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. Yellowstone is monitored for signs of volcanic activity by YVO scientists who detect earthquakes using seismographs and ground motion using GPS (Global Positioning System). YVO has not detected signs of activity that suggest an eruption is imminent.
The crust of North America continuously moves southwest over the Yellowstone hotspot as the Earth's crust stretches above it, promoting the ascent of heat and molten rock. These processes produce basaltic magmas within the Earth's mantle, which rise into the overlying crust and continue to heat the rocks beneath Yellowstone, maintaining and possibly adding to the rhyolite magma in the crust above.
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.