Answer:
The crust size remains constant because the older crust is melted at subduction zones.
Explanation:
The crust is constnatly created on Earth, but the crust is constantly getting destroyed as well. This situation leads to the total size of the crust being roughly at the same level, or rather constant, as one side a new one emerges, while at the same time, on the other side it gets destroyed.
The vast majority of the new crust is formed where there are divergent plate boundaries. Here, a gap opens up between the plates that move away and magma is constantly rising to the surface and creates new crust. When it comes to the destruction of crust, it occurs at subduction zones. Here, one plate moves below another plate, and as it does it reaches the upper mantle where it gets melted and recycled because of the high temperatures and pressure.
