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Why is the oldest ocean floor much younger than Earth itself?

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Map graphic showing the boundaries of the five oceans-Pacific, Atlantic, Indian, Arctic, and Southern-using color-coded regions. Four different map projections are included: North Pole view, South Pole view, International Date Line view, and Greenwich Meridian view, each displaying the oceans' extents around the continents. — ocean Earth's oceans and their marginal seas cover nearly 71 percent of the planet's surface.

The oldest ocean floor is significantly younger than Earth because it is continuously recycled through the process of plate tectonics.12

While Earth is estimated to be at least 4.6 billion years old, the oceanic crust is generally less than 200 million years old. This occurs because new ocean floor is constantly created at spreading centres along oceanic ridges, where magma rises to fill the gap between diverging plates. To maintain Earth's constant volume, an equivalent amount of older, denser oceanic crust is destroyed at subduction zones, where it descends into the mantle and is recycled.324

In contrast, continental crust is more buoyant and is not easily subducted, allowing it to be preserved over billions of years. Consequently, while the ocean basins are transient features that change shape and depth over geologic time, the continents contain much older rocks.321