Why is there melting and volcanism at subduction zones?

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Why is there melting and volcanism at subduction
zones?
Its relatively cold!
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Hydrous minerals warm up and dehydrate.
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Melting at subduction zones

• Hydration of oceanic floor at MOR
• More and deeper hydration due to outer rise
  faults (bending at trench)
• Dehydration of hydrous minerals
• Fluid moves up into warmer mantle which melts.

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Why are there deep Earthquakes?

• Normal stress across any fault plane is so high
  that you would need enormous stresses to cause it
  to move (i.e. high frictional strength.

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Negative volume change reduces friction
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