1. Why is there melting and volcanism at subduction zones?
It’s relatively cold!

3. Hydrous minerals warm up and dehydrate.

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8. 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.

9. 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.

10. Negative volume change reduces friction