Yellowstone National Park is volcanic in nature. That is what accounts for its geysers and hot pools and streams. But, if it is volcanic, where is its volcano? Well, the whole park is actually an active volcano sitting on top of a ginormous volcanic hotpsot and all 2.2 million acres is a caldera. Beneath the surface of Yellowstone is a magma chamber that is about 45 miles across and over 5 miles thick.
It is estimated that Yellowstone has erupted over 100 times in the past, beginning about 16.5 million years ago. Not all of its eruptions have been major ones; it has only had three truly enormous eruptions in history. The super-eruptions occurred 2.1 million years ago, 1.3 million years ago, and 664,000 years ago. The last super-eruption was about a thousand times more explosive than Mt. St. Helens. The Yellowstone eruption of 2 million years ago spewed enough ash to bury New York State to a depth of sixty-seven feet.
The last supervolcano eruption on Earth was about 74,000 years ago in northern Sumatra and is called the “Toba Eruption”. The Toba eruption caused six – ten years of “volcanic winter” on Earth due to all the ash it spewed forth and is thought to have affected earth’s climate for 1,000 years. There is some evidence that the eruption may have brought humans to the brink of extinction – reducing the global population of humans to a few thousand individuals. This helps explain the relative lack of genetic diversity among humans – we all came from a relatively small population base caused by the Toba eruption. Note that a recently published study in the journal Nature sets forth evidence that the Toba eruption did not cause the population bottleneck.
Super-eruption of Yellowstone would likely throw Earth into another volcanic winter for some period of time. The ash from the eruption would likely bury much of the midwest and west in ash, greatly impacting the Earth’s food supply immediately.
The ash in the atmosphere would partly block out the sun and cause the average temperature on Earth to drop. Once all the ash dissipated from the atmosphere the Earth would slowly heat back up. It could be devastating in terms of loss of life, plant, animal and human.
So, should we be worried? Probably not. According to the USGS “Given Yellowstone’s past history, the yearly probability of another caldera-forming eruption can be approximated as 1 in 730,000 or 0.00014%. However, this number is based simply on averaging the two intervals between the three major past eruptions at Yellowstone — this is hardly enough to make a critical judgment. This probability is roughly similar to that of a large (1 kilometer) asteroid hitting the Earth. Moreover, catastrophic geologic events are neither regular nor predictable. 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.“
Finally, an eruption of Yellowstone wouldn’t necessarily be a super-eruption. From the USGS: “Over the past 640,000 years since the last giant eruption at Yellowstone, approximately 80 relatively nonexplosive eruptions have occurred and produced primarily lava flows. This would be the most likely kind of future eruption. If such an event were to occur today, there would be much disruption of activities in Yellowstone National Park, but in all likelihood few lives would be threatened.”