Every hour or so the “Old Faithful” geyser in Yellowstone National Park, Wyoming, shoots a column of boiling water up to 50m into the air. This incredible natural spectacle, along with multiple other geothermal features and the one to two thousand earthquakes that occur every year, are just some of the signs that this region is sitting atop a whopping great volcano. The last super-eruption was nearly 640,000 years ago, but gentle swelling of the ground indicates that the underlying magma chamber is refilling, and Yellowstone will erupt again one day.

Most volcanoes occur at the boundary between two tectonic plates, but Yellowstone is unusual because it lies centrally on the North America plate. Many geologists believe that is because Yellowstone sits on top of a “hot spot” – a plume of warm mantle rising up from the edge of the Earth’s core. Hot spots create chains of volcanoes (like the Hawaiian island chain) as the tectonic plate above glides over it.

However, new research, published in Geophysical Research Letters, makes the hot-spot theory seem unlikely for Yellowstone. Recent images of the underlying mantle show clear anomalies between 400 and 1,000km deep, thought to be the remnants of an ancient ocean plate. By modelling the way the mantle might flow around this ancient plate, Tiffany Leonard and Lijun Liu from the University of Illinois have shown that this sinking slab would block the heat and get in the way of any mantle plume.

Unfortunately their model doesn’t provide any other explanations for the presence of Yellowstone; quite why this vast volcano exists remains a mystery.