1. (1)In the 1960s, while studying the volcanic history of Yellowstone National Park, Bob Christiansen of the United States Geologic Survey became puzzled about something that, oddly, had not troubled anyone before, he couldn’t find the volcano in Yellowstone. It had been known for a long time that Yellowstone was volcanic in nature, that’s what accounted for all its geysers and hot springs, and the one thing about volcanoes is that they are usually easy to find. But Christiansen couldn’t find the Yellowstone volcano anywhere. In particular what he couldn’t find was a structure known as a caldera.

2. Old Faithful Geyser Hot Springs

3. (2)When we think of volcanoes, most of us think of the classic composite volcanoes, also known as stratovolcanoes like Mt. Fuji or Mt. Etna, which are a collection of lava flows and pyroclastic material layered together to form a large mountain. There are roughly 10,000 of these stratovolcanoes visible on Earth, but only a few hundred of them are still active, the rest are extinct. However, there is another type of explosive volcano that doesn’t make a mountain, but instead forms a caldera. A caldera volcano is so explosive that their single powerful eruption causes the crust to collapse into the partially emptied magma chamber, leaving a caldera, or very large, basin-shaped crater. Yellowstone was a caldera type volcano, but Christiansen couldn’t find the caldera anywhere.

4. Stratovolcano Caldera Volcano

5. (3)Around the time Christiansen was searching for the Yellowstone caldera in the 1960s, NASA had taken high-altitude photos of Yellowstone National Park. As soon as Christiansen saw the NASA photos he realized why he hadn’t seen the caldera; the photos showed that almost the entire national park was the caldera. The explosion had left a crater more than 40 miles across, way too large to be seen from the ground. At some time in the past Yellowstone must have blown up with a violence far greater than anything known to humans.

6. Map of Yellowstone and its Caldera

7. (4)Yellowstone, it turns out, is a supervolcano. It sits on top of a huge hot spot. The heat from the hot spot feeds a magma chamber that powers all of Yellowstone’s geysers and hot springs. This magma chamber is about 5 miles below the surface of the crust and is about 45 miles across and has a maximum thickness of about 8 miles. Imagine a pile of TNT about twice the size of Bucks County and reaching into the sky well above the highest mountain peak in the world, and you have some idea of what visitors of Yellowstone are walking around on top of. This magma chamber puts so much pressure on the crust above, that Yellowstone and the surrounding area are about 1,700 feet higher in elevation than they should be.

8. Yellowstone Magma Chamber

9. (5)Hot spots are not that rare. There are about 30 active hot spots on Earth, and they are responsible for many of the world’s best known islands and island chains, such as Iceland, Hawaii, and the Galapagos islands. What makes Yellowstone different from all of these other hot spots is that it is located under continental crust instead of oceanic crust. No one is sure how or why the hot spot of Yellowstone ended up under a continental plate, but the continental crust makes a huge difference to its eruptions. Where the other hot spots, like Hawaii, tend to have nonexplosive eruptions with runny lava flows, Yellowstone blows explosively. It doesn’t happen often, but when it does it is a historic event.

10. Hot Spots around the World

11. (6)Evidence shows that the Yellowstone supervolcano has erupted about 100 times in the past 16.5 million years, but the three most recent eruptions are the ones that get the attention. The last eruption, 640,000 years ago, was 1,000 times greater than the Mount St. Helens eruption; the one before that, 1.3 million years ago, was 280 times bigger, and the one before that, 2 million years ago, was so big that we have nothing to compare it to. It was at least 2,500 times greater than St. Helens, but possibly much, much greater. The biggest blast in recent times was Krakatau eruption in Indonesia in 1883. Air Pressure recordings showed that the shock wave from the eruption reverberated around the globe 7 times. But if you imagine the volume of material ejected from Krakatau as being about the size of a golf ball, then the biggest of the Yellowstone blasts would be the size of a ball you could just about hide behind. On this scale, Mount St. Helens’ would be no more than a pea.

12. Yellowstone Supervolcano compared to Mount St. Helens and Krakatau

13. (7)The Yellowstone eruption of 2 million years ago put out enough ash to bury the entire state of Pennsylvania to a depth of almost 80 feet. The ash fall from the most recent Yellowstone eruption, 640,000 years ago, covered all or parts of 19 western states, plus parts of Canada and Mexico, nearly all of the United States west of the Mississippi River. Keep in mind this is the breadbasket of America, an area that produces roughly half the world’s cereals. And ash is not like a big snowfall that will melt in the spring. If you wanted to grow crops again, you would have to find some place to put all the ash. Imagine what it would take to clear the state of Kansas.

14. Ash Fall (Yellowstone Eruptions vs. Mount St. Helens)

15. (8)Now imagine the impact this type of eruption could have on Earth’s climate from the amount of volcanic dust and gases shot into the atmosphere. The last supervolcano eruption on Earth was Toba, in Sumatra, 74,000 years ago. No one recorded how big the eruption was, since humans were still hunter-gatherers back then, but there is evidence to show it was a whopper. Ice cores taken from the very old Greenland ice sheet show that the Toba eruption was followed by at least 6 years of “volcanic winter”. There is evidence to suggest this super eruption may have reduced the human population to a few thousand people. In more recent times, when Tambora in Indonesia erupted in 1815 it triggered “the year without a summer” in 1816. There is no doubt that when large explosive eruptions occur the whole planet can be impacted through climate change. No one knows for sure how long the volcanic winters lasted from the past three Yellowstone super eruptions, but they likely had a significant impact on life on Earth at that time.

16. Volcanic Winter Toba compared to more Recent Eruptions

17. (9)All this information from past Yellowstone eruptions and supervolcanoes was interesting as a history lesson until 1973, when an odd event made a Yellowstone eruption very real. Water in Yellowstone Lake, in the heart of the park, began to run over the banks at the lake’s southern end flooding a meadow, while at the opposite end of the lake the water mysteriously flowed away. Geologists surveyed the park and discovered that a large area of the park had been pushed upward. This uplift raised up one end of the lake causing the water to run out at the other, as would happen if you lifted one side of a child’s kiddy pool. By 1984, the central part of the park had risen more than 3 feet. Then in 1985, this part of the park began to drop 8 inches.

18. Yellowstone Lake Hot Springs overlooking Yellowstone Lake

19. (10)The geologists realized that only one thing could cause this, a restless magma chamber. Yellowstone wasn’t the site of an ancient supervolcano, it was the site of an active one. It was also at about this time that they were able to work out that the cycle of Yellowstone’s super eruptions averaged one massive explosion every 600,000 years. The last one, interestingly enough, was 640,000 years ago.

20. Yellowstone Caldera Magma Chamber

21. (11)By looking at the way earthquake waves travel through the crust below Yellowstone, scientists have discovered that the magma chamber is fed by a gigantic hot spot, rising from the Earth’s upper mantle. When the hot spot pumps more heat into the chamber, the crust rises upward. Small earthquakes allow gases and hydrothermal fluids to escape the magma chamber to the Earth’s surface, easing the pressure inside the magma chamber, which causes the ground to drop again. After Yellowstone dropped 8 inches from 1985 to 1995 it began to rise again, but faster this time. Since 2004, portions of the caldera have risen upward at a rate of nearly 3 inches a year, much faster than any uplift since close observations began in the 1970s.

22. Earthquakes during 1985 Mapping of the Hot Spot

23. (12)So, the big question is, will the Yellowstone supervolcano erupt again? Many scientists suspect that some kind of smaller eruption compared to the last three Yellowstone eruptions is highly likely at some point in the future. This type of eruption might be comparable to the eruption of Mount Pinatubo in the Philippines in 1991, which was still the second largest eruption in the last 100 years. The odds of a full scale, caldera-forming super eruption is not as likely anytime soon or perhaps never. Bob Christiansen, now retired, suspects the supervolcano magma chamber may be safely bottled up. For most of its history, the Yellowstone hot spot has formed caldera volcanoes in a thinner part of the continental crust along the western United States. As the North American plate has continued to move west the hot spot magma chamber is now lodged beneath a much thicker part of the continental crust among the Rocky Mountains possibly bottling up the hotpot from further super eruptions.

24. Pinatubo Eruption (1991) Plate Motion Are the Rockies thick enough to bottle up the Hot Spot?