In terms of where it emerges, I fully agree. Once it's on the surface, however, it flows like a river - until it hardens. Then that hardened flow becomes a barrier to subsequent flows - until it's built such significant barriers everywhere that the easiest path is back over the original flow.
In terms of where it's going to emerge, though, that can be bloody anywhere. The sort of dikes that feed these fissure eruptions are far more massive than most people realize. Most most times when there's magma movement all it does is make a dike, it never even breaks the surface. It's like the magma chamber is erupting into the ground itself. There's a great animated map of how the lava in the Bárðarbunga eruption moved from the magma chamber (lower left) dozens of kilometers underground, simply by plotting the earthquakes that occurred as the rock broke in its path:
https://www.youtube.com/watch?v=3PTEDxrIRoMIt ripped a channel up to a dozen meters wide, several kilometers tall, and dozens of kilometers long. It could have broken through to the surface at any time, or never. But obviously, once it does, that becomes an easier path to move rather than breaking more rock (so long as the rift is big enough for the flow rate), and it flows downhill from that point.
Oh great, now I'm going through all the old videos from the eruptions... I forgot about the fire tornadoes it made!
