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Why are the oldest ocean-floor sediments only about 200 million years old?

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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.

The reason ocean-floor sediments are no older than approximately 200 million years is due to the continuous process of seafloor spreading and the recycling of the oceanic crust.12

New oceanic crust is constantly created at mid-oceanic ridges through igneous activity. As this crust forms, it moves away from the ridge, carrying overlying sediments with it. Over time, this crust—along with its accumulated sedimentary record—is eventually destroyed as it is pushed into subduction zones and recycled back into the Earth's mantle.13

Because the oceanic crust is relatively young and is continuously being created and destroyed, the seafloor does not preserve a geological record older than about 200 million years. Scientific drilling projects, such as those conducted by the Glomar Challenger, have confirmed this by showing that sediments become progressively older and thicker with increasing distance from the ridge crests, but never exceed this age limit.124