A supervolcano is defined as a volcano that ejects more than a trillion tons of material when it erupts. When one explodes, it can cover an entire continent in ash. This type of explosion is about 30 times stronger than the strongest volcanic eruption in recent history, which occurred at Krakatoa.
The last time a supervolcano exploded was at Toba in Sumatra around 71,000 years ago. Ash blotted out the sun's rays, and humanity likely came close to extinction. Anthropologists estimate that only about 5,000 humans survived to reproduce in the aftermath of this event.
Another famous one is located directly underneath Yellowstone National Park, in the US. The caldera, a depression on top of the volcano, is 25 to 31 miles (40 to 50 km) long, about 12.4 miles (20 km) wide and about 6.2 miles (10 km) thick. It is so large that it is visible from space. This volcano is thought to explode about once every 600,000 years, but it last exploded more than 630,000 years ago.
A supervolcano will send hundreds of cubic miles (where about 240 cubic miles is equal to 1,000 cubic km) of ash into the atmosphere, changing the climate for hundreds to thousands of years. When Yellowstone last exploded, a pack of fossilized rhinos was discovered 621 miles (1,000 km) away from the blast zone, where they choked to death underneath the heavy ash. These massive volcanoes are the most powerful known destructive force on the planet, and only asteroids or other cosmic events are potentially powerful enough to exceed their magnitude.
The difference between regular volcanoes and supervolcanoes is in the way the magma underneath each comes to the surface. In a normal volcano, a thin magma chamber leads to a towering cone, with a relatively thin layer of rock shielding it from the surface. When pressure underneath builds up sufficiently, the magma is shot upwards.
In a supervolcano, magma comes up close to the surface, but a large mass of rock prevents it from breaking free. This rock forms the top of a large depression called a caldera. Over hundreds of thousands of years, magma from beneath builds up in a huge lake of tremendous pressure immediately under the caldera. When this pressure reaches a critical threshold, it blasts the entire thing sky-high, ejecting huge amounts of molten lava.