Volcanos (island arcs) and deep ocean trenches form at which boundary?

Volcanoes (island arcs) and deep ocean trenches typically form at convergent plate boundaries. Here's a step-by-step explanation:

1. Convergent plate boundaries are areas where two tectonic plates are moving towards each other.

2. When these plates collide, one plate is usually forced beneath the other in a process known as subduction.

3. The subducting plate moves into the mantle where it begins to melt due to the high temperatures and pressures.

4. This melting creates magma which can rise to the surface and form a volcano.

5. The area where the plate is being forced down into the mantle is where deep ocean trenches form.

6. If the process occurs under the ocean, it can lead to the formation of a chain of volcanic islands known as an island arc.

So, the answer is: Volcanoes (island arcs) and deep ocean trenches form at convergent plate boundaries.