Krakatoa

Krakatoa (also spelt Krakatau) is a small Indonesian island volcano famous for producing one of the largest and most violent eruptions in recorded human history. On 27 August 1883, Krakatoa exploded with the force of approximately 200 megatons of TNT (over 10,000 times the power of the Hiroshima atomic bomb). The eruption produced what is believed to be the loudest sound ever heard by humans: it was clearly audible 4,800 km away in Australia. The collapse of the volcano into the sea triggered tsunamis up to 40 m tall that killed at least 36,000 people. The eruption changed the global climate for years afterwards.

• LocationIndonesiaIn the Sunda Strait between Java and Sumatra
• Date of biggest eruption27 August 1883
• VEI6"Colossal"
• Loudest sound everHeard 4,800 km awayIn Mauritius, Australia and other places
• Tsunami heightUp to 40 metresAround 10 storeys tall
• DeathsApproximately 36,000Mostly from tsunamis, not the eruption directly

The 1883 eruption

Krakatoa had shown signs of activity for months before the main event. Then on 27 August 1883, in the space of just a few hours, the volcano produced four enormous explosions. The largest, at 10:02 in the morning, was so powerful that it shattered eardrums of sailors 60 km away and propelled an ash cloud approximately 80 km high.

The eruption blew apart approximately two-thirds of the volcano. The remaining hollow caldera then collapsed into the sea. The sudden displacement of huge volumes of water sent tsunamis racing outwards across the Sunda Strait at jet-aircraft speeds. The waves were up to 40 metres tall when they hit the coasts of Java and Sumatra. Nearby coastal towns were utterly destroyed.

The loudest sound ever heard

The 10:02 explosion is widely considered to be the loudest sound ever clearly heard by humans. Some details:

• It was clearly audible 4,800 km away in Mauritius and Western Australia.
• People in Perth (3,100 km away) reported hearing it as artillery fire.
• The sound wave was so powerful it travelled around the entire Earth 4 times, registering on barometers all over the world.
• The local sound near the volcano (over 200 decibels) would have killed any human within a few kilometres just from the pressure of the air.

Effects around the world

• Brilliant red sunsets: huge amounts of fine ash and sulphur dioxide in the upper atmosphere produced spectacular red sunsets around the world for over 3 years. They may have inspired the swirling sky in Edvard Munch's famous painting "The Scream" (1893).
• Global cooling: average global temperatures dropped by approximately 1.2 degrees C the following year.
• Strange light effects: the Moon appeared bluish or greenish in many parts of the world for months.
• First global news: the eruption happened just as the world had been linked up by telegraph cables. It was the first major natural disaster reported around the world in near real time.

Fact The 1883 eruption of Krakatoa was so loud that some people in places like Sri Lanka (3,000 km away) thought a major battle had broken out nearby. British ships in the Indian Ocean reported "tremendous artillery fire" and prepared for action, only to discover days later that there had been no battle, just a distant volcanic eruption almost completely unprecedented in human history.

The birth of Anak Krakatau

After 1883, the area where Krakatoa had stood was mostly empty ocean. But the volcano was not finished. In 1927, sailors noticed steam and ash bubbling up from the sea in the middle of the collapsed caldera. A new volcanic cone was building up. By the 1930s it had broken the surface and become a small island. It was named Anak Krakatau ("Child of Krakatoa").

Anak Krakatau has been growing steadily ever since, adding roughly 13 cm in height per week in some periods. It has had many small eruptions over the decades. In December 2018, the side of Anak Krakatau partly collapsed during an eruption, triggering a tsunami that killed 437 people in nearby coastal towns. The volcano lost approximately two-thirds of its height in the collapse but is now slowly rebuilding itself again.

Did you know? Edvard Munch's famous painting "The Scream" (1893) shows a figure on a bridge with a swirling red and orange sky in the background. Munch wrote that he had been walking with friends in Norway when "the sky suddenly turned blood red". Some art historians and atmospheric scientists believe Munch was painting one of the unusual red sunsets caused by Krakatoa, which lasted for years after the eruption and were spectacular in Scandinavia.

Deeper dive: how Krakatoa shaped modern volcanology

The 1883 eruption of Krakatoa was a turning point for the young science of volcanology. It happened just as global communications, instruments and scientific organisations were becoming capable of recording it in unprecedented detail.

Things scientists learned from Krakatoa:

• Volcanic tsunamis can be more deadly than the eruption itself. Most Krakatoa victims died from waves, not lava or ash. This led to better understanding of volcanic island collapse hazards, eventually informing modern tsunami warning systems.
• Eruptions can affect global climate. Detailed temperature records from the years after Krakatoa showed worldwide cooling, and helped develop our modern understanding of how volcanic aerosols affect the atmosphere.
• Sound waves can travel around the world. The Krakatoa pressure wave registered on barometers around the globe for days, providing the first solid evidence of how the atmosphere behaves on a planetary scale.
• New volcanic islands grow rapidly. The emergence and growth of Anak Krakatau has provided one of the best real-time observations of how a new volcanic island forms.
• Ecological recovery: in the months after the 1883 eruption, the remaining bits of Krakatoa were sterile. Scientists watched as life slowly returned: first bacteria and ferns, then insects and birds carrying seeds, then larger plants and small reptiles. By the 1920s a healthy ecosystem had re-established itself. This became one of the most important case studies in ecology.

Modern volcanology owes a huge debt to the careful international cooperation in studying Krakatoa's aftermath. The patterns it revealed have been recognised at many later eruptions.

For more, see volcanic eruptions, the Ring of Fire and tsunamis.