Crust is formed at a mid-ocean ridge, travels for millions of years, then is destroyed at subduction plate boundaries. Older oceanic crust subducts (dives) beneath continental crust at active continental margins where the subducted crust is destroyed. Passive continental margins are not associated with a type of plate boundary because the crust along these margins does not change much. We can infer that subduction occurs at active continental margins that have deep ocean trenches. We can make this inference because trenches form where subducting plates meet overriding plates. Topographic maps are a useful tool to determine if a margin has a trench. The trench suggests that subduction is present, which indicates the margin is active. Compare continental margins and ocean basins of the Atlantic and Pacific oceans using the map below. Deep ocean trenches are delineated with red lines, which indicates the margin is active.