When will Yellowstone erupt again?
We do not know. Future volcanic eruptions could occur within or near Yellowstone National Park for the simple reason that the area has a long volcanic history and because there is hot and molten rock, or magma, beneath the caldera now. USGS scientists monitor Yellowstone for signs of volcanic activity using seismographs (to detect earthquakes) and GPS (to detect ground motion).
There is no evidence that a catastrophic eruption at Yellowstone is imminent, and such events are unlikely to occur in the next few centuries. Scientists have also found no indication of an imminent smaller eruption of lava.
Yellowstone's 2-million-year history of volcanism, the copious amount of heat that still flows from the ground, the frequent earthquakes, and the repeated uplift and subsidence of the caldera floor also testify to the continuity of magmatic processes beneath Yellowstone and point to the possibility of future volcanism and future earthquake activity.
The Yellowstone Volcano Observatory website has information on Yellowstone history and hazards.