Why does seafloor spreading occur?
http://www.pmel.noaa.gov/vents/nemo/explorer/concepts/spreading.html
Seafloor spreading occurs due to the movement of tectonic plates, which make up the Earth's surface. These plates are constantly moving, though very slowly, and interact with each other at their boundaries. At certain boundaries called divergent plate boundaries, the tectonic plates move apart from each other.
Seafloor spreading specifically takes place at divergent plate boundaries, primarily along the mid-ocean ridges. As the plates move apart, molten rock called magma rises from the mantle beneath the Earth's crust. This molten rock fills the gap between the separating plates and solidifies, creating new oceanic crust.
The magma that fills the gap is basaltic in composition, which is relatively less dense than the surrounding rock. As a result, the newly formed crust becomes buoyant and pushes the existing crust away from the ridge, like a conveyor belt. This process of continuous creation and movement pushes the older crust away from the ridge, forming new oceanic crust as the plates continue to separate.
Over time, this seafloor spreading process creates new crust, allowing the oceans to expand and grow larger. It is an essential mechanism in the theory of plate tectonics, explaining the movement and formation of Earth's lithosphere, as well as influencing the overall shape and features of the Earth's surface.