Three extremely large explosive eruptions have occurred at Yellowstone in the past 2.1 million years with a recurrence interval of about 600,000 to 800,000 years. More frequent eruptions of basalt and rhyolite lava flows have occurred before and after the large caldera-forming events. For example, scientists have identified about 30 different rhyolite lava flows that erupted after the most recent caldera eruptions, about 640,000 years ago, from vents inside the caldera. The most recent was about 70,000 years ago. Many of these eruptions were separated in time by several tens of thousandsof years. Because the evidence of earlier eruptions may have been either buried or destroyed, we do not really know how often the volcano has actually erupted.

Source: USGS