Notices
Results 1 to 17 of 17

Thread: Seafloor Spreading and Paleomagnetism

  1. #1 Seafloor Spreading and Paleomagnetism 
    Forum Sophomore
    Join Date
    Sep 2005
    Posts
    106
    1) "...Finally, in 1965, 2 groups of scientists working independently of each other disccovered a previously unknown reversal in the earth's magnetic field. 1 group discovered the reversal in rocks on land, and the other group discovered the reversal in rocks on the ocean floor. The dates of both reversals were exactly the same. This was clear evidence that the earth's magnetic polarity does reverse itself and that the ocean floor does spread."

    I really don't get this paragraph! Why would this be the final and the most important evidence of reversing magnetic polarity and seafloor spreading. What does it MEAN if the dates of both reversals found in land rock and seafloor rock are the same? Can someone explain briefly, please?

    2) "Wegener studied the distribution of coal (deposited in old swampy areas), sand-dune deposits, and salt deposits and was able to determine climate belts on Pangaea"

    Why would this be a strong evidence of continental shift?

    I hope someone can explain to me! Thank you :wink:


    Reply With Quote  
     

  2.  
     

  3. #2  
    Forum Radioactive Isotope cosmictraveler's Avatar
    Join Date
    May 2005
    Location
    Key West, Florida, Earth
    Posts
    4,788
    It means that rocks that were examined from both locations were showing the same results and both those samples were taken from different locations, one on the land and the other under the sea, but had the same reversing results for their time periods they were made.


    Reply With Quote  
     

  4. #3  
    Universal Mind John Galt's Avatar
    Join Date
    Jul 2005
    Posts
    14,168
    First to expand on CT's reply: the banding effect on the ocean floor (with rocks of alternating polarity, evenly distributed on either side of mid-ocean ridges*) suggested that sea floor spreading was occuring. But some other mechanism could have been at work. Finding terrestrial reversals (again, from memory, in a vertical lava sequence) with the same timings, pretty well eliminated other mechanisms as an explanation, leaving sea floor spreading as the simple and elegant solution.

    By identifying the locations of particular climatic belts at different times in the Earth's history Wegener could show that their locations made little or no sense. If continental drift was allowed then the climatic belts fell into place.
    As an example, the Devonian period in the UK is represented by sandstones, shales and conglomerates of the Old Red Sandstone. These were laid down in a desert or semi-desert environment. We now know the UK was south of the equator at this time. It drifted across the equator during the Carboniferous, when the tropical forests flourished that would give rise to coal deposits. Then, as the northward drift continued, desert conidtions again prevailed in the Permo-Triassic period, with sediments in the UK assigned to the (imaginatively named) New Red Sandstone.

    *If my memory serves me correctly the first oceanic polarity reversals that were identified, off the north west coast of the US and Canada, did not show the mirror image distribution, as the mid-ocean ridge was no longer mid-ocean, but was being overidden by the North American continent.
    Reply With Quote  
     

  5. #4  
    Forum Sophomore
    Join Date
    Sep 2005
    Posts
    106
    Thanks cosmictraveler and Ophiolite!

    I think that the banding effect on the ocean floor is justing saying that rocks have alternating polarity, and they are mirror images on either side of the mid-Atlantic ridge. And seafloor spreading is saying that the seafloor is spreading, and when magama wells out, they are quickly solidified and the earth's magnetic polarity is recorded at that time in hisotry.

    But how would it be possible to have a LAND rock with reversed polarity? (it is no longer in the ocean) And why would the dates of both reversals (reversals on land rock and ocean rock) be exactly the same and what is its significance?

    Why would this be the clear evidence that the earth's magnetic polarity does reverse itself and that the seafloor does spread? (In my mind, Earth's reversed magnetic polarity is discovered earlier when scientists found ocean rocks pointing to the south (align with earth's magnetic field at that time). Seafloor spreading is discovered earlier when scientists found alternating polarity with mirror images on either side of the mid-Atlantic ridge.)
    Reply With Quote  
     

  6. #5  
    Forum Sophomore
    Join Date
    Sep 2005
    Posts
    106
    Quote Originally Posted by cosmictraveler
    It means that rocks that were examined from boyj locations were showing the same results and both those samples were taken from different locations, one on the land and the other under the sea, but had the same reversing results for their time periods they were made.
    I understand this part and but don't quite understand its meaning. :?
    Reply With Quote  
     

  7. #6 Re: Seafloor Spreading and Paleomagnetism 
    Forum Cosmic Wizard SkinWalker's Avatar
    Join Date
    Apr 2005
    Location
    Grand Prairie, TX
    Posts
    2,377
    Quote Originally Posted by kingwinner
    Why would this be the final and the most important evidence of reversing magnetic polarity and seafloor spreading. What does it MEAN if the dates of both reversals found in land rock and seafloor rock are the same? Can someone explain briefly, please?
    First, it is also significant that the bands of reversal are present on either side of the mid-oceanic ridges. This is where it was first discovered that the ridges themselves were a result of material from within the earth was being pushed up then out on either side and slowly being "spread" away from the ridge. The magnetic bands were created when the orientation of the magnetically influenced molecules were affected by the earth's current magnetic orientation at the time of formation when the rock was still molten.

    It is the same for the magnetic orientations in rock on land. Basically, what you do is use the sea-floor banding as the standard then match up the sequence of orientation found in rock on land. The gist of it is, the further from the mid-ocean ridge you are, the further back in time the terrestrial sequence is from.

    This is cross-referenced, compared and contrasted with other methods of dating and analyses and offers some very reliable methods of dating or corroborating the dates of various strata on land.

    The reason why you have the effect present in terrestrial rocks is because they orient when the rock is molten.
    Reply With Quote  
     

  8. #7  
    Forum Radioactive Isotope cosmictraveler's Avatar
    Join Date
    May 2005
    Location
    Key West, Florida, Earth
    Posts
    4,788
    Take a pebble and drop it into a bucket of water and notice that the ripples flow outward from the center where the pebble was dropped. That's the same thing that's happening with land masses all over, they are moving away from each other and they all started out at the same point wgich means some went into the sea and some went onto the land as it moved. I hope that helps.
    Reply With Quote  
     

  9. #8  
    Universal Mind John Galt's Avatar
    Join Date
    Jul 2005
    Posts
    14,168
    Quote Originally Posted by kingwinner
    But how would it be possible to have a LAND rock with reversed polarity?
    It doesn't matter where the rock is. When it solidifies from magma the orientation of magnetic minerals are set by the current magnetic field. Lavas from volcanoes on land are subject to this in exactly the same way as lavas from the mid-ocean ridges.
    If there is a reversal of the Earth's magnetic field during the lifetime of a volcano (and they have long lives) then we will be able to detect that difference in orientation as we move up (or down) the lava pile. This is why I emphasised the word vertical in my previous post.

    Quote Originally Posted by kingwinner
    And why would the dates of both reversals (reversals on land rock and ocean rock) be exactly the same and what is its significance?
    They have to be the same. The reversal of the Earth's magnetic field is global in scope. The whole planet is effected. Any lavas extruded under orientation A will be influenced in the same way, whether on land or sea. Then the orientation changes - at a specific date*. All lavas erupted after this date, wherever thay are, will show this new orientation. It was the fact that the reversals always matched up with the dates that showed we were dealing with a global effect, not some local mechanism.

    CT, your analogy left me completely confused, and I was a geologist before I grew up.
    Reply With Quote  
     

  10. #9  
    Forum Sophomore
    Join Date
    Sep 2005
    Posts
    106
    It is the same for the magnetic orientations in rock on land. Basically, what you do is use the sea-floor banding as the standard then match up the sequence of orientation found in rock on land. The gist of it is, the further from the mid-ocean ridge you are, the further back in time the terrestrial sequence is from.
    Why is it not sufficient to prove seafloor spreading without this reversed land rock?
    Is the reversed land rock and the reversed oceanic rock mentioned in the paragraph in equal distance from the mid-Atlantic ridge (because of the mirror images of the stripes)? I mean, are there in the mirror image regions or zones? (say, both are 50 stripes from the mid-ocean part where magma wells out?)
    Reply With Quote  
     

  11. #10  
    Forum Sophomore
    Join Date
    Sep 2005
    Posts
    106
    Finally, in 1965, 2 groups of scientists working independently of each other disccovered a previously unknown reversal in the earth's magnetic field
    What does it mean by a "previously unknown reversal"? Like pointing directions other than south and north?

    It doesn't matter where the rock is. When it solidifies from magma the orientation of magnetic minerals are set by the current magnetic field. Lavas from volcanoes on land are subject to this in exactly the same way as lavas from the mid-ocean ridges.
    If there is a reversal of the Earth's magnetic field during the lifetime of a volcano (and they have long lives) then we will be able to detect that difference in orientation as we move up (or down) the lava pile. This is why I emphasised the word vertical in my previous post.
    Why do we have to consider lavas welling out far away from the mid-Atlantic ridge to prove seafloor spreading?


    They have to be the same. The reversal of the Earth's magnetic field is global in scope. The whole planet is effected. Any lavas extruded under orientation A will be influenced in the same way, whether on land or sea. Then the orientation changes - at a specific date*. All lavas erupted after this date, wherever thay are, will show this new orientation. It was the fact that the reversals always matched up with the dates that showed we were dealing with a global effect, not some local mechanism.
    But the earth's magnetic field had reversed many times, why should they have to (reversed land and oceanic rock) be of the same time period?
    Reply With Quote  
     

  12. #11  
    Universal Mind John Galt's Avatar
    Join Date
    Jul 2005
    Posts
    14,168
    Quote Originally Posted by kingwinner
    Why is it not sufficient to prove seafloor spreading without this reversed land rock?
    Seafloor spreading has subsquently been demonstrated in a variety of ways. Remember that at the time the reversed polarity stripes on the seafloor were identified there was no mechanism available to explain them. I do not recall the particulars, but very likely several mechanisms were proposed to account for the observations. Seafloor spreading would have been one of these. But seafloor spreading required that the cause of the magnetic stripes was a periodic reversal of the magnetic field which would have to be global in extent. If it was global in extent we should expect to find it in terrestrial lava flows and to find that the timing of the polarity changes was the same in each instance. This is what was found and this eliminated the alternative explanations.
    Quote Originally Posted by kingwinner
    Is the reversed land rock and the reversed oceanic rock mentioned in the paragraph in equal distance from the mid-Atlantic ridge (because of the mirror images of the stripes)?
    The reversed land rock has no spatial relation, other than chance, to the mid-ocean ridges. There is no continental spreading occuring (that's a simplification, but any continental spreading either stalls - like the East African Rift Valley- or goes on to form an ocean - like the Red Sea.)
    Reply With Quote  
     

  13. #12  
    Forum Cosmic Wizard SkinWalker's Avatar
    Join Date
    Apr 2005
    Location
    Grand Prairie, TX
    Posts
    2,377
    Quote Originally Posted by kingwinner
    Why is it not sufficient to prove seafloor spreading without this reversed land rock?
    By 'reversed,' I'm assuming you mean the magnetic orientations of the minerals in the rock.

    I'm not sure that sea-floor spreading *isn't* sufficiently explained by simply examining the mid-oceanic ridges and the iron-bearing minerals like magnatite. Their orientations change in stripes or bands that run parallel to the ridge and switch orientation within each band to indicate north/south pole reversals in the planetary magnet. But of course, you already know this.

    These stripes/bands are identical on either side of the ridge, indicating that the sea floor spreads outward from the ridge where the rock is pushed up and out after reaching its Curie point (580 degrees C for magnatite), at which point the orientation of the minerals is "frozen" as the rock cools. If the rock were to be re-heated to 580 C, then the minerals would re-orient to whatever the current condition of the planet's poles are.

    The good thing about the relationship of sea-floor spreading to land-based geology is that you have a master-sequence to which you can compare and derive dates of strata.

    ****+++*******+***++***++++++ MAR ++++++***++***+*******+++****

    Think of the above as the sequence for the Mid-Atlantic Ridge. "MAR" refers to the ridge itself, "+" is a north pole orientation, "*" is a south.

    If you found a geologic strata that had this partial alignment:

    ***++***+*******+++

    You could, theoretically, match it to a portion of the master sequence found with the oceanic ridges. It wouldn't be perfect and precision/accuracy would depend upon how much of a sequence you can get from the strata you are examining. Obviously, the more data you can get from the strata, the easier it would be to match it to the sea-floor sequence. But even with small data sets, it is possible to make several possible matches then use deduction to rule out all but one (perhaps the strata above or below are already known).

    Quote Originally Posted by kingwinner
    Is the reversed land rock and the reversed oceanic rock mentioned in the paragraph in equal distance from the mid-Atlantic ridge (because of the mirror images of the stripes)?
    I think you can probably infer the answer to this from the above text, but let me try anyway in case I wasn't clear enough (I tend to ramble about such things).

    The opposing sides of the ridge will match, mirror-like because of the spreading out effect from the middle. But with strata involved in volcanic events (lava flows, for instance), you might only have one set. However, a volcano will often spew lava relatively equally from the vent, so you can get multiple sequences in various localities surrounding the volcano that match. These sequences are measured from the volcano's pipe or neck.

    These are all great questions, kingwinner! And welcome to The Science Forum! We hope you'll stick around.
    Reply With Quote  
     

  14. #13  
    Forum Sophomore
    Join Date
    Sep 2005
    Posts
    106
    "...Finally, in 1965, 2 groups of scientists working independently of each other disccovered a previously unknown reversal in the earth's magnetic field. 1 group discovered the reversal in rocks on land, and the other group discovered the reversal in rocks on the ocean floor. The dates of both reversals were exactly the same. This was clear evidence that the earth's magnetic polarity does reverse itself and that the ocean floor does spread."

    So they are just comparing the reversals times of oceanic and land rock to see if they are matched, right? Before they have found the "previously unknown reversal ", the reversal timescale is incompleted and there exists slight contradictory from land data and oceanic data, true?

    So does this paragraph simply mean that the reversal timescale data obtained from land rock and oceanic rock are found to be exactly the same? That is, every time period of reversals throughout the earth's history matched exactly(according to the 2 sets of data from land rock and oceanic rock). Therefore, the earth's magnetic field must have been reversed itself many times which further supports the hypothesis of seafloor spreading. Am I right?

    I searched the web and have found a paragraph stating this, which draws me to the above conclusion:

    "Vine and Wilson published a paper in October 1965 proposing a model for seafloor spreading in the northeastern Pacific, using as evidence bands of reversed magnetism that marched out from either side of the ridge. Shortly thereafter, a slight discrepancy between the seafloor reversal bands and the timing of known field reversals on land was smoothed out by a new land-based field reversal discovered by Doell and Dalrymple. With this addition, the two sets of data matched astonishingly well.
    "
    http://www.beyonddiscovery.org/conte...page.asp?I=233
    Reply With Quote  
     

  15. #14  
    Forum Sophomore
    Join Date
    Sep 2005
    Posts
    106
    http://www.beyonddiscovery.org/conte...page.asp?I=233

    But another thing that bothers me is that the web seems to be talking the Pacific Ocean instead of the Atlantic Ocean. I thought Seafloor Spreading is just talking about the mid-Altantic ridge because the Atlantic Ocean is spreading, or getting wider, throughout the earth's history. However, the Pacific Ocean must be getting smaller becuase the area of the surface of the earth stays the same. So there is no way to have the Pacific Ocean spreading. How can seafloor spreading be occuring at the Pacific Ocean, or seas other than the Atlantic Ocean?
    Reply With Quote  
     

  16. #15  
    Universal Mind John Galt's Avatar
    Join Date
    Jul 2005
    Posts
    14,168
    Kingwinner, I need to find you a good site that explains this in a straightforward fashion for you. In the meantime here are some quick responses.
    Seafloor spreading is occuring along all the mid-ocean ridges, in the Atlantic, the Indian and the Pacific oceans. If you refer back to my first post (I think) you will see I refer to the occurence of banded zone of the North West of North America (i.e. the NE Pacific). When this was first observed we didn't know what it meant. The explanation of seafloorspreading and magnetic reversals came later to account for the observations. You have found the references to that already.

    The North American continent is riding up over part of a Paciifc mid-ocean ridge, which isn't mid-ocean anymore.

    Look, the Earth's surface is divided into plates. These plates meet at one of three kinds of boundary.
    1. Transform - the plates slide past each other
    2. Divergent - this is the mid ocean ridge type where the plates separate and lava wells up from the mantle
    3. Convergent - two plates collide. The oceanic plate plunges down below the continental plate, melts and generates volcanoes above it - the Andes for example. Two continents collide and you get rather large mountains - e.g. the Himalayas.

    Apologies for very rushed reply - I'm fitting this into a break at work. Will attempt a fuller response later.
    Reply With Quote  
     

  17. #16  
    Forum Sophomore
    Join Date
    Sep 2005
    Posts
    106
    "...Finally, in 1965, 2 groups of scientists working independently of each other disccovered a previously unknown reversal in the earth's magnetic field. 1 group discovered the reversal in rocks on land, and the other group discovered the reversal in rocks on the ocean floor. The dates of both reversals were exactly the same. This was clear evidence that the earth's magnetic polarity does reverse itself and that the ocean floor does spread."

    So they are just comparing the reversals times of oceanic and land rock to see if they are matched, right? Before they have found the "previously unknown reversal ", the reversal timescale is incompleted and there exists slight contradictory from land data and oceanic data, true?

    So does this paragraph simply mean that the reversal timescale data obtained from land rock and oceanic rock are found to be exactly the same? That is, every time period of reversals throughout the earth's history matched exactly(according to the 2 sets of data from land rock and oceanic rock). Therefore, the earth's magnetic field must have been reversed itself many times which further supports the hypothesis of seafloor spreading. Am I right?
    Can somebody see if I have made the right conclusions? (I hope I have not mistaken these...)

    Besides, thanks Ophiolite for your patience and your detailed explanations to all of my questions! Thank you again! :wink:
    Reply With Quote  
     

  18. #17 Enough 
    Forum Freshman
    Join Date
    Dec 2005
    Posts
    5
    Quote Originally Posted by kingwinner
    1 group discovered the reversal in rocks on land, and the other group discovered the reversal in rocks on the ocean floor.
    Yeah, you're right, but you can only do that for so long.. d'you know what i mean?
    Reply With Quote  
     

Bookmarks
Bookmarks
Posting Permissions
  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •