Final answer:
Seafloor spreading involves new crust forming at mid-ocean ridges and moving away from these ridges. The old crust is pushed towards and eventually into subduction zones, where it melts and is recycled by mantle convection.
Step-by-step explanation:
The old crust moves away from the mid-ocean ridge when the seafloor spreads. This process occurs because at the mid-ocean ridges, such as the Mid-Atlantic ridge, tectonic plates are pulling apart from each other. This creates a space that is filled with basaltic lava rising from the mantle, forming new crust. As new crust is continuously formed, the older crust is pushed away. When it reaches a subduction zone, the denser oceanic plate is forced beneath a less dense plate into the upper mantle. This old crust then melts due to the high pressure and temperature, being recycled in the mantle's convection currents. Eventually, it balances the creation of new crust at rift zones.Closer to the coastline, volcanism occurs due to partial melting of the asthenosphere above the subducting slab. This volcanic activity often creates a corresponding volcanic arc parallel to the coast and to the trench marking the subduction zone.