In the mantle there is a layer that explains the heat anomaly found under the glacier of Greenland. It has been calculated and is shown here above.
The lines are obviously approximately the same. Usually geologists regard Vatnajökull as the site of hot spot, so therefore someone has drawn the line over Iceland a little bit eastwards of the calculated division line between convection rolls.
This line represents the main division of the second row of convection rolls counted from above.