krakatau tour

Krakatoa

Global aftermath

The last eruption from Krakatoa occurred around 2.30pm on Tuesday 28 August, after which silence finally returned to the Sundra Straits. However, the aftermath of the eruption was felt around the globe for many years.

Initially, getting close to the island was difficult. Great floating rafts of pumice up to 3 metres thick made it almost impossible for ships to push through the water. For many months after the eruption huge swaths of floating debris were reported by ships sailing thousands of kilometres away in the Indian Ocean.

When a detailed survey of Krakatoa was finally possible, it revealed that two-thirds of the island had disappeared. Where once was land, now was sea – 300 metres deep. The emptying of the volcano's magma chamber resulted in the near total collapse of the island into a giant underwater caldera. The rest of the volcano was blown apart and much of it distributed by winds over an area of nearly four million square kilometres.

A considerable amount of Krakatoa ended up high in the stratosphere, producing some dazzling effects. For several years this high altitude ash and dust resulted in spectacular haloes forming around the sun and moon as the particles scattered the incoming light. At sunset, the sky turned an incredible fiery red and orange colour as far away as London, as depicted in the paintings of William Ascroft. It's estimated that over 70% of Earth's atmosphere was impregnated with the remains of Krakatoa, so much so that the average global temperature cooled by half a degree Celsius.

But this is not the end of the story. In December 1927, a group of fishermen were alarmed by steam belching from the ocean above the submerged caldera. The following year, Anak Krakatoa (Child of Krakatoa) raised itself above sea-level, building a cone of ash and pumice. While Anak Krakatoa has erupted almost every year since, the activity has been mild and harmless. But as scientists and local people know all too well, volcanoes have Jekyll and Hyde personalities.

krakatau Volcano tour

Krakatoa

Harbour wave

Krakatoa represents the most violent volcanic eruption in recent human history, resulting in an estimated death toll of 36,000. Though Krakatoa was uninhabited, the adjacent islands felt the force of the eruption, with devastating consequences.

The greatest loss of life came not from the volcano itself but giant tsunamis up to 40 metres high, generated when the flanks of the volcano slid into the sea. The closest islands to the eruption were completely submerged by a series of giant waves. To the northeast of Krakatoa lay the island of Sebesi. Here, all vegetation was uprooted and 3000 people were washed out to sea. Over 80 kilometres away, on the Thousand Islands, a wall of water 2 metres high swept across the land.

Within the Sunda Straits, numerous ships accurately recorded the eruption and the tsunamis. One such vessel was the Gouverneur Generaal Loudon. On the morning of the Monday, she was sheltering from the ferocity of the eruption in the harbour of Lampong Bay near Telok Betong in Sumatra, 65 kilometres northeast of Krakatoa. The following is an eyewitness account from onboard the ship when the tsunami struck:

'Suddenly we saw a gigantic wave of prodigious height advancing toward the seashore with considerable speed. Immediately, the crew managed to set sail in face of the imminent danger; the ship had just enough time to meet with the wave from the front. The ship met the wave head on and the Loudon was lifted up with a dizzying rapidity and made a formidable leap. The wave continued on its journey toward land, and the benumbed crew watched as the sea in a single sweeping motion consumed the town. There, where an instant before had lain the town of Telok Betong, nothing remained but the open sea.'

Pyroclastic flows

While tsunamis were responsible for the majority of fatalities around Krakatoa, over 4500 deaths were attributed to the volcano's deadly pyroclastic flows. Meaning 'fire-broken', pyroclastic is the name volcanologists give to the dense hot flows of ash and rock that can travel down the flanks of a volcano at frightening speeds. Pyroclastic flows form when the eruption column, which contains vast amounts of dust, ash and rock (collectively know as tephra), collapses under its own weight when gases erupting from the volcano can no longer support the column.

The amount of tephra generated by the eruption of Krakatoa is thought to be to the order of 20 cubic kilometres. That's 20 times the amount generated by the 1980 eruption of Mount St Helens in the US.

The pyroclastic flows on Krakatoa were truly awesome. Once the column began to collapse, nothing could stop the ensuing calamity. At over 100kph, the tumultuous incandescent cloud would have raced down the sides of the volcano. Incredibly, the flows were able to travel across the waters of the Sunda Straits and remain hot enough to kill 2000 people 40 kilometres away in southern Sumatra. It's thought that when the base of the pyroclastic flows came into contact with the sea, a considerable amount of steam was generated which greatly enhanced the fluidity of the flow.

The Gouverneur Generaal Loudon, was blasted by pulses of hot searing winds and tephra that roared across the sea at near hurricane force. Luckily, the ship was just far enough away to escape spontaneous combustion due to intense heat. Closer towns, villages and ships were not so

krakatau tour

Contrary to the 1969 Hollywood film Krakatoa, East of Java, the remains of the island of Krakatoa lie to the west of Java, in the Sunda Straits, Indonesia. Indonesia is famed for its number of volcanoes – over 130 in all. These beautiful islands are part of a great volcanic belt which circles the Pacific Ocean, from New Zealand through Indonesia, northward to the Philippines and Japan, across the Aleutian Islands in Alaska, then southward along the western seaboard of North and South America. The belt forms the notorious 'Pacific Ring of Fire', a region of Earth that has spawned many volcanoes of extreme violence.

Ring of fire

A geological map will show that volcanoes typically occur in long belts similar to the Pacific Ring of Fire. The reason lies within Earth's crust. The crust of our planet is not a perfect and unbroken spherical shell. It's composed of eight separate major 'tectonic plates' which jostle each other around the globe, driven by heat from within Earth. At the 'plate boundaries' where the plates meet, the crust is typically being torn apart or is sinking. Both of these plate boundary processes result in the production of magma (molten rock). An active volcano must have a source of magma which eventually escapes and erupts at the surface. Hence the formation of volcanic belts that lie along plate boundaries.

The volcanoes of the Pacific Ring of Fire, including the island of Krakatoa, exist because deep beneath them two plates meet and the crust of one is sinking below the other. As the crust sinks into Earth's interior it heats and melts, forming magma. Similar to a lava lamp, the hot magma rises until it stalls just below the surface. Over time, pressures in a magma chamber can build phenomenally. If the overlying crust is unable to contain the pressure, a volcanic eruption occurs spewing out huge quantities of magma, fractured rock and ash.

The great eruption

Volcanoes have Jekyll and Hyde personalities. They can lie dormant for hundreds or even thousands of years before springing into life with little warning. Krakatoa's awakening occurred on 20 May 1883 with a series of powerful earthquakes and explosions that were heard over 150 kilometres away. In the days that followed, an ominous black cloud of ash rose over 10 kilometres high and gently fell on neighbouring islands. Unbeknownst to those who inhabited the nearby islands, this was just the prelude to the most fearsome eruption in recent history.

Krakatoa continued to rumble and belch harmlessly for three months; so much so that most Indonesians grew complacent to the volcano's stirrings. That was until the events of two fateful days, Sunday 26 and Monday 27 August.

A little after lunchtime on the Sunday, great explosions began at 10-minute intervals. They steadily grew louder. By 5pm enormous booms were heard all over Java. In Batavia (now Jakarta), 160 kilometres from Krakatoa, the din was compared to close range artillery shells exploding, with shockwaves that rattled windows and shook chandeliers.

As darkness fell on the Sunday, the volcano's huge eruption column lit up like a Christmas tree. Tremendous turbulence in the air caused violent discharges of static electricity, forming brilliant, almost continuous flashes of lightning. Krakatoa's crescendo was about to follow. On the morning of the Monday, an almighty eruption blew a column of ash 80 kilometres high, to the very edge of space. Hot rocks the size of pumpkins rained down around the Sunda Straits, hundreds of kilometres from the volcano. Ash fell as far as the Cocos Islands, 1850 kilometres away. The loudest explosions were so phenomenal that they woke people in south Australia 3224 kilometres away and were heard 4000 kilometres away.