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Stages in the creation of a calderaStages in the creation of a calderaA supervolcano is a volcano that erupts more than 1000 cubic kilometres of magma. Such a volcano would have a devastating effect over a very wide area, but, luckily, supervolcanoes are extremely rare. The last one erupted at what is now Lake Toba in Sumatra, Indonesia, about 74,000 years ago. Unlike “normal” volcanoes, which rise as steep-sided mountains, supervolcanoes are usually large depressions. Some supervolcanoes come into existence when magma in the Earth's mantle rises into the crust from a hot spot, but cannot break through the crust. Pressure builds up in a large underground magma pool, until the crust is unable to contain it any longer. When the eruption eventually happens, the magma pool drains away and the land above it collapses. This creates a caldera, a vast crater many kilometres across (the Yellowstone Caldera in Wyoming was created in this way). Supervolcanoes can also form above a subduction zone, where the edge of one tectonic plate boundary is sliding under another. Lake Toba is an example of this.

A supervolcanic eruptionA supervolcanic eruption
A view of the Deccan Traps, in central India, today.A view of the Deccan Traps, in central India, today.


Supervolcanic eruptions cover vast areas with ash, destroying vegetation. They may even change world climates, as clouds of dust swirling round the Earth block out the sun for years on end.

The Deccan Traps are a huge area of igneous rock in central India. They were formed after a supervolcano erupted there about 66 million years ago. This coincided with the extinction of the dinosaurs at the end of the Cretaceous Period. An asteroid impact probably finished the dinosaurs off, but a supervolcanic eruption around this time may have already affected the global climate, devastating dinosaur populations.

Cross-section through a calderaCross-section through a caldera


The magma chamber beneath Yellowstone National Park is about 80 km (50 miles) long and 20 km (12 miles) wide. Only about 6–8% is filled with molten rock. Scientists believe that the proportion of melted rock in the chamber is much too low to trigger another supervolcanic eruption.

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