Sir, please elaborate last line

As can be seen from figure above, subduction is at a shallower angle, because this hotter crust is more buoyant than the usual case. The reason that partial melting of subducted basalt is unusual is that this kind of subduction and magma generation is, geologically speaking, short-lived. Subduction will end when the overriding plate crashes into the mid-oceanic ridge. Most subduction zones are a long distance from the divergent boundaries of their plates, so steep subduction and magma production from the asthenosphere are the norm.