The Oxararfoss waterfall in Thingvellir, Iceland. The falls occur where the Oxara river plunges into the Almannagja fissure, a rift 50 metres wide and up to 30 metres deep. The Almannagja is located along the North Atlantic constructive plate margin, the boundary between the North American and the European tectonic plates which runs north- east to south-west across the island. Continental drift causes the two plates to slowly move apart, widening the Atlantic. Iceland is one of the few places in the world where the oceanic ridge rises above sea level. The island, which is entirely volcanic in origin, continues to exhibit intense seismic and volcanic activity.