What is seafloor spreading?

Seafloor spreading is a geologic process in which tectonic plates large slabs of Earth’s lithosphere split apart from each other. Seafloor spreading occurs along mid-ocean ridges (large mountain ranges rising from the ocean floor). The Mid-Atlantic Ridge is formed in the Atlantic ocean because of seafloor spreading. The Atlantic Ocean growing by a few centimetres every year because of seafloor spreading. 

Further Reading:

• Geomorphic Processes
• The Origin And The Evolution Of The Earth