There's something I’m still not quite getting about why island arcs are arcs. In the examples I’ve seen, the subducting plate is compared to a knife slicing across the surface of an orange - but it makes sense for the slice of the orange to be an arc, because the knife is a flat plane while the orange is a sphere.
Subducting plates, on the other hand, are parts of a spheroid "slicing into" another part of the same spheroid. I’m assuming that because subducting plates are part of the Earth’s crust, they must also be curved, unlike the knife.
So it seems that if the knife is creating an arc because it's flat relative to the shape of the orange, something must flatten the subducting plate so that it's flatter relative to the plate it subducts under. Does the compressing force of the upper plate on the subducting plate flatten it relative to the shape of the upper plate?