Why does sediment thickness generally increase away from a ridge in sea-floor spreading?

Correct answer: Older floor had more time for deposition

The direct answer is A: older ocean floor had more time for deposition. A mid-ocean ridge is a place where new oceanic crust forms. Fresh basalt at the ridge is very young, so only a thin layer of marine sediment has had time to collect on it. As the crust moves away from the ridge by sea-floor spreading, it becomes older. During this longer period, dust, shells, clay and other particles settle from seawater and build a thicker layer. Therefore sediment thickness generally increases with distance from the ridge. Option A is correct because it gives the age-and-time explanation. Option B is wrong: distant crust is not necessarily always free from volcanic activity, and volcanism is not the main reason for sediment thickness. Option C is wrong because water is present over oceanic crust away from the ridge. Option D is wrong because ocean-floor sediment is not made of continental ice. Memory cue: young ridge means thin sediment; older, farther crust means thicker sediment.