Here's why:

* Iceland sits on the Mid-Atlantic Ridge: This is a massive underwater mountain range where the North American and Eurasian tectonic plates are pulling apart (diverging).

* Magma rises: The separation creates a space where magma from the Earth's mantle can rise to the surface.

* Volcanic activity: This rising magma leads to volcanic eruptions, like the one that created Eldfell in 1973.

So, the divergent plate boundary is the driving force behind the volcanic activity that formed Eldfell.