How Krakatoa made the biggest bang

It was 6am on August 26 in 1883, when the volcano on Krakatoa, a small island in Indonesia, catastrophically erupted. This earth-shattering event became the greatest natural disaster of the 19th century: the sky was bathed in an unearthly red glow and the fallout was felt around the world.

The force of the eruption created the loudest noise ever recorded: it was heard 4,653km away on Rodriguez Island in the Indian Ocean and some 4,800km away in Alice Springs; shock waves travelled around the world seven times; and the force of the blast was some 10,000 times greater than that of the hydrogen bomb dropped on Hiroshima. The volcano left 36,000 people dead and the survivors battled to cope with tsunamis, further eruptions and superheated ash clouds.

As the volcano erupted, a plume of ash swept 80km into the sky, the hot gas became unstable and raced across nearby islands at 150km. "Those who weren't killed by the intense heat," says Dr Dave Rothery, from the Department of Earth Sciences at the Open University, "would have been sandblasted to death. It was hot enough to carbonise everything in its path."

The real killers, though, were the giant tsunamis that were unleashed, reaching heights of 40m and which were so violent that they flung sections of coral reef ashore, some weighing as much as 600 tons. Like the Indonesian tsunami on Boxing Day 2004, these tsunamis destroyed everything in their path.

This Sunday, the BBC re-examines the impact of one of the world's largest volcanoes in a factual drama, Krakatoa - The Last Days, and in an accompanying documentary, Krakatoa Revealed. Human tragedy aside, Krakatoa was important because it was declared a "modern" disaster and became one of the first global media events.

It produced a wealth of first-hand survivor accounts: a German quarry manager recounted being hurled by a wall of water from the top of his office block and swept into the jungle below. To his astonishment, he saw a crocodile being carried alongside him and incredibly he leapt on the animal's back and rode it for 3km before being deposited, unharmed, on the rainforest floor.

The rest of the world heard such stories almost instantly because a series of underwater telegraph cables had been recently laid traversing the globe. For the first time, operators were able to communicate stories to their counterparts across the globe using morse code. As Professor Nick Petford, from the School of Earth Sciences and Geology at Kingston University, London, and presenter of the BBC documentary explains, "This is the first time a volcano had exploded and was known about instantly. The underwater telegraph cables were a network for communication, the precursor to the internet."

In these days of instant global access, it's easy to forget how revolutionary this was: Krakatoa has been erupting irregularly since 250 AD; the last previous explosion was even more powerful and had happened only 200 years before the one in 1883, yet few had heard of this catastrophe.

This new technology meant that Krakatoa also generated the first modern scientific study of a volcanic eruption. "At the time geology was a lively academic discipline," says Petford. "A Dutch scientist, Rogier Verbeek, got there very quickly and recorded everything and his report was an astoundingly inspired piece of work."

Verbeek was quickly followed by a team of geologists from London's Royal Society and what was immediately obvious was that while two-thirds of the island of Krakatoa had been destroyed, a string of new islands had formed where the sea had been 36m deep. Many were ephemeral but Anak Krakatoa (child of Krakatoa), which formed in the remnants of its parent, was here to stay: it's been erupting since 1927 and the last recorded explosion was in 2001.

So what made Krakatoa explode so powerfully? Indonesia has many active volcanoes (132 have exploded in the past 10,000 years) because the islands lie above two tectonic plates, one of which is being pulled underneath the other at a rate of 6cm per year. Originally Verbeek thought that Krakatoa was so fierce because sea water flooded into the volcano, reacting with molten lava; the build-up of pressure from the resulting steam would have led to an enormous blast.

Petford has an alternative theory. He uses computer models combined with sandcastles - volcanoes made of sand - into which he pumps air to mimic the build up of pressure from gas deep within the volcano. His model of Krakatoa showed that gas pressure leads to "massive sector collapse": a large portion of the volcano collapses under the internal pressure. His model left a crater and landslide scar very like the remains of Krakatoa.

"Our best idea about what happened is that the final stage of the Krakatoa eruption was marked by a massive internal failure," says Petford, "perhaps due to a landslide that exposed lots of magma to atmospheric pressure."

The best way of predicting a blast is to record seismic activity within a volcano. Small earthquakes indicate that the volcano is becoming unstable. However, many countries do not have the money to monitor volcanic activity. Conversely, volcanoes in wealthy areas are also a problem. "Look at Mount Vesuvius," says Petford, "It erupted in the Second World War and it will erupt again. But people have built houses further and further up the slopes. You have a penthouse view across the Bay of Naples, your house cost a fortune, would you abandon it?"

Mount Vesuvius is not the only slumbering supervolcano. Krakatoa, or rather, its child, is also bubbling away. Professor Richard Fiske, from America's Smithsonian Institute, and one of the authors of a scientific book on Krakatoa, says, "It's not going to erupt in our lifetime." But given Krakatoa's history, it seems highly likely that it could explode in our children's.

Krakatoa - The Last Days, a factual drama, BBC1, Sunday 7 May, 8-9.30pm; Krakatoa Revealed, BBC1, Sunday 7 May, 10-10.50pm

Angry mountains: the power of nature

As you read this, 20 volcanoes are erupting throughout the world. In the past 10,000 years 1,300 have erupted and estimates suggest that there have been more than a million underwater explosions. These are five of the worst explosions that humankind has recorded.

Montserrat

In 1902 Mount Pelée on the French Island of Martinique, 150 miles south of Montserrat erupted, killing 28,000 people in a pyroclastic flow. This is a mixture of hot lava turned into a fluid by expanding volcanic gas and air, which is hot enough to melt glass and flows at speeds of around 150km. Volcanic activity has now moved to Montserrat, which has been active since 1995 and may be in danger of erupting at any point.

Mount St Helen's

On Sunday morning, May 18, 1980, Mount St Helen's in Washington State, erupted. The north face of the mountain collapsed, causing a massive rock avalanche. Almost 230 square miles of forest was blown down or buried beneath ash deposits. A column of ash rose hundreds of metres into the air and the eruption lasted for nine hours. Around 24,000 animals and 56 people died.

Tambora

The explosion in 1815 of the Indonesian Tambora volcano was the largest recorded. About 150 cubic kilometres of ash (150 times more than was produced when Mount St Helens erupted) was blown 44km into the sky and spread for 1,300km. Around 92,000 people were killed.

Toba

Toba, in Sumatra, exploded 74,000 years ago. The blast was so strong that a mixture of ash and sulphur dioxide was thrown into the stratosphere, blocking out the sun's rays and causing the temperature to plummet. Some scientists believe that it almost led to the extinction of humanity as only a few thousand people survived.

Vesuvius

Vesuvius has exploded a number of times in the past 17,000 years; the most famous eruption was on 24 August 79AD when a pyroclastic flow destroyed the Roman cities of Pompeii and Herculaneum, burying thousands beneath the lava and preserving their bodies.