There are three ways of generating magma:
1. Heating a rock
2. Reducing the pressure
3. Lowering the solidus (melting point)

Subduction arcs generate magma through the third process. Subducting oceanic crust is rich in water and clay minerals (also rich in water). When this material is subducted, water and other volatiles are released, and rise into the mantle wedge above the subducting crust. This addition of volatiles lowers the solidus of the mantle wedge, causing melting.

Because this melted material is less dense, it rises, and eventually causes volcanoes at the surface. This melt is basaltic or intermediate in composition.

Also, a lot of deep-sea sediments are scraped off the subducting slab, and form an accretionary wedge on the edge of the continent. These sediments can be folded and faulted upwards to form mountain ranges (eg. The Lachlan Fold Belt in south-eastern Australia)

When the plate subducts under another place (always ocean plate subducting under another ocean or continental plate) it causes melt. This melting magma can find it’s way to the surface through heat and pressure and erupt under the water (because it is near ocean crust) and over time it will build up enough layers to reach out of the water.

And the Rockies were formed from thrust faulting of colliding plates, not by subductions and magma intrusions to the surface.

subduction zones cause volcanoes because one plate moves under another. the heavier one is the sinking one, usually because its oceanic crust which is more dense and has water. this water causes melting which more or less shoots up and causes volcanoes which when they cool form mountains. think of the Rocky Mountains. if this were to happen over the oceans, you would have the same result but appear as islands.

when one plate subducts the other, the plate melts by the process of friction and partial melting which is caused by the oceanic liquid( water) trapped within the plate, it melts part of the lithosphere on which the plate floats on, and produces magma (molten rock) this magma rises because it is more buoyant than the surrounding rock that isn’t molten, but is has very high temperatures. this molten rock rises to form volcanoes around the plate margin where the two plates are subducting. this usually involves two oceanic plates but in some cases the more denser basalt oceanic plate subducts below the more less dense granite continental plate to cause a large arc of volcanoes into a huge mountain range such as the Andes mountain range on south America. the magma produced in these regions are viscous (thick) and are highly volatile, the process of Assimilation can also cause the magma to change it’s properties to very andesitic to ryholitic. Assimilation is the process in which other materials and lithosphere matter surrounding the magma ,in the magma chamber or conduit mix and cause the magma to be more viscous, therefore the eruption of the volcano or volcanoes will be more explosive due to its high gas content. Islands are also formed in the same way. an example is the Indonesian archipelago, which is home to the most volcanoes on earth.