Geophysics give us some insight into what is happening at Reykjanes, now when the eruption at Geldingadalir has started. Force and Stress is the key to understanding the situation. You can learn about it here: http://kurd-brave.weebly.com/uploads/5/3/8/0/53800263/3130122022015_lec._2.pdf. Then this map can be studied:
If you look at this carefully, you see symmetric vectors, marked with black arrows. The northern half of the red polygon drifts rather freely along with the North American Tectonic Plate, but the southern half is more trapped between the drift effect from the Reykjanes Ridge and the division line between the two tectonic plate of the Reykjanes Oblique Rift Zone (RORZ). That is where the eruption is taking place.