Krakatoa, Sunda Strait

Four explosions in twenty hours

The 1883 eruption of Krakatoa — a small volcanic island in the Sunda Strait between Java and Sumatra — was the largest volcanic eruption of the modern instrumental period.

It produced four climactic explosions over the course of twenty hours.

The first at 05:30 on 27 August 1883. The second at 06:44. The third at 08:20. The fourth and largest at 10:02. The fourth explosion is the loudest sound documented in recorded human history. Its acoustic energy was measured by barographs at six weather stations around the planet, including the British colonial observatory at Rodrigues Island in the western Indian Ocean — 4 800 kilometres from Krakatoa — where the chief of police reported it as "the distant roar of heavy guns".

The atmospheric pressure wave from the fourth explosion travelled around the planet seven times in the next four days before fading below detectability.

A volcano that swallowed itself

The eruption destroyed two-thirds of the island of Krakatoa.

The volcano had been producing minor steam eruptions and small ash columns since May 1883 — a three-month buildup that European scientists in Batavia (modern Jakarta) had been observing without alarm. The climactic phase began on the afternoon of 26 August and ended at 10:02 on 27 August.

In those twenty hours, about 21 cubic kilometres of magma and rock were ejected from the volcanic chamber. The chamber, emptied of support, collapsed inward. The central part of the island of Krakatoa — including the entire 800-metre-tall Perbuwatan peak — fell into the resulting caldera, leaving only the southern third above sea level as a still-existing islet.

The chamber collapse generated the climactic 10:02 explosion and the giant tsunami that followed.

Thirty-six metres of water

The tsunami was the deadliest part of the eruption.

A wave between 30 and 36 metres high — about the height of a ten- storey building — propagated outward from Krakatoa at roughly 700 km/h. Within an hour it had struck the south coast of Sumatra at the port of Telok Betong (modern Bandar Lampung) and the west coast of Java at the port of Anyer.

The cumulative casualty figure was around 36 000 dead, almost all of them from the tsunami rather than from direct effects of the eruption. The Dutch colonial gunboat Berouw, which had been moored at Telok Betong, was carried 3 kilometres inland and deposited on a hillside 9 metres above sea level. Its wreck remained on the hillside for twenty years before it was finally broken up for scrap. The displacement of the Berouw is still cited in textbooks as a benchmark of tsunami energy.

The wave continued outward across the Indian Ocean. Tide gauges in Aden (5 800 km away) and Le Havre (17 000 km away — almost on the other side of the planet) recorded measurable wave action over the following 24 hours.

Three years of sunsets

For three years after the eruption, sunsets across the world were unusually vivid.

The volcanic ash from Krakatoa — particularly the sulphur dioxide that oxidised to sulphate aerosols in the stratosphere — circulated around the planet at the 25-kilometre altitude and remained suspended for roughly three years. The sulphate haze reflected and scattered sunlight, particularly at the red end of the spectrum at low solar angles.

The dramatic red sunsets of late 1883, 1884 and 1885 were observed and reported across Europe and North America. The London skies of November 1883 were described in a Royal Society report as showing "the most extraordinary appearance after sunset, the whole western horizon being illuminated with a deep scarlet glow".

The Norwegian painter Edvard Munch wrote in a diary entry from January 1892 — eight and a half years after the eruption, with the visible atmospheric effects long faded but the cultural memory intact:

Suddenly the sky turned blood red... I stood there trembling with anxiety, and I sensed an infinite scream passing through nature.

The visual memory of the Krakatoa sunsets is generally accepted as the inspiration for Munch's The Scream (1893), one of the most recognisable paintings of the late nineteenth century.

The volcano itself returned. In 1927, a new volcanic cone began to rise from the seabed in the centre of the collapsed Krakatoa caldera. It was named Anak Krakatau — Child of Krakatoa — and has continued to grow and erupt periodically ever since. Its most recent major eruption was December 2018, when a flank collapse produced a tsunami that killed about 430 people in coastal Java.

The volcano is not finished.