Mike Poland is the scientist-in-charge of the Yellowstone Volcano Observatory. During his monthly update, he debunked the misconception that if Yellowstone were to erupt, it would be Armageddon.
In fact, he says, the most common form of eruptive activity at Yellowstone has been a lava flow, and even those aren’t that common. They happen only once every few tens of thousands of years.
The last big explosive eruption at Yellowstone was 631,000 years ago and resulted in the caldera that you now know from your visits to Yellowstone. Since that time, there have been about 20 or more lava flows that have occurred in the caldera region.
These lava flows are very thick, pasty rhyolite flows, not like what you’d see in Hawaii. They are very thick flows that don’t move very quickly, but they are huge. So, lava flow activity is much more common than explosive eruptions, but still somewhat rare, with the last one being about 70,000 years ago.