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Why do ocean-floor sediments become thicker farther from a spreading centre?

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Earth's lithosphere and upper mantle — oceanic crust A cross section of Earth's outer layers, from the crust through the lower mantle.

Ocean-floor sediments become thicker with increasing distance from a spreading centre primarily because of the age of the oceanic crust.

According to the theory of seafloor spreading, new oceanic crust is continuously created at the crests of oceanic ridges through volcanic activity. As this crust forms, it moves laterally away from the ridge, carrying any overlying material with it.123

Because the crust at the spreading centre is the youngest, it has had the least amount of time to accumulate marine sediment deposits. As the seafloor moves farther away from the ridge, it grows progressively older, providing a longer duration for sediments—such as the skeletal remains of marine organisms and land-derived particles—to settle and build up on the seafloor. Consequently, the sedimentary layer is thinnest at the ridge crest and becomes progressively thicker as the distance from the spreading centre increases.123