How It Happens
The closer material gets to the earth’s core, the hotter it gets. We know that heat rises. So, when the magma closest to the core heats up, it rises to the top of the mantle. Once it gets there, it starts to cool down. The cooler that it gets, the denser it gets. Once it is dense enough, it sinks again. This cycle creates convection currents in the form of mantle plumes.
Convection currents are what cause divergent boundaries. At divergent boundaries, tectonic plates pull away from each other. Magma rises to fill the gap between plates. As more magma fills the gap, it’s pushed onto either side of the boundary. Eventually, it spreads and hardens.
This process continues to add new floor to the bottom of the ocean.
Ridges are underwater mountain ranges formed at divergent boundaries. The magma that rises here piles up the highest near the boundary itself. This creates a slope from the center of the boundary to its edges. Ridges differ in height, narrowness, and steepness. These features depend on the age of the ridge and how fast the magma comes out of it. Still, they’re all made out of igneous rock.
What Happens to Older Sea Floor
As magma rises to create new layers, old layers get pushed away. The constant outward movement of old layers makes room for new layers. This is why it’s called sea floor “spreading”.