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Here's what I know:

  • The lithosphere is hard because it is cold and low-pressure.
  • The asthenosphere is more deformable because of higher temperatures and pressures.
  • The mesosphere is below the asthenosphere and presumably experiences higher temperatures and pressures than the asthenosphere.

Why does the asthenosphere act like a lubricant for tectonic plates to slide over? Why is the mesosphere more viscous than the asthenosphere?

What's causing the differences in the viscosity between these regions, and is there more than viscosity going on?

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  • $\begingroup$ I read somewhere that the asthenosphere may have lens of partial melt, which in turn affects it's mechanical problems. $\endgroup$ – Gimelist Mar 31 '17 at 9:29
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The introduction to Morgan et al., 2013 has a good summary of the features you are asking about, as well as proposing a mechanism for why it happens.

From the intro:

The Earth’s mantle underneath the tectonic plates, commonly referred to as asthenosphere, is known to be the lowest viscosity region of the upper mantle. Several potential mechanisms have been suggested to be responsible for this low viscosity zone. These mechanisms...include: (1) the temperature and pressure dependence of the mantle rheology may lead to a viscosity minimum between 70 and 200 km depth; (2) a small and immobile fraction of partial melts may weaken the mantle; (3) wet mantle below its dry solidus (deeper than about 70 km) would be expected to be weaker than shallower mantle that dehydrated during partial melting at mid-ocean ridges (MOR)—but this mechanism does not explain the physical origin for the base of the asthenosphere; (4) a reduction in mantle grain size within the asthenosphere.

The paper's proposed cause for this is that:

the sub-oceanic asthenosphere forms because it is the ‘graveyard’ for rising (i.e. hotter-than-average mantle) plumes. In this view, below the plate-age-dependent 60–100 km-thick oceanic lithosphere there exists a pool of hot plume material that has risen as far as the overlying thermal and/or compositional lithosphere will allow.

Whether that mechanism is correct or not, only time will tell. In conclusion, due to the temperature and pressure characteristics of the asthenosphere, it is less viscous than the chemically identical mesosphere. This may be because of its place on temperature and pressure curves, because it contains pockets of partially molten rock, because rising mantle plumes make the asthenosphere actually hotter than the mesosphere, or otherwise.

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  • $\begingroup$ I'm not sure it's hotter because of the partially molten rock. AFAIK the partial melt is there because it's easier to melt at those conditions. $\endgroup$ – Gimelist Mar 31 '17 at 9:30
  • $\begingroup$ @Michael Well if you read that linked paper, they suggest that plumes of hotter material rise up from deep within the mantle, and are trapped by the lithosphere. Since these plumes are hotter, once they get to a low enough pressure zone, they become partially molten. If you don't think that is true, I suppose you should take it up with Morgan and company. $\endgroup$ – kingledion Mar 31 '17 at 13:01
  • $\begingroup$ By "easier to melt" I mean that the pressure was low enough. We're agreeing on the same thing. $\endgroup$ – Gimelist Mar 31 '17 at 20:49
  • $\begingroup$ "due to the temperature and pressure characteristics of the asthenosphere, it is less viscous than the chemically identical mesosphere....(and so on)" This is a classic instance of circular reasoning, or, "begging the question", and highlights a common misconception when it comes to "chemically identical" to describe mantle rocks throughout the mantle's depth. The referenced paper assumes an understanding of the term "rheology" in the context of high pressure mineralogy. $\endgroup$ – Knob Scratcher Apr 4 '17 at 4:25
  • $\begingroup$ @KnobScratcher I don't understand what you think is wrong with my answer, the paper or both. $\endgroup$ – kingledion Apr 5 '17 at 0:39

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