How can subduction change the average age and elevation of oceanic crust?

Subduction zones act as the primary mechanism for recycling oceanic crust, which directly influences its average age and elevation within ocean basins.12

Impact on Age

New oceanic crust is continuously generated at oceanic ridges and spreads outward. As this crust moves away from the spreading center, it cools and becomes denser. Eventually, this older, denser crust is consumed at subduction zones, where it descends into the mantle. Because subduction continuously removes older crust, the seafloor is effectively recycled, explaining why the oldest oceanic crust is only about 200 million years old, despite the much greater age of the Earth.1234

Impact on Elevation

The elevation of oceanic crust is fundamentally linked to its age and thermal state.24
* Thermal Contraction: Crust at spreading centers is warm and thermally expanded, causing it to rise about 2 km above the surrounding seafloor.24
* Subsidence: As the crust ages and moves away from the ridge, it cools and undergoes thermal contraction, causing it to sink deeper into the ocean basin.4
* Subduction: By the time oceanic crust reaches a subduction zone, it is significantly cooler, denser, and deeper than the newly formed crust at ridge crests.23