<p>Gimelist has given a good and fairly thorough answer, I would add that the insulative quality of rock is also a major contributor to it's extended survival at high temperatures. While the surface of the slab undergoes thermo-chemical reactions, like those that liberate carbon dioxide and water from sedimentary clays in their surface layers the inner material is insulated from heat transfer for decades because solid rock is a very poor conductor of heat, inner being anything more than a few millimeters below the surface.</p>

CC BY-SA 4.0
2018-07-02T19:45:19.957Z
Millenia, not decades. But yes.
Gimelist
CC BY-SA 4.0
2018-07-02T12:16:05.137Z
2018-07-02T19:49:23.120Z
2018-07-02T19:49:23.120Z
Ash
<blockquote>
  <p>...the submerging plate is always drawn as deep (seemingly tens of km) into molten rock...</p>
</blockquote>

<p>It's actually hundreds of kilometres, not tens of them. But let me correct a common misconception:</p>

<p><strong>The mantle is solid rock. It is <em>not</em> molten rock.</strong></p>

<p>So subduction leads solid rock into more solid rock.</p>

<blockquote>
  <p>the surrounding medium is hot enough to melt rock</p>
</blockquote>

<p>The surrounding medium is hot enough to melt rock only under very special circumstances, the two most common being:</p>

<ol>
<li>If the rock gains water somehow (yes, water under pressure helps rock to melt), or</li>
<li>If even hotter rock from down below rises up and decompresses.</li>
</ol>

<blockquote>
  <p>Why are so many textbooks drawing it submerged and unmelted to such depths?</p>
</blockquote>

<p>Because it doesn't melt. The subducting plate goes down faster than it heats up. Therefore, pressure increases faster then temperature. This does not allow melting of the subducting slab.</p>

<p>What does happen? The slab loses water. Remember - this was oceanic crust and it's wet. On moderate heating, the water from hydrous minerals (serpentine etc) is lost to the mantle rocks above it. Because the mantle rocks above the slab <em>are</em> hot, the addition of water causes them to melt. We see this all the time in the form of arc volcanoes. The Pacific "rim of fire" is the most famous example of this process.</p>

<p>I would say that some very limited amount of melting may actually happen, but in terms of volume % of the entire slab it's likely to be negligible. The contribution of this melting to global volcanism is also likely to be negligible.</p>

CC BY-SA 4.0
2018-07-02T09:54:17.440Z
2018-07-02T09:54:17.440Z
Gimelist
<p>If you take a <a href="https://www.google.ca/search?hl=en&amp;biw=1280&amp;bih=541&amp;tbm=isch&amp;sa=1&amp;ei=8Yw2W8eUDovSwQKM34PwBQ&amp;q=tectonic+plate+submerged&amp;oq=tectonic+plate+submerged&amp;gs_l=img.3...3902.7998.0.8115.15.15.0.0.0.0.256.1343.2j5j2.9.0....0...1c.1.64.img..6.3.528...0j0i8i30k1j0i24k1.0.txdLbVOS9Dk" rel="nofollow noreferrer">look</a> at a <a href="https://en.wikipedia.org/wiki/Asthenosphere#/media/File:Subduction-en.svg" rel="nofollow noreferrer">typical</a> <a href="https://e9350ade-a-62cb3a1a-s-sites.googlegroups.com/site/platetectonicsamyg/home/plate-boundaries/Convergence1.gif?attachauth=ANoY7cqjMuTj1xeLGnjqRrebI07r5UK-zFch-VYiTho0UY0-HnV7CsNaEgSuOz9HYtMzoYBWn0Tr-yhPeApLOzyq26ngTf9Cm1DkcuyeVGKliY2YmHG6Yw4hZd4uxe0-lgjcvPSCSJTQVH2GhcUxNzTPXxyi4puQbe2_iPMMuu3MvxlB-oubcxF0XQ0d6FbLUb2eWVcWpXrattVUO1xRH1eNhNy3GXVo2Cd60yihtQC7DnfxVl--6T61JbzTqOlB1g-yWVVRuAxw&amp;attredirects=0" rel="nofollow noreferrer">picture</a> tectonic plate subducting under another plate, the subducting plate is always drawn as deep (seemingly tens of km) into molten rock, but the subducted part of the plate itself is not molten. Why are the subducted plates not drawn as shallower into the magma?</p>

<p>My logic looking at this is the following: the surrounding medium is hot enough to melt rock. Alright, rock may take time to melt. A typical continental plate moves 10cm/year which yields 100,000 years / 10km. Is an order of a hundred thousand years really not enough time to melt that subducted plate? Why are so many textbooks drawing it subducted and unmelted to such depths?</p>

CC BY-SA 4.0
2018-06-29T23:28:10.610Z
What molten rock? The Earth's mantle is not liquid. It's a visco-elasto-plastic solid-like substance.
David Hammen
CC BY-SA 4.0
2018-07-02T19:46:35.820Z
Perhaps part of the problem is that you seem to be expecting those drawings to be exact representations of reality, while instead they're simplified for teaching purposes, and use exaggerated scales &c.  Also, here's a real-world example of two streams of the same liquid (water) running side by side without mixing: https://en.wikipedia.org/wiki/Meeting_of_Waters  Much easier for a "visco-elasto-plastic solid-like substance" to do that :-)
jamesqf
<thermodynamics><geophysics>
Why doesn't subducted earth crust melt?
Yaroslav
# Why doesn't subducted earth crust melt?

(Asked by: username_0 on 2018-06-29)

If you take a [look](https://www.google.ca/search?hl=en&biw=1280&bih=541&tbm=isch&sa=1&ei=8Yw2W8eUDovSwQKM34PwBQ&q=tectonic+plate+submerged&oq=tectonic+plate+submerged&gs_l=img.3...3902.7998.0.8115.15.15.0.0.0.0.256.1343.2j5j2.9.0....0...1c.1.64.img..6.3.528...0j0i8i30k1j0i24k1.0.txdLbVOS9Dk) at a [typical](https://en.wikipedia.org/wiki/Asthenosphere#/media/File:Subduction-en.svg) [picture](https://e9350ade-a-62cb3a1a-s-sites.googlegroups.com/site/platetectonicsamyg/home/plate-boundaries/Convergence1.gif?attachauth=ANoY7cqjMuTj1xeLGnjqRrebI07r5UK-zFch-VYiTho0UY0-HnV7CsNaEgSuOz9HYtMzoYBWn0Tr-yhPeApLOzyq26ngTf9Cm1DkcuyeVGKliY2YmHG6Yw4hZd4uxe0-lgjcvPSCSJTQVH2GhcUxNzTPXxyi4puQbe2_iPMMuu3MvxlB-oubcxF0XQ0d6FbLUb2eWVcWpXrattVUO1xRH1eNhNy3GXVo2Cd60yihtQC7DnfxVl--6T61JbzTqOlB1g-yWVVRuAxw&attredirects=0) tectonic plate subducting under another plate, the subducting plate is always drawn as deep (seemingly tens of km) into molten rock, but the subducted part of the plate itself is not molten. Why are the subducted plates not drawn as shallower into the magma?

My logic looking at this is the following: the surrounding medium is hot enough to melt rock. Alright, rock may take time to melt. A typical continental plate moves 10cm/year which yields 100,000 years / 10km. Is an order of a hundred thousand years really not enough time to melt that subducted plate? Why are so many textbooks drawing it subducted and unmelted to such depths?

**Tags:** thermodynamics, geophysics

> **Comments:**
> **username_1**: What molten rock? The Earth's mantle is not liquid. It's a visco-elasto-plastic solid-like substance.  
> **username_2**: Perhaps part of the problem is that you seem to be expecting those drawings to be exact representations of reality, while instead they're simplified for teaching purposes, and use exaggerated scales &c.  Also, here's a real-world example of two streams of the same liquid (water) running side by side without mixing: https://en.wikipedia.org/wiki/Meeting_of_Waters  Much easier for a "visco-elasto-plastic solid-like substance" to do that :-)  

---

## 2 answers

### Answer by username_4 on 2018-07-02. Score: 7 | Accepted answer

> ...the submerging plate is always drawn as deep (seemingly tens of km) into molten rock...

It's actually hundreds of kilometres, not tens of them. But let me correct a common misconception:

**The mantle is solid rock. It is *not* molten rock.**

So subduction leads solid rock into more solid rock.

> the surrounding medium is hot enough to melt rock

The surrounding medium is hot enough to melt rock only under very special circumstances, the two most common being:

1. If the rock gains water somehow (yes, water under pressure helps rock to melt), or
2. If even hotter rock from down below rises up and decompresses.

> Why are so many textbooks drawing it submerged and unmelted to such depths?

Because it doesn't melt. The subducting plate goes down faster than it heats up. Therefore, pressure increases faster then temperature. This does not allow melting of the subducting slab.

What does happen? The slab loses water. Remember - this was oceanic crust and it's wet. On moderate heating, the water from hydrous minerals (serpentine etc) is lost to the mantle rocks above it. Because the mantle rocks above the slab *are* hot, the addition of water causes them to melt. We see this all the time in the form of arc volcanoes. The Pacific "rim of fire" is the most famous example of this process.

I would say that some very limited amount of melting may actually happen, but in terms of volume % of the entire slab it's likely to be negligible. The contribution of this melting to global volcanism is also likely to be negligible.


---

### Answer by username_3 on 2018-07-02. Score: 0

username_4 has given a good and fairly thorough answer, I would add that the insulative quality of rock is also a major contributor to it's extended survival at high temperatures. While the surface of the slab undergoes thermo-chemical reactions, like those that liberate carbon dioxide and water from sedimentary clays in their surface layers the inner material is insulated from heat transfer for decades because solid rock is a very poor conductor of heat, inner being anything more than a few millimeters below the surface.

> **Comments:**
> **username_4**: Millenia, not decades. But yes.  

---

