The West Volcanic Zone is narrower than the East Volcanic Zone. It is not as active either. Thingvellir has the calculated alignment of convection rolls, and we can then analyse the formations due to horizontal forces of tectonic drift.
The polygon of Thingvellir is subject to pulling to NW, and a compromise of tension is drawn with blue lines. This compromise also is the decisive factor for the width of the volcanic zone. Lake Thingvallavatn fits into the resulting pattern. The rift valley extending to the NE from the end of Thingvallavatn has then the exact alignment of convection rolls underneath.
The convection roll under this polygon rolls from west to east, opposing the general tectonic drift of the North American tectonic plate extending all the way to the Pacific Coast of N-America, thereby causing rifting like seen in the graben of Thingvellir.