Sea floor spreading
Seafloor spreading is a process on land in which plate tectonic large slabs of earth's lithosphere and fell apart from each other. Tectonic plates slowly move away from each other. Heat from the mantles convection currents make crust more plastic and less dense. Mantel convection is as motion of earth's mantel. Convection currents carry heat from the lower mantle and core to the lithosphere. Convection currents also "recycle" lithosphere materials back to the mantle. The way you know the age based on seafloor is when it gets subducted that's old land and when you would " recycle" land it would be new.
"Geologic process in which plate tectonic large slabs of earth's lithosphere split apart from each other"
"mantel convection is the slow, churning motion of earth mantel. Convection currents carry heat from the lower mantel and core to the lithosphere" Sea floor spreading www.national.org