The plate boundary that forms arcs of volcanoes and deep-ocean trenches is a convergent boundary, specifically one where oceanic crust subducts beneath continental crust or oceanic crust subducts beneath oceanic crust.

Here's a breakdown:

* Convergent Boundary: Two tectonic plates collide.

* Subduction: When one plate (usually the denser oceanic plate) slides beneath the other (either continental or oceanic), this process is called subduction.

* Volcanic Arcs: As the subducting plate sinks, it melts, generating magma. This magma rises to the surface, forming volcanoes. These volcanoes often form a curved line, or arc, parallel to the trench.

* Deep-Ocean Trenches: The collision of the plates creates a deep depression in the ocean floor called a trench.

Let me know if you'd like more details on how this process works!