The differentiation of the magma entering the eruption site of Geldingadalir in Iceland tells a story. The magma did become more primitive quite rapidly as this graph shows:
This tells us that the lava ascending at first to the surface must have had time for differentiation, whereas what follows has not been altered in the same way. This makes sense if the magma did flow some distance horizontally before entering the dike and eventually the eruption site. This is one argument for speculating about the existence of another, lower dike, being aligned perpendicularly to the upper dike.
The eruption is developing stepwise, and now higher fountains of lava are measured to have reached over 200 meters hight.