When new crust is made at mid-ocean ridges, the old crust moves away from the ridge. This is due to a process called sea floor spreading. Here's how it works:

1. Magma rises: Molten rock (magma) rises from the Earth's mantle at mid-ocean ridges, where tectonic plates are pulling apart.

2. New crust forms: The magma cools and solidifies, creating new oceanic crust.

3. Old crust moves: As new crust is formed, the existing crust is pushed away from the ridge, like a conveyor belt.

4. Subduction: Eventually, the older, denser oceanic crust will collide with a continental plate and be forced beneath it in a process called subduction.

So, the old crust doesn't disappear. It moves away from the mid-ocean ridge, gets older and denser, and eventually gets recycled back into the Earth's mantle through subduction.