Iceland is part of the North Atlantic Igneous Province- and example of Volcanic Rifted Margins. Large Igneous Provinces (LIPs) are often linked to active hotspots by linear chains of volcanic islands or volcanoes, leading to models that connect their origins to mantle plumes. At some point all of the Icelandic volcanoes share a mantle plume and caldera- how deep or how shallow we don't know.
This makes the island of Iceland the largest known super volcano on earth- even larger than Yellowstone.
Eruptions of super-volcanoes and the emplacement of LIPs appears to be coincide with four mass extinctions in geological history. The most important being the Permian mass extinction which more or less wiped the planet and reset the evolutionary clock.
There is no telling how long this volcano will erupt. It could last from days to decades. Other volcanoes in the caldera could become involved. Given the depth, type and duration of the magma plume fueling the eruption, it is possible that the entire Iceland caldera could become involved.