The Eyjafjallajökull volcano is a stratovolcano, which means it is built up of layers of lava flows, ash, and volcanic debris. This results in a variety of rocks being formed, but some of the most common include:

* Basalt: This is the most common type of rock found in volcanic eruptions, and it's formed from the cooling and solidification of lava flows.

* Andesite: This type of rock is also common in stratovolcanoes. It's a darker, more silica-rich volcanic rock than basalt.

* Rhyolite: This rock is even more silica-rich than andesite, and it's often found in explosive eruptions.

* Tuff: This is a rock formed from compacted volcanic ash, which can be deposited in layers during eruptions.

The specific types of rocks formed at Eyjafjallajökull depend on the composition of the magma and the eruption style. However, the rocks mentioned above are all likely to be present in some form.