photo by Thursday Review

Yellowstone is Bigger Than We Thought
| published April 28, 2015 |

By Thursday Review staff writers

Scientists and geologists have known for most of the 20th century that Yellowstone’s caldera area—the more-or-less circular area which was once the upper part of a large volcano—was massive, one of the largest in all of the Americas, and the single biggest caldera north of the Isthmus of Panama.

The Yellowstone volcano, though highly active in thousands of small areas now visited by tourists every day, is—however—relatively inactive as actual volcanos go. Its last eruption was roughly 621,000 years ago, and geologists believe that it only erupts on a 700 thousand year cycle. That means humans have a bit of time to spare before we get our bags packed and move out Yellowstone’s path.

Still, the recent announcement that scientists and geologists have discovered that the magma chamber under Yellowstone is even larger than originally thought requires a few analogies. Somewhere deep underground and ranging from 12 to 28 miles below the Wyoming surface, sits a chamber of molten lava four times larger than the most generous previous estimates. That enormous underground cavern contains enough lava to fill all of the Grand Canyon, from Roundy Creek in the north, to the Ruby Rapids in the west.

Another image might help: if all that molten lava were spread out in a large puddle on a relatively flat surface, six feet thick, it would cover all of the Dakotas, Nebraska and Kansas, with some lava to spare. We use the five foot image to spare the ability of people six feet tall to look around after they discover themselves standing in, say, Sidney, Nebraska, wondering how they landed neck-deep in lava.

The Yellowstone rim—or caldera—sometimes described as the Yellowstone Supervolcano, is massive, stretching from the southern edge of Montana down to the Teton Range of the Rocky Mountains, and from eastern Idaho (in the west) over to east-central Wyoming. The rim is actually multiple overlapping rims, including the remnants of eruptions ranging from roughly 1.3 million years and 2.1 million years ago. The so-called Third Caldera is the cyclical one which scientists believe last erupted 621 thousand years ago. The wider rim measures approximately 34 miles from north to south, and 55 miles east to west.

Even the rim’s enormous diameter belies the vast quantity of magma underneath.

Humans were not around to witness the power and intensity of Yellowstone’s last eruption. But according to the U.S. Park Service, the best, closest comparison to something seen in human history is the April 1815 eruption of Mount Tambora, on the island of Sumbawa, now a part of Indonesia. Mount Tambora erupted on April 5, 1815, sending shockwaves through the earth, causing the ground to shake 400 miles away and sending shockwaves of sound more than 850 miles away. People heard the thunderous explosions in Australia, in New Guinea, and in Borneo. Nearby towns and villages were destroyed instantly, incinerated by the heat and incandescent ash.

A few nights after the eruption began, people more than 20 miles away watched as Tambora lit up again, this time with enormous columns of molten lava and fire stretching up into the sky, puncturing the heavy smoke and ash cloud cover which had been forming for days. Some of those eyewitnesses reported that five minutes later, debris began raining down on them from the sky—stones as large as eight inches in diameter which had been hurled in all directions by the explosions. A few minutes after the debris began falling, a powerful shockwave of air, gas and ash slammed into villages, towns, and other islands in an ever-expanding circle of destruction. The toxic wind levelled houses, destroyed boats and ships, uprooted trees, and stripped the sand and soil of most vegetation. Thousands were killed within hours.

But Tambora was not finished with its temper tantrum. Early on April 16, eleven days after its eruptions began, another series of explosions were triggered as the rock and ground around the island became so fractured and fragmented that it became unstable. Millions of gallons of seawater poured inward, into the molten chambers, essentially making the pressure cooker explode from within. The new round of explosions were so powerful that the sound travelled for more than 1500 miles in all directions. People in Sumatra could hear the series of booms and feel the ground shake, and sailors as far away as New Zealand and the Philippines heard what seemed like endless thunder. The mountain was collapsing, its walls and slopes shattering and tumbling down into the superheated caldron, now battling with the rush of incoming seawater and triggering explosions on a scale larger than atomic explosions. A tsunami was set in motion, and more shockwaves of hot air, ash and gas were now exploding outward in a concentric circle. Within 30 minutes, more than 88,000 people were dead in a wide circle stretching some 350 miles in all directions. Over the next days, ash fell for hundreds of miles, in some places (in what is now Indonesia) forming a layer two feet thick. The ash cloud stretched unbroken for 850 miles, and over the next weeks and months, that thick dust soon dispersed throughout the atmosphere and covered the planet.

Tambora’s 1815 display of superheated anger was the most powerful volcanic event in recorded history, and its legacy lasted for several years: unusual and eerie sunsets in many parts of the world; disruptions to ship navigation for lack of night sky visibility; and a worldwide drop in temperature which affected both the Southern and Northern Hemispheres, and stalled or stunted agricultural production for more than three years. Some areas of the world were substantially cooler, and farmers dubbed 1816 as the “Year Without Summer,” a reference to the heavily clouded skies and cool median temperatures.

For humans, the Tambora eruption was the big one, eclipsing all volcanic events for thousands of years of recorded history.

How big was the last Yellowstone eruption by comparison? Geologists and scientists say that the next time Yellowstone erupts, in about 70 thousand years, it will exceed Tambora by a factor of 17. Yellowstone’s last eruption produced a volcanic output of rock and soil of roughly 600 cubic miles (that’s miles, mind you).

Want a contemporary comparison? The next Yellowstone eruption will be 2500 times larger than the 1980 eruption of Mount St. Helens.

Related Thursday Review articles:

Australia, Seen From Space; Thursday Review staff; Thursday Review; April 8, 2015.

Hubble's Birthday: Celestial Fireworks; Keith H. Roberts; Thursday Review staff; April 24, 2015.