There is quite a lot of recent corroborating evidence in support of sea floor spreading. Even so, there is no oceanic crust older than about 200 Ma. Thus the ocean floor must be continually recycled. Plate Tectonics is the theory that unifies the continental drift and sea-floor spreading theories. Tectonics is the study of large-scale deformation and movement of the Earth's layers.
Plates are rigid crustal blocks that move over semi-molten mantle material.
The elements of plate tectonics. There are six large crustal plates and several small plates that comprise Earth's solid surface. Below that is a plastic layer of earth materials, the asthenosphere, that flows in convection cells that extend to a depth of 300 km or so. The Earth's interior is very hot, and the most efficient way for it to transfer heat to the surface is through convection. Undersea mountain ranges, the mid-ocean ridges, are over the rising parts of convection currents in Earth's mantle.