Consider the following statements:

1. Oceanic trenches constitute the deepest parts of the oceans.
2. They occur mainly on the crests of mid-oceanic ridges and are generally remote from island arcs.
3. They may be approximately 3–5 km deeper than the surrounding ocean floor.
4. Their association with active volcanoes and strong earthquakes makes them important for the study of plate movements.

Which of the statements given above are correct?