A supervolcano produces eruptions and explosions of catastrophic magnitude. It is a major event that generally decimates the entire population of a region, capable of killing tens of millions of people in a few minutes! No one can imagine such an eruption because none have ever been seen or recorded.
The explosivity index scale
Professor Bill McGuire, volcanologist and geophysicist at the Benfield Greig Centre of the UCL in the United States reminds us that super eruptions are often called "VEI 8" which represents a degree of 8 on the volcanic explosivity index scale, the VEI index varying between 0 and 8. Each degree represents an eruption 10 times more powerful than the previous one. Mount St. Helens, for example, was classified as VEI 5: paroxysmal. To give you an idea of its power, it released energy equivalent to 27 000 times that of the Hiroshima bomb (12.5 kT), i.e. over 337 MT of TNT! It spewed out 470 million tonnes of ash, earth and stones over a radius of 30 km and unfortunately caused 57 deaths.
If we represent the quantity of material ejected as a cube, with the eruption of Mount St Helens represented by a small cube of 1 cm3. When you see the damage it caused, pulverising an entire mountainside, you understand that its energy was phenomenal. Going to the next higher class, a VEI 6 such as the explosion of Santorin, would be represented by 125 cm3, i.e. a cube of side 5 cm. Its eruption would be described as colossal. The dust that it released, called a plinian spine, rose up to the stratosphere at an altitude of 36 km, the level of the ozone layer!
Now hold on to your seat. If now we represent a VEI 8 eruption, we are talking on quite a different scale. This is a "mega-colossal" eruption equivalent to 1000 volcanoes like St. Helens simultaneously. The energy released is 22 times that of the most powerful American H bomb, Castel Romeo, which exploded on 26 March 1954 at Bikini! In our scaled down model, the quantity of ejected material would be represented by ... a cubic metre! We could fit a million of those little VEI 5 cubes in this volume!
Today there are 1500 active volcanoes in the world. Every year there are 50 or 60 volcanic eruptions in the world. They are mainly between 0 and 3 VEI. Nearly all volcanoes release harmful gases such as carbon dioxide and sulphur dioxide, and sometimes just ash. Most eject relatively fluid lava containing gas that can easily escape without causing an explosion. Others explode releasing pyroclastic surges (gas, steam and solid debris at high temperature flowing at several hundred km/h at ground height), but supervolcanoes are completely different.
We do not know exactly why, but supervolcanoes form a vast reservoir of molten rock. This is what is known as active caldera. The magma is so thick and viscous that it traps the gases, leading to a colossal increase in pressure over a few thousand years. When the magma chamber can no longer contain this pressure there is an explosive eruption hundreds of times more powerful than the normal flow from the reservoir through fissures in the rock. This effect destroys the roof of the chamber forming an enormous crater which can reach a diameter of a hundred kilometres. The volcano collapses and is transformed into a caldera.
If the catastrophe occurs at sea, it creates a tsunami and what remains of the volcano is likely to be engulfed by the water. This is what happened at Santorin and at Toba and probably in other supervolcanoes that today are extinct and undetectable.
The main factor determining the size of the eruption of a supervolcano is the amount of magma available. If an enormous amount of magma accumulates under the crust, we have the potential risk of seeing an extremely large explosion.
Across the world there are a few places with the right geological conditions to create such vast magma chambers. They can be counted on the fingers of one hand and researchers are not even sure they have been able to find all the supervolcanoes that still exist. We know of at least two. In Sumatra there is a supervolcano currently located in the middle of a lake called Toba. There were no witnesses to its eruption, which occurred 74 000 years ago.
The second is more surprising: it is the Yellowstone National Park in the United States. It is well known mainly for its "landscapes from hell": the rainbow lake of Grand Prismatic Spring that owes its colours to cyanobacteria, the hot springs at Mammoth Hot Spring locally covered in slime and its geyser Old Faithful, which spouts every 65 to 92 minutes up to a height of 55 metres. But from time to time, some trails are closed to the public because the thermal activity is too intense, proof if it were needed that though the place has its attractive aspects, it still remains capricious. It remains to be known why.