Iceland and Hawaii are both located on large scale division lines of lower mantle. That explains the volcanic activity within both areas. This can be found because convection rolls of equal hight and width fit into the layer, and the horizontal curvature of the rolls can be calculated.
The red curved lines show up-welling division between large scale convection rolls, spanning 30° from east to west. The blue curved lines show down-welling division between large scale convection rolls. Iceland and Hawaii are both found on such up-welling lines. This is the most basic feature of the convection rolls system.