Question: where does new crust come from in sea floor spreading?
Answer: basaltic magma, volcanic activity

Seafloor spreading is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge. Seafloor spreading helps explain continental drift in the theory of plate tectonics. When oceanic plates diverge, tensional stress causes fractures to occur in the lithosphere.The motivating force for seafloor spreading ridges is tectonic plate pull rather than magma pressure, although there is typically significant magma activity at spreading ridges. At a spreading center basaltic magma rises up the fractures and cools on the ocean floor to form new seabed.  Hydrothermal vents are common