A supervolcano is defined as a volcano that ejects more than a trillion tons of material when it erupts. Supervolcanoes can cover an entire continent in ash. A supervolcano 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 came close to extinction. Anthropologists estimate that only about 5,000 humans survived to reproduce in the aftermath of this event.

Another famous supervolcano is located directly underneath Yellowstone National Park, in the USA. The caldera is 40-50 kilometers (25-31 miles) long, about 20 kilometers (12.4 miles) wide and about 10 kilometers (6.2 miles) thick. It is so large as to be visible from space. The supervolcano explodes about once every 600,000 years. It last exploded more than 630,000 years ago.

A supervolcano sends hundreds to thousands of cubic kilometers 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 1000 km (621 miles) away from the blast zone. They choked to death underneath the heavy ash. A supervolcano is the most powerful known destructive force on the planet. Only asteroids or other cosmic events are potentially powerful enough to exceed their magnitude.

The difference between 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 the magma 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 caldera sky-high, ejecting hundreds of cubic kilometers of molten lava.