree that recent growth has occurred, I don't agree that significant net growth has as other factors wear down continental crust and the material needs to come from somewhere... If it came from oceanic crust, we should see oceanic crust shrinkage.
    I shall stay well out your personal dispute, but on the point about the material needing to come from somewhere I don't see the problem. There is, so far as I know, nothing to say the amounts of the sial and sima minerals are any any way fixed. There is, I understand, a huge degree of overlap in chemical composition between the two types of crust, and also between the two of them and the rocks of the mantle. And chemical reactions go on all the time interconverting minerals.

    So if there is - or has in the past been - a fractionation process, it seems perfectly possible that continental crust has grown by a gradual concentration over time of those mineral constituents that lead to lower density rocks.
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    Quote Originally Posted by John Galt View Post
    Quote Originally Posted by kojax View Post
    I've always a bit of a problem with the idea that there ever was a "Pangea". I'm not doubting Pangea existed. Nor that it encompassed all the existing continents.

    My problem is the idea that there were no other land masses.,
    That is illogical. If Pangea encompassed all existing continents what were these landmasses? Apart from miniscule oceanic islands and marginally larger, but still insignificant island arcs, land is continental in character. so if you agree that Pangea encompassed all continents, then there was no other significant land.

    And by the way, the Earth doesn't really care whether or not you have a problem with how it does things.
    Good thing the Earth's opinion doesn't matter very much. .....since it doesn't hold any opinions.

    Quote Originally Posted by kojax View Post
    How could we have all the land on one side of the Earth and none at all on the other side, without balance issues?
    .
    We pretty much have all the land on onse side of the Earth at present, or haven't you noticed the Pacific Ocean. We don't seem to have a balance problem. You might want to compute the problem that a skinny little crust, less than 1% of the Earth's diameter will cause. You don't seem to have heard of isostasy.
    You could bring the topic of isostasy up, then maybe post a link to some site that explains the concept, or even just the wiki page.

    I don't know why you think it's productive to berate people instead of just telling them what you know. Honestly I don't.

    While we're on the topic of posting useful information/links for the enjoyment of others - I found a cool online globe to look at. And it confirms that yes.... the Pacific ocean does take up a rather disproportionately large amount of space.

    Terrestrial Globe - Flash 3D Earth Viewer Application


    Quote Originally Posted by kojax View Post
    It seems more plausible that there were other land masses at the same time as Pangea, but they vanished utterly..
    Why do you find that more plausible? It seems it's because you overestimate the significance of a minor difference in crustal mass disposition.
    It's because the Earth is spinning and has a moon orbiting around it - which means there are some forces in play here that would tend to maintain a symmetry.

    I'm not aware of any forces that would tend to act against that symmetry, so the emergence of a single Pangea is an interesting phemonena. It doesn't outright demand an explanation, because there's certainly quite of lot of randomness involved in a planet's formation and behavior, but an explanation would be nice. And in attempting to find that explanation we might stumble onto something interesting.

    Quote Originally Posted by kojax View Post
    Perhaps they were subducted. Perhaps ice sheets formed on the top of them that pushed them down. Perhaps they found some other way to sink.
    .
    In general continental crust is reluctant to subduct and oceanic crust does not produce significant land. Ice sheets would not pushe them down to the extent they ceased to exist and once the ice melted they would rebound.
    Did they find another way to sink? Like Lemuria and Atlantis and other mythical land masses? Yeah, sure.

    Quote Originally Posted by kojax View Post
    If there's nothing left of them now, then naturally we will have a hard time proving they ever existed, but it stands to reason. I guess that's all I'm saying here.
    No, it does not stand to reason. You have offered no evidence to support this suggestion, only your own misunderstanding,
    I'm curious what makes continental shelves stay at the height they are at. They're floating on top of magma aren't they? Why don't they sink?

    Where do they get their buoyancy? Are the ones that have land on them made of (slightly) lighter materials than the ones that do not?
    Some clocks are only right twice a day, but they are still right when they are right.
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    Quote Originally Posted by exchemist View Post
    I shall stay well out your personal dispute
    -Sigh- Really?!
    In spite of John Galts Claim; There is no personal dispute.
    Just because I disagree with something he says does NOT make it personal. I disagreed with Sculptor- actually... and while it is true I did not word myself very well, either- John Galt was the one that started the posts between him and us.
    Quote Originally Posted by exchemist View Post
    but on the point about the material needing to come from somewhere I don't see the problem. There is, so far as I know, nothing to say the amounts of the sial and sima minerals are any any way fixed. There is, I understand, a huge degree of overlap in chemical composition between the two types of crust, and also between the two of them and the rocks of the mantle. And chemical reactions go on all the time interconverting minerals.

    So if there is - or has in the past been - a fractionation process, it seems perfectly possible that continental crust has grown by a gradual concentration over time of those mineral constituents that lead to lower density rocks.
    I do see your point. It does seem possible, but how likely is it? Let's take another look at what I quoted from the material in the link earlier:
    It is a matter of debate whether the amount of continental crust has been increasing, decreasing, or remaining constant over geological time. One model indicates that at prior to 3.7 Ga ago continental crust constituted less than 10% of the present amount.[9] By 3.0 Ga ago the amount was about 25%, and following a period of rapid crustal evolution it was about 60% of the current amount by 2.6 Ga ago.[10] The growth of continental crust appears to have occurred in spurts of increased activity corresponding to five episodes of increased production through geologic time.[11]
    I re-quote this and use bold because I want to make it clear that this is not a Well Supported Model like say, Relativity.
    There is a great deal of uncertainty in this one.
    This is what Sculptor said that got my attention:
    Quote Originally Posted by sculptor View Post
    I cannot think of instances of continental crust being subducted, so the land nasses are growing, and have been for at least 3.5 billion years.
    There was far less land when algea colonized fresh water lakes, whose clade then colonized the land.
    This is not certain. It is possible, but not certain. There are certain problems with the idea, though you did make a good point about one of them, exchemist.
    Other problems, I've already pointed out.

    The land is a dynamic system, constantly changing. To make an assertion that one particular aspect remained consistent requires Strong Evidence.
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    This link is to a .doc download...
    ftp://ftp.gps.caltech.edu/pub/djs/St...l%20MS&ZBA.doc

    I have not finished reading it... Maybe others can weigh in on it.

    From page 12:
    3. Models of crust formation and continental growth
    General
    Models that describe the mechanism of continental crust growth and the evolution of continental masses can be divided into two major groups: (1) models of early crustal growth and subsequent recycling; (2) models of continuous or episodic crustal growth.
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    Quote Originally Posted by seagypsy View Post
    You're words aren't very clear. And you have both lost me. But first you say there has been significant growth over time. Ok That I understand. But then you say "It is uncertain when that significant growth stopped.".... so does that mean that currently there is no significant growth? Because your statement almost directly says that growth stopped, but it is uncertain when it stopped. Is that what you meant?
    The first thing to remember is that this is a moving target. Views on continental growth have changed over the course of more than a century. The underlying theories have undergone dramatic shifts, not just the emergence of plate tectonics, but sub-sets of the theory, such as the nature of terranes, back-arc evolution, etc. Techniques of seismic investigation, isotope analysis and the like have supplied vast volumes of ne evidence from new perspectives.

    Two things emerge clearly:
    The present continental crustal mass is substantially greater today than it was in the past.
    The growth of that mass has not been steady.

    What is less clear and what is debated is whether the net addition of crustal materal ended in the Phanerozoic, or even earlier, or if it continues to day at a significant, but perhaps lower rate.

    What is not disputed, what should be known to every undergraduate geology student, what is just so baseline ordinary that it does not require citations to justify, is that there is more land around today than there was 3 billion years ago. The only caveat is that to a geologist 'land' will often be a proxy for continent. If some of the continent is covered by a shallow sea such as the South China Sea, well that's just a temporary aberration.
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    Quote Originally Posted by John Galt View Post
    What is not disputed, what should be known to every undergraduate geology student, what is just so baseline ordinary that it does not require citations to justify, is that there is more land around today than there was 3 billion years ago. The only caveat is that to a geologist 'land' will often be a proxy for continent. If some of the continent is covered by a shallow sea such as the South China Sea, well that's just a temporary aberration.
    While I think there have been misunderstandings and vague assertions on all sides- I think that you have more knowledge on the topic than I do. I think that my initial impressions were in error as I see you asserting far more of what I've read and less of what I've seen as unsubstantiated.

    But this last bit is not of geology but of principle and I still don't agree with it. Nothing is above providing a reference for. NOTHING.
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    Quote Originally Posted by exchemist View Post
    There is, so far as I know, nothing to say the amounts of the sial and sima minerals are any any way fixed. There is, I understand, a huge degree of overlap in chemical composition between the two types of crust, and also between the two of them and the rocks of the mantle. And chemical reactions go on all the time interconverting minerals.

    So if there is - or has in the past been - a fractionation process, it seems perfectly possible that continental crust has grown by a gradual concentration over time of those mineral constituents that lead to lower density rocks.
    You are absolutely correct and have nailed it.

    A major means, probably the major means, of generating new continental crust is through andesitic vulcanicity, which arises from partial melting of subducting oceanic slabs. I've mentioned other mechanisms earlier, such as obducted ophiolites, accreted island arcs and oceanic plateaus.

    These mechanisms are not disputed, but some researchers maintain that recyling of continental crust and sub-crustal erosion, now balance any additions. The evidence is not clear cut, but it is tending in that direction. As I've noted in other posts if such a balance exists it is also not clear when it was reached. Estimates vary from roughly half a billion years ago to two and a half billion. At present I am sifitng through as many review and key papers as I can find on the subject from the last three decades and hope to present a summary in a week or two.

    As a side note, the terms sial and sima, which I always found useful, seem to have fallen into disuse.
    Neverfly likes this.
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    Quote Originally Posted by kojax View Post
    Good thing the Earth's opinion doesn't matter very much. .....since it doesn't hold any opinions.
    Well, technically since nearly every one has many opinions - let's say at least fifty per person - then the Earth holds 350 billion opinions, give or take.


    Quote Originally Posted by kojax View Post
    You could bring the topic of isostasy up, then maybe post a link to some site that explains the concept, or even just the wiki page.

    I don't know why you think it's productive to berate people instead of just telling them what you know. Honestly I don't.
    Then I will explain it to you. Over the years, when I provided you with correct information in a friendly and helpful manner you chose to ignore, or declare you didn't believe it, or concoct some piece of nonsense, unsupported by evidence, that would disagree with it. After a while I got fed up with that and started treating you like the fool you so often present yourself as.

    If you had said something like "What I don't quite get is why Pangea didn't create some sort of imbalance in the Earth, with all the land on one side. Is it possible that there was actually some balance of land on the other side which has since disappeared in some way?"

    That would have been a reasonable question and I would have been happy to give a fuller explanation. But what you basically said was "I don't believe that there was only Pangea. It just doesn't make any sense". That's an Argument from Incredulity and I don't respond well to posters who use it routinely. Now if your intention was more akin to my first example, then you need to learn to write more clearly.

    Despite that, I still gave you all the information you needed to understand why your argument was mistaken. I pointed out that the thickness of the crust was very small, so we were dealing with minor effects and I mentioned isostasy. I am confident that wikipedia has a good entry on it. You don't encourage me to do the work for you, because you have traditionally ignored it when I do, but here's the simplified info.

    The continents are low density, but they are thicker than oceanic crust. They sink down into the asthenosphere until they are 'floating'. So the result is that the total mass at any one point around the Earth is much more equal than the elevations of land or depressions of oceans would suggest.

    Quote Originally Posted by kojax View Post
    I'm curious what makes continental shelves stay at the height they are at. They're floating on top of magma aren't they? Why don't they sink?

    Where do they get their buoyancy? Are the ones that have land on them made of (slightly) lighter materials than the ones that do not?
    The mantle is not magma. The mantle is solid. Solids can flow. The mantle flows. Very slowly. At comparatively shallow depths some portions of the mantle undergo partial melting. The melt migrates through the mantle and may gather together in magma chambers that then initiate igneous intrusions and eruptions.
    Isostasy will answer your other questions.
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    Quote Originally Posted by John Galt View Post
    Quote Originally Posted by exchemist View Post
    There is, so far as I know, nothing to say the amounts of the sial and sima minerals are any any way fixed. There is, I understand, a huge degree of overlap in chemical composition between the two types of crust, and also between the two of them and the rocks of the mantle. And chemical reactions go on all the time interconverting minerals.

    So if there is - or has in the past been - a fractionation process, it seems perfectly possible that continental crust has grown by a gradual concentration over time of those mineral constituents that lead to lower density rocks.
    You are absolutely correct and have nailed it.

    A major means, probably the major means, of generating new continental crust is through andesitic vulcanicity, which arises from partial melting of subducting oceanic slabs. I've mentioned other mechanisms earlier, such as obducted ophiolites, accreted island arcs and oceanic plateaus.

    These mechanisms are not disputed, but some researchers maintain that recyling of continental crust and sub-crustal erosion, now balance any additions. The evidence is not clear cut, but it is tending in that direction. As I've noted in other posts if such a balance exists it is also not clear when it was reached. Estimates vary from roughly half a billion years ago to two and a half billion. At present I am sifitng through as many review and key papers as I can find on the subject from the last three decades and hope to present a summary in a week or two.

    As a side note, the terms sial and sima, which I always found useful, seem to have fallen into disuse.
    Thanks John. I'd noticed sial and sima seem not to be used much. Do people now use felsic and mafic as equivalents, perhaps? Anyway, look forward to the results of your investigation of the literature.
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    first(way the hell off topic):as/re
    Given the anti-anthropogenic global warming arguments you often give
    You have grossly misunderstood my words. I dispute the models that seem a tad negative to my tastes, and find all of them incomplete.
    given: We are in an ice age, Ice ages occupy a small fraction of earth's existance,
    Then, sooner or later, this current ice age will most likely end(it may have already done so?). I predict cities flooded, inland shallow seas, and a more warm and moist and verdent earth with less variation from the poles to the tropics.
    Meanwhile, if we are indeed still in an ice age(most likely), then it is unlikely that any greater thaw and sea level rise will do much more than to flood low lying coastal cities and islands, and maybe turn florida back into a reef, etc. ...
    ....................
    Back on topic:

    I have read variously, that 75% of the current continental crust is less than 1billion years old, and that up to 65% of continental crust was created circa 3+ gybp
    If both of these are accurate, then either much(most?) continental crust is recycled, or the (current?) steady state for our planet would be for continued expansion of continental crust volume.

    If, indeed(as quoted above) 75% is under 1 billion yrs old: Then continent building may not have slowed nearly as much as some have postulated?
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    Quote Originally Posted by John Galt View Post
    Quote Originally Posted by kojax View Post
    Good thing the Earth's opinion doesn't matter very much. .....since it doesn't hold any opinions.
    Well, technically since nearly every one has many opinions - let's say at least fifty per person - then the Earth holds 350 billion opinions, give or take.


    Quote Originally Posted by kojax View Post
    You could bring the topic of isostasy up, then maybe post a link to some site that explains the concept, or even just the wiki page.

    I don't know why you think it's productive to berate people instead of just telling them what you know. Honestly I don't.
    Then I will explain it to you. Over the years, when I provided you with correct information in a friendly and helpful manner you chose to ignore, or declare you didn't believe it, or concoct some piece of nonsense, unsupported by evidence, that would disagree with it. After a while I got fed up with that and started treating you like the fool you so often present yourself as.

    If you had said something like "What I don't quite get is why Pangea didn't create some sort of imbalance in the Earth, with all the land on one side. Is it possible that there was actually some balance of land on the other side which has since disappeared in some way?"

    That would have been a reasonable question and I would have been happy to give a fuller explanation. But what you basically said was "I don't believe that there was only Pangea. It just doesn't make any sense". That's an Argument from Incredulity and I don't respond well to posters who use it routinely. Now if your intention was more akin to my first example, then you need to learn to write more clearly.

    Despite that, I still gave you all the information you needed to understand why your argument was mistaken. I pointed out that the thickness of the crust was very small, so we were dealing with minor effects and I mentioned isostasy. I am confident that wikipedia has a good entry on it. You don't encourage me to do the work for you, because you have traditionally ignored it when I do, but here's the simplified info.
    I don't think you and I have as much posting history as you seem to think we do. There is only one topic on this forum about which I consistently express denialism and that is Big Bang Cosmology. I don't see a lot of posts from you on those threads, so I don't really know where this is coming from. Maybe this stems from the now-long-dead 9/11 threads? That was forever ago.

    I figured out a long time ago that the future of this site depends on the ability of lurkers to come on here and get useful information. Even for the most trivial question - if you answer it, and give your explanation in a way so someone doesn't have to be an expert in order to understand it, then you're helping the site.

    I'm pretty sure Google drops a lot of people in here off of search terms, so they're probably looking for information rather than idiotic banter and man-children frothing at the mouth. A lot of these threads degenerate into pairs of egotistical jerks going back and forth about who is or isn't a crank, and I would think that you of all people would want to rise above that. .... but as in other matters that could just be another thing I'm wrong about.


    The continents are low density, but they are thicker than oceanic crust. They sink down into the asthenosphere until they are 'floating'. So the result is that the total mass at any one point around the Earth is much more equal than the elevations of land or depressions of oceans would suggest.
    Yeah. From what I can tell, "Isostasy" is a fancy word for "buoyancy" when applied to geology.


    Isostasy - Wikipedia, the free encyclopedia


    [/quote]


    Quote Originally Posted by kojax View Post
    I'm curious what makes continental shelves stay at the height they are at. They're floating on top of magma aren't they? Why don't they sink?

    Where do they get their buoyancy? Are the ones that have land on them made of (slightly) lighter materials than the ones that do not?
    The mantle is not magma. The mantle is solid. Solids can flow. The mantle flows. Very slowly. At comparatively shallow depths some portions of the mantle undergo partial melting. The melt migrates through the mantle and may gather together in magma chambers that then initiate igneous intrusions and eruptions.
    Isostasy will answer your other questions.

    The continents are low density, but they are thicker than oceanic crust. They sink down into the asthenosphere until they are 'floating'. So the result is that the total mass at any one point around the Earth is much more equal than the elevations of land or depressions of oceans would suggest.
    Wiki's answer to my question appears to be "yes".

    Quote Originally Posted by wiki - isostasy
    In the simplest example, isostasy is the principle of buoyancy where an object immersed in a liquid is buoyed with a force equal to the weight of the displaced liquid. On a geological scale, isostasy can be observed where the Earth's strong lithosphere exerts stress on the weaker asthenosphere which, over geological time flows laterally such that the load of the lithosphere is accommodated by height adjustments.
    Last edited by kojax; June 22nd, 2013 at 05:38 AM. Reason: To Fix the Wiki Quote
    Some clocks are only right twice a day, but they are still right when they are right.
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  47. #46  
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    Remember that I used to be Ophiolite. Most of our interactions took place when I had that persona.

    In most instances I provide either an explanation, or links, or suggestions for further searching, or some combination. I stop doing that when I perceive my efforts are in some way disregarded by the direct recipient. Since you have been reasonable in this thread I shall put ouy back opn my Christmas Card list.

    Could you change the last quote box - I'm not sure if it was an error, or some 'between the lines' message, but you are showing my words on isostasy as being a wiki quote.
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  48. #47  
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    Quote Originally Posted by John Galt View Post
    Quote Originally Posted by seagypsy View Post
    You're words aren't very clear. And you have both lost me. But first you say there has been significant growth over time. Ok That I understand. But then you say "It is uncertain when that significant growth stopped.".... so does that mean that currently there is no significant growth? Because your statement almost directly says that growth stopped, but it is uncertain when it stopped. Is that what you meant?
    The first thing to remember is that this is a moving target. Views on continental growth have changed over the course of more than a century. The underlying theories have undergone dramatic shifts, not just the emergence of plate tectonics, but sub-sets of the theory, such as the nature of terranes, back-arc evolution, etc. Techniques of seismic investigation, isotope analysis and the like have supplied vast volumes of ne evidence from new perspectives.

    Two things emerge clearly:
    The present continental crustal mass is substantially greater today than it was in the past.
    The growth of that mass has not been steady.

    What is less clear and what is debated is whether the net addition of crustal materal ended in the Phanerozoic, or even earlier, or if it continues to day at a significant, but perhaps lower rate.

    What is not disputed, what should be known to every undergraduate geology student, what is just so baseline ordinary that it does not require citations to justify, is that there is more land around today than there was 3 billion years ago. The only caveat is that to a geologist 'land' will often be a proxy for continent. If some of the continent is covered by a shallow sea such as the South China Sea, well that's just a temporary aberration.
    Ok that makes it more clear. But remember not all readers are geology students of any level. Some just have a strong interest but no formal training. So references to further more detailed reading material about what you are stating is very helpful to those who are just wanting to learn but aren't in school.
    Speaking badly about people after they are gone and jumping on the bash the band wagon must do very well for a low self-esteem.
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  49. #48  
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    at the risk of seeming petulant, may we revisit the bolides?

    from the above, here is a link to vicki's article:
    http://www.d.umn.edu/unirel/homepage/07/e-hanseng.pdf
    wherein is stated:
    One might argue that calling on impact
    processes violates uniformitarianism, and thus
    the very roots of geology. However, perhaps
    ignoring impact events represents a modern
    form of geocentric provincialism. Both secular
    and uniformitarian philosophies, each temporaland
    spatial-dependent approaches, are critical to
    understanding Earth’s formation and evolution.
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  50. #49  
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    So what you really wanted to ask was "Do you think that the growth of continents over times, through plate tectonics, was initiated by bolide impact that triggered plate tectonic activity."

    Does that accurately reflect your question? If not what is your root question? And, for me the most important, why the heck could you not have asked it up front, instead of asking a completely different question about continental growth?

    Anyway, I don't see why HAnsen is hung up on "the inviolability of uniformitariansim". I would have thought modern geologists have abandoned that concept, sensuo stricto. One also tends to doubt her underlying rationale for favouring bolides, given the obvious impact of her location.
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    Yeh, John, that's one of the questions, and V. Hansen is one of the persons whose articles I've read.
    Like I said:
    This sprang from looking for a machanism to support or deny the concept of increased axial tilt vs "snowball earth".
    There are, for me, a few unanswered questions along this path.
    Understanding simple earth bound drivers for the growth of continents, and mechanisms for redistribution over the face of the earth would form a better platform from which to formulate some of the other questions. Or, if (at least) partially driven by impacts, and quantifiable, we would have a different platform. ...

    Are you familiar with G.C. Herman and the "Far-Field Bolide-Impact Strains on Earth"?
    Far-Field Bolide-Impact Strains on Earth
    and
    ImpactTectonics.org

    credible of crackpot?
    ............
    http://jgs.lyellcollection.org/conte...expansion.html
    ......
    are you familiar with "thermochronology.” for dating rocks?
    .....

    too many questions?
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  52. #51  
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    Quote Originally Posted by sculptor View Post
    Yeh, John, that's one of the questions, and V. Hansen is one of the persons whose articles I've read.
    Like I said:
    This sprang from looking for a machanism to support or deny the concept of increased axial tilt vs "snowball earth".
    There are, for me, a few unanswered questions along this path.
    Understanding simple earth bound drivers for the growth of continents, and mechanisms for redistribution over the face of the earth would form a better platform from which to formulate some of the other questions. Or, if (at least) partially driven by impacts, and quantifiable, we would have a different platform. ...

    Are you familiar with G.C. Herman and the "Far-Field Bolide-Impact Strains on Earth"?
    Far-Field Bolide-Impact Strains on Earth
    and
    ImpactTectonics.org

    credible of crackpot?
    ............
    Estimates of yet-to-find impact crater population on Earth
    ......
    are you familiar with "thermochronology. for dating rocks?
    .....

    too many questions?
    Sculptor, I've taken a brief look at these references, but failed to detect any theory of how these impacts are supposed to affect plate tectonics, still less how they might be supposed to lead to spurts in growth of continental crust, which seems to be what interests you. But it was only a cursory look so I may have missed the point, I suppose.

    Assuming you have read them in more depth, can you please summarise for us all what the argument is and what supporting evidence is adduced?
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    really, this ain't my forte

    From what i understood of what i read:
    The early earth had a relatively uniform crust---likely covered by a shallower ocean
    Then, something led to the formation of protocontinents(bolide impact throwing up rings of matter, or cooling and concentration of less dense crust?)
    with the beginnings of plate tectonics, the protocontinents wandered aimlessly(?) about bumping into each other forming uplift mountain chains, and growing as land vulcanism produced a less dense rock(granite,etc.)than the seafloors, further growing the land masses.

    Herman's far field impact strains, seems to need the plates to have been relatively well formed. Then the impacts would create strains far away from the impact site, increasing and altering the speed and direction of the tectonic plates.

    any gross mistakes here?
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    Any gross mistakes? Only that Herman's ideas are not, as far as I am aware, accorded much attention. You present them as though they were likely, perhaps the likeliest explanation. There are other more accepted and to my mind more plausible explanations. I don't rule it out, I suspect a large impact would certainly have an influence, but I doubt it would be primary. Also you seem to be under the impression that plate tectonics was initiated at an early time. This is not universally agreed. I continue to try to fit in a reasonable literature search and summary, but it will take time.
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  55. #54  
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    Quote Originally Posted by John Galt View Post
    Any gross mistakes? Only that Herman's ideas are not, as far as I am aware, accorded much attention. You present them as though they were likely, perhaps the likeliest explanation. ... .
    oops, That was not my intention.

    I only see possibilities so far(no conclusions), and present that which i read for others' opinions.
    Thanks fo yours.
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  56. #55  
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    Quote Originally Posted by John Galt View Post
    Remember that I used to be Ophiolite. Most of our interactions took place when I had that persona.
    I didn't know you used to be Ophiolite. I admit I wondered what had happened, and why he put "goodbye" in his signature. I was worried it might mean he had terminal cancer or something. You know: that kind of "goodbye".

    I am glad to know you are still alive.

    Anyway. I like to hope that I've become more well behaved in recent years than when I first started here.

    Also I followed the links that Seagypsy gave for past continental changes, and I can see that Pangea wasn't the original state of the continents. It's just a state they occupied temporarily. That makes more sense. There's no reason the continents should have to balanced all the time. Maybe after they finish drifting around and reconnecting on the other side of the world, that will be a new kind of Pangea, which then splits up again, and recombines again, and splits up again..... and so on.
    Some clocks are only right twice a day, but they are still right when they are right.
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  57. #56  
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    snakestone
    Funny, how appropriate the name "ophiolite" would have been to the building of continental land mass part of this thread.

    ...................
    My sculpting mentor absolutely loved carving serpentine.
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    Yeah. I should have put 2 and 2 together. Both Ophiolite and Galt have lots of knowledge about geology. I had thought it was just a coincidence.
    Some clocks are only right twice a day, but they are still right when they are right.
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    Quote Originally Posted by John Galt View Post
    I used to be Ophiolite.
    Oh, hi.

    I recall you once said something to the effect that geology owes more to the biosphere than most know. Could that help explain pulses or lulls in crust formation?
    A pong by any other name is still a pong. -williampinn
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  60. #59  
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    John?
    You any closer to posting here?
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    Quote Originally Posted by sculptor View Post
    John?
    You any closer to posting here?
    To go back on topic. The age of continental crust can be roughly measured using proxies. You can even look at it on Google Earth (See HERE). So yes, continental crust has been growing, but not steadily.
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  62. #61  
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    Quote Originally Posted by sculptor View Post
    John?
    You any closer to posting here?
    Further away than ever. I've got about thirty heavy papers and fifty or so abstracts to wade through and the numbers keep growing. My original contention stands, but the detail is fascinating. Florian's summary statement is accurate. What is in dispute is the rate and the variations in that rate and the causes of those variations. Excellent arena - I'm almost ready to switch my allegiance from basalts to granites.

    I recall you once said something to the effect that geology owes more to the biosphere than most know. Could that help explain pulses or lulls in crust formation?
    Interesting thought. Offhand I doubt it could be a primary driver, but in moderating the carbon cycle and erosion rates it would certainly have an influence of a kind.
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    The biosphere changes surface chemistry (oxidation state) but I have a hard time to link it to pulses of continental growth. These pulses should be related to whole mantle events.
    I would say that continental growth is all about migration of volatile rich fluids in the mantle. That is happening for arc volcanism for sure, but there are also some evidence for orogens as well.
    I mean, for the latter, the tibetan plateau is just above two large fluid-rich channels that are flowing eastward, then into Indonesia thru the himalayan eastern syntaxis. Regarding that point, Bai et al (2010) Nature Geoscience is a must read.
    Last edited by florian; July 3rd, 2013 at 11:02 AM.
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    Correction:

    At no time did John Galt endorse steady or constant growth. A PM from John Galt reminded me of that reading comprehension error on my part.
    He had said that the consensus is that growth has occurred over time- there is a difference and I was in error.
    KALSTER likes this.
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