Geologically, where are most diamond deposits found?

• A. On mid-ocean ridges
• B. In volcanic arcs
• C. In gravel deposits in river systems
• D. On cratons or in the areas surrounding them

Answer: (D)

Diamonds form deep in the mantle under high pressure and are carried to the surface by kimberlite volcanism. For these diamonds to be preserved and concentrated into economically workable deposits, they need very old, stable continental crust—the cratons—along with their thick, cold, rigid lithosphere. Kimberlite systems often tap into these preserved mantle domains, so many primary diamond occurrences occur in cratons or in areas surrounding them.

Mid-ocean ridges involve younger, dynamic lithosphere where such kimberlite conduits and long-term preservation aren’t favorable. Volcanic arcs are related to subduction zones and also don’t typically host the primary diamond-bearing settings. River gravels can host diamonds too, but they are secondary deposits formed by erosion and transport from primary sources and do not represent where most diamond deposits originate.