How did seafloor spreading provide key evidence for the theory of plate tectonics?

The basic theory of plate tectonics is that along seafloor spreading zones, the continents are separating from one another. As they spread apart, magma comes to the surface and becomes new continental crust. As the tectonic plates move away from spreading zones, they collide with one another. In some cases, the edges of two different plates will grind against each other in a horizontal fashion