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  1. #1 D'' 
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    At the bottom of the mantle there is a mysterious layer -- it is called D" (pronounced 'dee double prime') after Bullen's 1949 classification for the layers of the Earth, but was first discovered in 1924 by a graduate student named Dahm. Some say this layer is comparable to the lithosphere, it being the "other" boundary to the vast mantle. It may indeed possess its own tectonics, with large provinces comparable to continents and oceanic regions. It may even be part of the plate tectonic cycle; it is a working hypothesis that tectonic plates penetrate right down to this layer.


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    Forum Freshman jlhredshift's Avatar
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    Quote Originally Posted by billiards View Post
    At the bottom of the mantle there is a mysterious layer -- it is called D" (pronounced 'dee double prime') after Bullen's 1949 classification for the layers of the Earth, but was first discovered in 1924 by a graduate student named Dahm. Some say this layer is comparable to the lithosphere, it being the "other" boundary to the vast mantle. It may indeed possess its own tectonics, with large provinces comparable to continents and oceanic regions. It may even be part of the plate tectonic cycle; it is a working hypothesis that tectonic plates penetrate right down to this layer.
    My Bold

    Tell me more. Are there Papers. Fascinating!


    I'm sorry, my responses are limited. You must ask the right question.

    "The track of a glacier is as unmistakable as that of a man or a bear, and is as significant and trustworthy as any other legible inscription"
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    "From observations upon living glaciers, and from the known nature of ice, we may learn to recognize the track of a glacier as readily and unmistakably as we would the familiar foot-prints of an animal." G. F. Wright 1891 (108-109)

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    I can't elaborate too much right now.

    To first order there are two distinctive classes of regions, these are distinguished primarily on their velocities. There are the Large Low Shear Velocity Provinces (known by the catchy acronym LLSVP) and the as yet unnamed "faster regions" (my ad hoc name).

    The LLSVPs are interesting, they are denser, and hotter, and they are (approx) equatorial and there are two of them that are antipodal. One sits beneath Africa and the Atlantic and the other is central Pacific. As the name suggest they are large, they occupy almost half the available area. They are correlated with anomalies in the geoid (spherical harmonic degree two), and the geomagnetic field. They are also correlated with hotspots. Another name that is given to them is "super plumes". These regions are also associated with "ultra low velocity zones" which are small pockets of very low velocities seen just above the core -- one working hypothesis is that these are pockets of melt.

    The "faster regions" are correlated with sites of palaeo subduction. These also show up quite clearly in the geoid. It just so happens that seismologists better sample these regions as there are more favourable ray paths -- as a result of this there are actual migrated seismic images of reflections and diffractions from layers in these regions (similar to the imaging employed by the oil and gas industry). These images have shown "lenses" of material, which have been hypothesised to be crumpled and folded slabs, or perhaps (more compellingly due to experimental evidence) a double crossing of post-perovskite phase transition. The anisotropy of these areas have also been studied in some detail. The data fit a dipping fabric that may indeed trace out the patterns of flow.

    That turned out to be a little more than I had anticipated, I have to go now, I can recommend some papers later if you'd like.
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    Thanks for these Ophiolite.

    The British Geophysical Association have their "Bullerwell Lectures" each year. Mike Kendall in 2003 talked about the tectonics of the lowermost mantle:

    2004A&G....45b..28C Page B33

    Also Dave Dobson more recently (2010) gave a talk entitled Tectonics of the Lowermost Mantle 2, but I'm not sure if it has been written up and published.

    The story of D" is developing right now. In 2004 a major discovery was made by mineralogists which really shook up the science. MgSiO3 perovskite (the most volumetrically important mineral in the lower mantle) undergoes a phase transformation to the structure of "post-perovskite". This was seen as a major break through and is reflected in the literature. A lot of stuff was "rethought" with post-perovskite in mind. The questions are still unravelling now. It is a very active arena right now.
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    Now geochemists are telling us that certain elements are missing from the reservoirs that we can "see" relative to the chondritic meteorites which formed from the same "stuff" as the planet. They say "there must be a hidden reservoir" in the Earth.

    Meanwhile there was this theory that the whole mantle was molten after a giant impact with a Mars sized object which went on to form the moon. Some people calculated that the "magma ocean" crystallised from the middle so that there were separate magma oceans at the top and at the bottom of the mantle. The melt was rich in "incompatible elements" (elements that prefer to stay in the melt) and at the top formed a light continental crust. At the bottom it formed a dense continental anti-crust.

    Seismologists see large "provinces" beneath Africa and the Pacific corresponding to massive geoid highs. These provinces appear to be dense and chemical anomalies.

    Is it too much of a stretch to attribute the "hidden reservoirs" from geochemistry, to these large "LLSVP" anomalies found by seismology?


    Garnero and McNamara. Structure and dynamics of Earth's lower mantle. Science (2008) vol. 320 (5876) pp. 626-628

    Labrosse et al. A crystallizing dense magma ocean at the base of the Earth's mantle. Nature (2007) vol. 450 (7171) pp. 866-869

    Tackley. Dynamics and evolution of the deep mantle resulting from thermal, chemical, phase and melting effects. Earth-Science Reviews (2012) vol. 110 (1-4) pp. 1-25
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    Life-Size Nanoputian Flick Montana's Avatar
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    Very cool stuff. Proof that Geology isn't a completed science. I'm working with a couple geologists and I'll have to ask them about this. I've honestly never heard of it before.
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    Just what is a "completed science" as opposed to incomplete science? I've never heard of this manner of defining science.
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    Quote Originally Posted by MrMojo1 View Post
    Just what is a "completed science" as opposed to incomplete science? I've never heard of this manner of defining science.
    I like to joke with our geologists that they work in a completed science. That we already know everything there is to know about rocks. When you work together every day, you have to keep things fun.
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    Quote Originally Posted by Flick Montana View Post
    I'm working with a couple geologists and I'll have to ask them about this.
    Sure, let me know what they reckon.
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