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If you take a look at a typical picture 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?

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  • $\begingroup$ "....but the submerged part of the plate itself is not molten. Why are the submerged plates not drawn as shallower into the magma?" - I've noticed that in the diagrams, too. I've always assumed that it was due to the fact that the plates are so thick and that thermal diffusion becomes a slower and slower process as objects get thicker and thicker since the characteristic distance that heat diffuses into an object in a time t goes as sqrt(t). $\endgroup$
    – Samuel Weir
    Jun 29, 2018 at 20:58
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    $\begingroup$ What molten rock? The Earth's mantle is not liquid. It's a visco-elasto-plastic solid-like substance. $\endgroup$
    – David Hammen
    Jun 29, 2018 at 23:28
  • $\begingroup$ @SamuelWeir: right, but we are talking about hundred of thousand of years as well. Also, as the crust melts into the surrounding state, it should start getting thinner and melt faster, no? $\endgroup$
    – Yaroslav
    Jun 30, 2018 at 4:07
  • $\begingroup$ @DavidHammen: I don't know if that is a technicality or not as I have never taken geophysics. But does it really take over a hundred thousand of years for crust to reach that state? $\endgroup$
    – Yaroslav
    Jun 30, 2018 at 4:11
  • $\begingroup$ 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: en.wikipedia.org/wiki/Meeting_of_Waters Much easier for a "visco-elasto-plastic solid-like substance" to do that :-) $\endgroup$
    – jamesqf
    Jul 2, 2018 at 19:46

2 Answers 2

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...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.

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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.

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    $\begingroup$ Millenia, not decades. But yes. $\endgroup$
    – Gimelist
    Jul 2, 2018 at 19:45

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