One of the most popular sites in Iceland is Thingvellir. Here is a simple drawing of Lake Thingvallavatn, showing the combined tectonic forces that make the area so special:
As can be seen on this simple drawing, the edge of the Western Volcanic Zone of South Iceland, Hengill Volcanic System and a large scale rift zone combine around the NS-axis of the relevant polygon of convection cells underneath. This results in a very prominent line of cliffs including the famous Almannagjá. A similar combination occurs at Hrafnagjá.
Of course, tectonic drift is responsible for this, but the division line of the large convection cells is found at the eastern edge of the Western Volcanic Zone.
The tectonic directions can be calculated one by one, as explained in an earlier post.