Describe what happens in the ocean with plate tectonics

As tectonic plates slowly move away from each other, heat from the mantle's convection currents makes the crust more plastic and less dense. ... This rock (basalt) becomes a new part of Earth's crust. Mid-Ocean Ridges. Seafloor spreading occurs along mid-ocean ridges—large mountain ranges rising from the ocean floor.

what image you forgot to include it.

a "theory" a suggestion that is not 100 % accurate, or a figure. some people say its a theory because they of all the passed theory's about one subject. it depends sometimes theory might be accurate and sometimes they are.