Reynisfjall

Reynisfjall mountain lies within the Katla volcanic system and was formed in a subglacial and/or submarine eruption during the latter stages of the Ice Age. Mountains formed in these types of eruptions typically have pillow basalt at their base, followed by volcanic tuff, often intersected by several intrusions, and finally layers of lava at their summit. 

Pillow lava is often formed at the onset of such eruptions, if there is deep water above the eruption vent, but once the water pressure decreases above the vent, the interaction of water and lava leads to an explosive eruption. The lava then fragments, creating fine ash tephra that is subsequently ejected from the vent. As the tephra begins to settle around the vent, the sides of the volcano build up and quickly solidify into palagonite tuff. The tephra continues to build up the volcano and eventually, if water is no longer able to access the eruption crater, lava begins to flow, building layer upon layer.

Lava intrusions are common and are created when lava forces its way into the tuff during an eruption. If the tuff prevents water from coming into contact with the lava during an intrusion, the lava often forms basalt columns as it slowly cools and cracks, often creating hexagonally shaped columns as the surface area decreases. These remarkable columns always stand perpendicularly to the cooling surface and the direction of the columns can often vary due to multiple cooling surfaces. Powerful erosional forces, especially in the form of waves from the sea have created many beautiful formations and have contributed to the shape of Mt. Reynisfjall.