<p>The introduction to <a href="http://geophysics.wustl.edu/seminar/2013_Morgan_EPSL.pdf" rel="nofollow noreferrer">Morgan et al., 2013</a> has a good summary of the features you are asking about, as well as proposing a mechanism for why it happens. </p>

<p>From the intro:</p>

<blockquote>
  <p>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.</p>
</blockquote>

<p>The paper's proposed cause for this is that: </p>

<blockquote>
  <p>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.</p>
</blockquote>

<p>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. </p>

CC BY-SA 3.0
2017-03-31T09:30:44.613Z
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.
Gimelist
CC BY-SA 3.0
2017-03-31T13:01:14.740Z
@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.
kingledion
CC BY-SA 3.0
2017-03-31T20:49:02.703Z
By "easier to melt" I mean that the pressure was low enough. We're agreeing on the same thing.
Gimelist
CC BY-SA 3.0
2017-04-04T04:25:50.717Z
"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.
Knob Scratcher
CC BY-SA 3.0
2017-04-05T00:39:57.917Z
@KnobScratcher I don't understand what you think is wrong with my answer, the paper or both.
kingledion
CC BY-SA 3.0
2017-03-30T20:30:58.063Z
2017-03-30T20:30:58.063Z
kingledion
<p>Here's what I know:</p>

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

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

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

CC BY-SA 3.0
2017-03-31T09:29:34.800Z
I read somewhere that the asthenosphere may have lens of partial melt, which in turn affects it's mechanical problems.
Gimelist
<geology><geophysics><mantle><crust>
What's up with the "jelly sandwich" analogy of the lithosphere, asthenosphere and mesosphere?
jvriesem
# What's up with the "jelly sandwich" analogy of the lithosphere, asthenosphere and mesosphere?

(Asked by: username_0 on 2017-03-30)

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?**

**Tags:** geology, geophysics, mantle, crust

> **Comments:**
> **username_1**: I read somewhere that the asthenosphere may have lens of partial melt, which in turn affects it's mechanical problems.  

---

## 1 answers

### Answer by username_2 on 2017-03-30. Score: 2 | Accepted answer

The introduction to [Morgan et al., 2013](http://geophysics.wustl.edu/seminar/2013_Morgan_EPSL.pdf) 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.

> **Comments:**
> **username_1**: 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.  
> **username_2**: @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.  
> **username_1**: By "easier to melt" I mean that the pressure was low enough. We're agreeing on the same thing.  
> **username_3**: "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.  
> **username_2**: @username_3 I don't understand what you think is wrong with my answer, the paper or both.  

---

