Here's why:

* Iceland sits on the Mid-Atlantic Ridge: This is a divergent plate boundary where the North American and Eurasian plates are pulling apart.

* Iceland also has a hotspot: This is a plume of hot magma rising from deep within the Earth's mantle.

* The combination: The hotspot interacts with the divergent plate boundary, creating a unique situation where both divergent and convergent forces are at play.

While the Mid-Atlantic Ridge is a divergent boundary, Hekla itself is located on a zone of crustal compression within Iceland, caused by the interaction of the divergent plate boundary and the hotspot. This compression leads to the formation of volcanoes like Hekla.