Final answer:
Sea-floor spreading occurs at a divergent boundary where two tectonic plates move away from each other, forming new oceanic crust.
Step-by-step explanation:
According to the plate tectonics theory, sea-floor spreading occurs at a divergent boundary. At divergent boundaries, two tectonic plates move away from each other, creating a gap in the Earth's crust. As the plates separate, molten rock called magma rises to fill the gap, forming new oceanic crust and causing the sea floor to spread.
Learn more about Plate tectonics