Final answer:
Sea-floor spreading occurs at a divergent boundary where two tectonic plates move away from each other.
Step-by-step explanation:
According to the plate tectonics theory, sea-floor spreading occurs at a divergent boundary. This is where two tectonic plates move away from each other, creating a gap. Magma rises up from the mantle to fill the gap, solidifies, and forms new oceanic crust.
Learn more about Plate tectonics