The largest feature of Iceland is indeed the Reykjanes Ridge. But it is below the ocean so we can not see it directly. It shows that two mantle currents must be pulling the ocean apart, and it is also clear that those are convection currents. These currents must be long, and it means they have to be convection rolls.
It is also logical that the convection rolls do not end where the red line ends. Therefore the full 3D analysis of the mantle currents has been made to explain the different features found on the surface. The geology of Iceland is quite complicated, partly because it is found within the intersection zone of different layers of convection rolls. The simplicity and regularity of the Reykjanes Ridge is helpful for us to derive the whole system and understand the basics of the convection rolls system of Earth’s mantle.