Krakatoa

Krakatoa

"Suddenly it became pitch black.
The last thing I saw was the ash" -From A. Scarth, 1999

Overview

On August 27th, 1883 a series of blasts on the island of Krakatoa culminated in a colossal explosion that blew the island apart in one of the largest eruptions in recorded history. Below are a few brief facts about the blast and its effects.

1. The 23km square island of Krakatoa existed at a height of 450m above sea level. The blast leveled most of the island to 250m below sea level.
2. Pyroclastic flows traveled as far as 40km from the island consuming traversing ships in fire and ash.
3. The sound of the final explosion was heard over 4500km away and covered 1/13th of the Earth's surface.
4. The eruption generated tsunamis 40m high that devastated nearby coastlines.
5. The final death toll from pyroclastic flows, volcanic bombs, and tsunamis was calculated to be a devastating 36,417.

Geography and Geology

The island of Krakatoa is located in the Sundra Straits between the islands of Java and Sumatra off the southwestern coast of Indonesia. Krakatoa is one of many active volcanoes along the Indonesian Island Arc, including the famous Tambora volcano. This arc, containing over 130 active volcanoes, is produced by the subduction of the Indo-Australian plate as it moves northward towards mainland Asia.

The island of Krakatoa itself is lies next to two other islands: Lang and Verlaten. These three islands together are the remains of an ancient volcanic island. Ancient Krakatoa, as it is called, exploded in an enormous eruption around 416 A.D., according to Javanese scriptures. In that eruption the majority of the island collapsed into a 7km long caldera beneath Rakata on Krakatoa forming the three present day islands.

The map on the right shows Krakatoa as it was before the great eruption of 1833. Three major volcanoes existed on the island, Perboewetan, Danan(not shown but in the middle), and Rakata. Since the eruption and collapse of Ancient Krakatoa the chamber beneath these three volcanoes had begun to fill, heat up, and create enormous pressure. Of late, Perboewetan had been the most active.

The Beast Awakens

On May 20, 1883, a plume of smoke was seen rising over 10km above the island of Krakatoa. During the following summer months military and commercial vessels would also report seeing the cloud. By August, ash and pumice bars were found everywhere in the Sundra Straits. The minor eruptions on Krakatoa from May-August were from Perboewetan volcano and by the time of the main eruption Perboewetan had nearly been completely destroyed widening the caldera beneath the islands and building more pressure.

At around 1:00pm in the afternoon, Krakatoa delivered the first in a series of blasts that would continue throughout the day until the climactic eruption on August 27th. This opening blast generated and defining shockwave that alerted the nearby coastal villages on the islands of Java and Sumatra. A column of black volcanic gas rapidly ascended to a height of over 25km above Krakatoa. In the coming hours as blasts intensified it would reach at least 35km. Later in the day the coastlines were hit with a series of tsunamis generated by the ocean impact of pyroclastic flows coming off the flanks of the island. Nothing of this would compare to what the world would witness the following day.

The World Explodes, August 27, 1883

At 5:30am the first of four cataclysmic explosions began on the island of Krakatoa. Tsunamis lashed out from the island pounding nearby coastlines and ash and pumice fell in droves on surrounding islands. At 6:44am a second massive blast came from Krakatoa unleashing similar effects. Finally at 10:02am the colossal blast took place that blew the island apart. Perboewetan and Danan erupted and fell into the emptying caldera 250m below sea level. Adding to the empty chamber was Rakata as half of the erupting volcano slid into the ocean displacing large volumes of and generating a massive tsunami. In total, 23 square kilometers of the island fell into the 6km wide caldera. The ground shook in the wake of the blast which was heard over 4500km and was estimated to be equal to the detonation of over 21,000 atomic bombs.

"...all of a sudden there came a great noise. We...saw a great black thing, a long way off, coming towards us. It was very high and very strong, and we soon saw that it was water" - From A. Scarth, 1999

Upon the third and catastrophic explosion of Krakatoa, enormous tsunamis were generated by the displaced water as the island collapsed into the caldera. These waves moved with great speed across the Sundra Straits reaching a height of around 40m tall before slamming into the nearby coastlines. Smaller tsunamis had pounded the local villages in the previous days eruptions but nothing compared to this mammoth wave. Many small coastal islands were completely submerged and as the wave hit the mainland islands of Java and Sumatra it ravaged towns and villages while stripping away nearly all vegetation. In some cases, whole towns of several thousand people were washed away in a flash destroying and sign they had ever been there. Accounts exist of villagers scrambling up inland hills to escape the waves. Often only the small top of a hill would be spared by the enormous current leaving former neighbors in great struggle with one another, while trying to maintain a safe position from the waves.

Many ships in the Sunrda Straits at the time of the eruption met a similar fate as the villagers on the coasts. Many were caught unaware in the torrent and thrown across the sea. Once such ship, the Berouw(seen right), was carried over a mile inland and deposited on a position 10m above sea level. However, some ships were fortunate enough to be maneuvered head-first into the swells only receiving minor injury onboard.

According to Dutch authorities, 90 percent of the over 36,000 deaths were attributed to the devastating tsunamis. Bodies lined the coasts for weeks and thousands of livestock were found throughout the Sundra Straits.

When Krakatoa exploded hot ash and tephra were sent sprawling down the flanks of the volcano and into the Sundra Straits. These pyroclastic flows, essentially avalanches of fire and rock, stormed across the straits to distances of up to 40km engulfing passing ships and coastal villages. The flows traveled at speeds of over 100km/hr leaving little time for people to evacuate from the advancing surge. The flows were able to move quickly and a great distance for two reasons. These pyroclastic flows may have reached temperatures of over 700 degrees Celsius, which overland would scorch anything in its path. However, in the open ocean the pyroclastic flow will flash boil the water as it comes in contact with it allowing the entire surge to ride on a cushion of air. This lack of friction with water or land, often compared to and air hockey table, allows the flow to move with great speed and for a long time. It was the southern coast of Sumatra that was hit hardest by the flows. Of the over 36,000 deaths, around 4500 are contributed to the deadly pyroclastic flows which would have arrived just after the tsunamis. Most likely the 4500 who met their fate with the flows had reached high ground or shelter to avoid the rushing water only to be engulfed by fire and ash.

The Skies Change

In the weeks following the eruption, fine fragments of tephra and dust that were propelled kilometers into the stratosphere began to make a ring around the equator. They would remain suspended there for years causing remarkable solar effects and atmospheric hazing as they bent the incoming light. Also the enormous volumes of sulfur dioxide gas molecules that were ejected into the atmosphere combined with water to make sulfuric acid. These acidic aerosols sufficiently blocked enough sunlight to drop the Earth's temperature by several degrees for a few years. There presence in the atmosphere also created spectacular effects over 70% of the Earth's surface. Effects such as halos around the sun and moon, and amazing sunsets and sunrises were seen. For years these particles would remain suspended in the atmosphere being the final reminder of the massive and fatal blast that occurred in Sundra Straits. At least for a time...

On December 29, 1927 debris began spouting out of the ocean surface above the collapsed caldera of Krakatoa. The debris continued to spout till finally on January 26, 1928 the rim of a new volcanic cone emerged from below sea level. Indeed, after 44 years of silence Krakatoa began erupting anew giving birth to Anak Krakatoa, the child of Krakatoa, which has been erupting steadily ever since. To this day the many eruptions of Anak Krakatoa have been mild and expanded the island to base of 2km and over 200m above sea level. Today you may go to the Sundra Straits and step foot on young volcano's rim. And perhaps there as you look across the sea to the coasts of Java and Sumatra you may be able to feel, ever so slightly, the power of the awesome events that took place there just over a century ago.

Cited Sources

1. Winchester, Simon. Krakatoa. HarperCollins Publishers Inc. Copyright 2003.