Here's why:

* Subduction zones are areas where one tectonic plate slides beneath another.

* The denser plate (usually oceanic crust) sinks into the mantle.

* As the descending plate goes deeper, it melts due to the heat and pressure.

* This molten material rises to the surface and erupts as volcanoes, often forming island arcs or volcanic mountain ranges.

New crust is formed at mid-ocean ridges, where tectonic plates move apart, allowing magma to rise and solidify, creating new oceanic crust. This process is called seafloor spreading.