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Plate Tectonics

  1. Nov 4, 2011 #1
    The Asthenosphere and Lithosphere

    We can all agree that the Earth is rotating on its axis, making one rotation about every twenty-four hours. We can also agree that the Earth is revolving around the sun in its happy 364-day elliptical orbit. Correct? But why? We have proof to support these ideas. We can observe night and day and the changing positions of the sun as the Earth rotates and we can see the change in seasons as it revolves (1). Would you believe that the Earth makes its own movements within?

    The Earth is composed of four layers: the inner core, outer core, mantle, and crust. The crust (or lithosphere) is divided into sections called plates. The mantle, also referred to as the asthenosphere, is a hot liquid layer, made up of magma, or molton rock. The magma closest to the core is the hottest, and thereby less dense than the magma near the crust. What happens next? The less dense magma cannot support the denser magma so the hot, less dense magma shifts upward while the denser molton rock near the core. This is all repeated again, continuously as the cooler magma is heated. This process is called convection currents (2). The plates in the lithosphere glide over the top of this magma, causing them to reposition themselves over the globe.

    How can we accept this theory? If we go back to the motions of the Earth through space, we can make daily observations that will support the idea. With this plate techtonic theory, proposed by Alfred Wegener, we cannot go outside and see the ground in motion, nor can we obtain a satillite picture and see a change in the continents from what they looked like yesterday, or even last decade. So we would naturally be forced to think this is false, which is what scientists did when Wegener conceived this hypothesis. After initially being dismissed, evidence came forth that facilitated the proving of plate techtonics. It was found that the continents have a "puzzle-like" fit, telling us that they could have one day been together and broken apart. Also the same fossils of organisms only thought to have been in Africa were found as well in the Americas. These creatures could not have swam across the ocean, so there must have been land connectiong them. The same rock layer have even been found in different continents, indicating that they were once joined. Given this evidence, we can now support this theory, as scientists today do.

    So, what happens to these continents? They obviously do not all travel in the same direction. This is where we get into the effects of plate techtonics.

    The Earth's season are caused by its 23.5 degree tilt on its axis, making either the northern or southern hemisphere closer to the sun. The seasons change as the Earth rotates around the sun. With the Earth's axis always pointing in the same direction (Polaris, or the North Star), different hemispheres will be closer to the sun with each few months.

    2. For a good demonstration of convection currents, fill a large, clear container with cold water and two small cups with boiling water. Place the two cups side by side, about five or six inches apart (depending on the size of the container). Place the large container of cold water on top of the two cups. Put a few drops of food coloring (blue, red, or green) in the water and you will see how the water closer to the heat source rises and the cooler water falls, etc.

    The Effects of Plate Tectonics

    As we have stated earlier, the plates do not move in the same directions, making way for other plates. For example, The plate that contains North America is colliding with the plate that contains the Pacific Ocean, and at the same time, it is moving away from the Atlantic plate.

    There are different interations between plates. Collisions occur when two plates of the same density (Oceanic plates are more dense than continental plates) collide, resulting in a mountain range. Diversions occur when any two plates move away from eachother, resulting in a rift, ridge, or rise. Subductions, a kind of collision, occur when two plates of different densities collide. The denser plate, the oceanic plate, collides with the continental plate and begins to sink under it. These plate collisions sometimes cause volcanic eruptions. An example of a subduction that we already mentioned is North America's collision with the Pacific plate. The oceanic plate is sinking below the continent, causing great deals of earthquakes and volcanoes, like Mt. St. Helens on the west coast of North America. Another way that plates may interact is when they move against one another, side to side, creating faults. An example of this is the San Andreas fault in California.

    The moving of these plates do not occur over night. It has taken millions of years since the continent of Pangea broke apart (3). Pangea was a super continent that was thought to have existed in the Mesozoic Era in the geologic time era. Given this, we will not see major changes in the continents in our lifetimes.

    So, the next time someone ask you why volcanoes and earthquakes always occur in one area, you will tell him/her: That area is located on a plate boundary, a place where much stress in built up in the collision/ diversion motion of the plates all caused by plate tectonics.

    3. A long time ago, more than 65 million years, a super continent existed. This was the time that all of today's continents were joined into one, creating one continent, Pangea, and one ocean Panthalassa. Later, the continent was split into two, Gondwanaland and Laurasia. From there, caused by plate techtonics, we got our modern-day continent positions.
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  3. Nov 4, 2011 #2


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    was there a real question in all that or was it just a huge statement? :)

    you seemed to be answering some of your own questions and a few false assumptions ;)

    Ahhh ... but we can over years and decades. EVERY earthquake defines plate tectonics in motion. The plate motion is measureable over just 1 year across an active plate boundary region. You DONT need millions of years to see the results.
    Satellites do see the changes for us. GPS satellites gave geologists a whole new insight into tectonics. We can very accurately measure motion and changes of position of fixed objects by GPS triangulation.

    Many of the plate boundaries have quite substantial motion eg in the SW Pacific, the Tonga Kermadec Trench area has motions of upwards of 8 cm / year That is very easily measurable with today's technology. This motion builds up substantial stress across the area and when a major quake happens eg the Mw9+ offshore NE Honshu, Japan earlier this year. That stress resulted in a motion of some 20 metres of movement across the fault line.

    Back in my old home country of New Zealand, long term motion of a fault line is observable just by going through some farmland paddocks. ~ every 250 - 300 years the Waiarapa Fault in the southern North Island lets rip in a major quake. Offsets for the last 3 events are visible as individual 12 metre offsets with each event. And down on the south coast of the Nth Island you can go back some 5000 years and see multiple events as each event has raised the beach several metres. There is evidence of 6 - 7 major events along the coastline. The last Waiarapa event was in 1855 and was estimated at ~ Mw8.0.

    Tho Alfred Wegener did the early proposals of plate tectonics, it took Harold Wellman, of New Zealand, to put it all together with practical observations of what was occurring in New Zealand as he criss-crossed the country doing mapping during the early - mid 1900's.

    Others around the world confirmed these ideas with the work done on mapping seafloor magnetic banding anomalies etc. Plate tectonics moved out of the realm of just a theory and into the real world of a practical explanation of observations.

  4. Nov 5, 2011 #3
    Please take the following positively. My objective is to help you develop your understanding of this fascinating topic.

    There are several errors here.

    The mantle and the asthenosphere are not equivalent. The asthenosphere is a thin part of the mantle immediately below the lithosphere. It is closely associated with the LVZ (Low Velocity Zone).

    The mantle is not liquid or molten. There are portions of the mantle where partial melting has occured. The proportion of molten material in these zones is generally less than 20%.

    [Molten is not spelt molton.]

    Some of the rock, not magma, and not all of the rock, near the core is hotter and less dense than the adajcent rock. Not less dense than the rock (not magma) near the crust, just less dense than the adjacent rock. The load of thousands of kilometres of overlying rock increases its density through compression and phase changes.

    You have, wisely, not indicated whether the plates are moved by the convection currents, cause the convection currents, or are independent of them. Slab pull, where a plate sinks into the mantle is thought to be an important cause of plate movement; more important than the push from the mid-ocean ridges.

    Wegner did not propose plate tectonics, but continental drift. While there are similarities there are also, arguably significant differences. Plate movement related to convection currents was first proposed in 1928(?) by Arthur Holmes. It took the work of the likes Hess, Diaz, Pinchon, etc in the 1950s and 1960s to evolve the present theory - a theory, that as far as internal mechanisms is concerned, is still an area of active research and strong disagreements.

    As davenn has pointed out we can measure the movements of the plates, perhaps not on a daily basis, but certainly on an annual one.

    Even if we could not measure the motion annually why would we think it is false when there is such an abundance of independent evidence supporting it? And scientists did not reject the idea on those grounds, but rather on the grounds that 1) there was no obvious mechanism, 2) meterologists don't understand geology.

    These were the very facts that Wegner based his ideas on, so I ask again why would we reject an idea that has other supporting facts? Your argument is confusing me. I hope you will clarify it.

    You seem to have misunderstood why the winter hemisphere is colder. It is not due to distance to the sun, but the height of the sun above the horizon.

    @PhysicsPost - Clearly you have a real interest in this subject. I applaud that. But I encourage you to continue your reading in the field, because many of your facts are faulty and your understanding is therefore compromised. I and others here would be happy to answer any questions to help develop that understanding and interest.
  5. Jan 31, 2012 #4
    This is more of a very basic lecture than an open dialogue or question.

    I will add, that one of the ways this was proven was using submarines to explore the sea-floor during the Cold War era, for defense purposes. It was found that the floor was symmetrically stretching in between Europe and U.S.
  6. Feb 10, 2012 #5
    I suspect you are confusing two separate things.

    Seafloor spreading was identified from magnatic anomalies by Vine and Mathews off of the Pacific coast of the US and Canada. (Vine, F.J. & Matthews, D.H. (1963). Magnetic anomalies over oceanic ridges. Nature 199, 947–9.) As far as I am aware the data were gathered from magnetometers towed by surface craft not submarines. W.J.Morgan may have also have identified the phenomenom around the same time and that might have been in the Atlantic, however I can't recall exactly where and when he published.

    Submarines come into play with Venning Meinez persuading the Dutch government to 'lend' him a submarine with which he made extensive gravity measurements, thereby identifying the Bouger anomalies associated with submarine trenches. (Meinesz, V.F.A. (1932) Gravity Expeditions at Sea: 1923 – 1930 Vol. 1 The Expeditions, the computations and the results. Netherlands Geodetic Commission.)
  7. Feb 18, 2012 #6
    Nice post! It really explains a lot and give me more information to understand. What I just know about plate tectonics are the land which has some boundaries that can cause earthquake when it moves.
  8. Feb 18, 2012 #7


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    FYI the username PhysicsPost is actually not a person. Or rather it is, but it is the admins moved a bunch of articles and stuff from another part of the website and into PF, this article being one of them.
  9. Feb 18, 2012 #8
    Almost, but not quite.
    During World War II, the US Navy (and likely other naval forces) used ship town magnetometers as a means to detect submarines. During their measurement they detected magnetic anomalies from the magnetized seafloor. Detecting seafloor magnetism was not unexpected, they knew how strongly basalt could be magnetized, but they did note unusual patterns in the magnetic anomalies. Not being interested in the anomalies and in the middle of a war, nothing was done with data and much of it was classified.

    And as Ophiolite points, Vine and Matthews published their seminal work on the topic in 1963. But it was Lawrence Morley who published the same idea at the same time, not Jason Morgan. There is a bit of story behind it all, but Morley had his paper crudely rejected in early 1963 by an editor who described it as pure speculation and not worthy of publication. As a result Vine & Matthews got their paper out first.

    Morley got his article published the following year:
    Morley, L. W., and Larochelle, A., 1964, Paleomagnetism as a means of dating geological events, R. Soc. Can. Spec. Publ., vol 8, 39-50.
  10. Mar 17, 2012 #9
    If you like geology, do not read this.

    Ha ha of course I'm kidding. :b
  11. Apr 29, 2012 #10
    I think I have just messed up an attempt to post this question elsewhere. If so, that could be a good thing, as this seems a good place for it.

    Looking at a plate tectonics world map, it seems that plates are converging on Africa from both East and West, with no area of subduction between. How can this be? Where is the crustal material going? Please don't say "up or down". :)
  12. Apr 29, 2012 #11
    It is being subducted under the Eurasian plate. The Mediterranean Sea is getting smaller and in about half a million years the strait of Gibraltar will close and the Mediterranean will dry up. In maybe ~ 50 million year the relic sea will be gone completely. Then, during a phase of mountain building as the continental crusts collide... Africa will go up! :smile:
  13. Apr 29, 2012 #12
    Thanks geo101. That seems to make sense, now it's back to the map to make sure I really have got my head round it.
  14. May 1, 2012 #13
    Geo101, I thought you might be interested to know what I had done with your information.

    I was involved in a discussion in another forum. I posted

    Obviously, I have not reproduced the map here.

    The response I received started:

    At times its very difficult to avoid being drawn into "ad hominem" exchanges with this poster, but I try. Just wish me luck, and be on hand when I run into trouble>
  15. May 1, 2012 #14
    Well its good to know that my amateur status is also secure. :)
  16. May 1, 2012 #15


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    hahaha likewise ;)


    then ask those other suspects to expain the volcanics of the Mediterranean region and NO they are NOT hotspots!!!

    its a mixture of subduction and collision hence all the volcanics and also the Swiss Alps and other thrusted up areas of the region
    there was a time so very long ago when the opposite was true and the Mediterranean was opening up (extensional tectonics) which what initially allowed Africa and Europe to separate at Gibraltar and allowing the Mediterranean basin to fill with water
    Hmmmm .... wonder if anyone has ever calculated what that would have done to worldwide seal levels ??

  17. May 1, 2012 #16


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    delving a bit into some tectonic maps found this nice one ....

    It shows that there is possibly still extensional and strike-slip faulting around the Gibraltar and out to the Mid Atlantic Ridge region. But it clearly shows that the eastern Mediterranean from Sicily to Middle East/Turkey region is a subduction/collision zone.

    This was backed up by a couple of other sites I visited :)
    so :tongue: to that guy on the other forum haha

  18. May 1, 2012 #17


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    wow here's a cool site.. lots of animated movies on mantle convection etc by CIT ( Caltech)

    just stumbled across it when looking for those other sites
    enjoy!! :)

  19. May 1, 2012 #18
    Thanks Dave. Lots of interesting looking stuff. I know in advance that none of it will make any difference to the chap on the other forum, his mind is made up, but I find the hunt for counter arguments is a good exercise.
  20. May 1, 2012 #19


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    A closed mind is such a sad thing :( unfortunately you will meet many like that as you go through life

    My advantage, like many others, is I never claim to be an expert. That leaves me open to new ideas/ different ways to look at things

    do you have any univ. geology studies ? or just a passing interest?

  21. May 1, 2012 #20
    In 1983 I started Earth Science with the OU, but my circumstances changed drastically and I stopped at 2 credits. Only recently have I started dabbling a little again. (in geology, not the OU) So I guess that makes me a dabbler, of sorts.
  22. May 1, 2012 #21


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    OU ? which uni is that ?
    dont forget we are worldwide on the forum :)

    I did my studies at OU = Otago University, Dunedin, NZ

  23. May 1, 2012 #22
    And it is good to know my professional status is very secure! I'm a geophysicist, not specializing in tectonics, but a basic knowledge is useful to say the least :smile:

    What else was his argument, other than "wrong, wrong, wrong!!"??

    I guess he is either one of two groups...
    1) Plate tectonics doesn't exist since the world is only 5000 years old
    2) The Earth is flat!

    [I say this jokingly and mean no disrespect]

    A third option does exist, which tentatively supports the idea the Africa is not subducting under Eurasion....

    At the moment subduction has largely stalled. And there is some seismic evidence to suggest that a new subduction zone might be forming along the northern coast of Africa (northern Algeria I think). It may be possible that Eurasia will be subducted under Africa.

    This was presented at a conference a couple of years ago, but I'm not too sure if it has been published yet. I am not too sure how reliable the data, so I would treat it as an interesting, but still speculative, future possibility.

    The OU, Open University, UK??
  24. May 1, 2012 #23


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    what's your main field of interest ? I didnt go into geology professionally, am already a full time electronics tech.
    I did geol at univ for the pure enjoyment

    Now that would be an interesting situation where the subduction/ collision zone reverses direction!! Imagine a few volcanoes popping up in nthrn Africa!
    Back in my home area, Dunedin, New Zealand was not a subduction area but there was a distinct change in tectonics ~ 5 Ma where it went from extensional with lots of volcanics to collision that produced the Southern Alps that we still see growing today.

  25. May 2, 2012 #24
    My focus is magnetics, mainly rock magnetism and paleomagnetism. I don't work on tectonic applications of paleomag, mostly the evolution of the ancient field
  26. May 2, 2012 #25
    "By George, he's got it!" Open University, UK, is what I meant. Sorry to be vague; could I be more insular than I thought? :)

    As for the other poster; he is a ferocious supporter of Kevin Mansfield’s “collision” theory. When Pre-Earth and the “old moon” collided Pangaea broke up and the continents moved to their present positions. He tends to present his arguments “loudly” and repetitively in three different threads, so it is easy to avoid actually answering questions.

    I am genuinely interested to know how Mansfield’s theory can be supported in the light of modern knowledge, but he seems to assume that anyone who asks questions is a brain washed supporter of plate tectonics. Only recently have there been signs that my persistence may be paying off. One has to walk a delicate line.
    Last edited: May 2, 2012
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