This week in science history saw, in 1883, the catastrophic eruption of Krakatoa in Indonesia. The eruption and subsequent pyroclastic flows and tsunami killed at least 40 thousand people, destroyed towns and villages and had effects on global climate for several years. It is considered to be one of the most important eruptions in the history of volcanology, as well as being one of the largest eruptions in recorded history, it was the first where we have enough global accounts and measurements to understand what happened.
There are around 1500 active volcanoes in the world, 75% of which are found on the Pacific Ring of Fire around the Pacific Ocean. Krakatoa is located in the Sunda Strait between Sumatra and Java – at an area known as a subduction zone, where the Indo-Australian tectonic plate is pushed down underneath the Eurasian plate, causing large amounts of volcanic and seismic activity. It was an earthquake caused by this subduction that caused the devastating Boxing Day Tsunami in 2004. What makes Krakatoa so violent is that it is exactly over the point where the subduction zone changes direction, turning northwards, creating extra tension and fissures in the rocks.
The volcanoes on Krakatoa are stratovolcanoes – steep-sided and made up of alternating layers of solidified lava, ash and rock – they erupt periodically and often violently. They are the most common sort of volcano and if you ask someone to picture a volcano, chances are this is the sort they will imagine. Other examples are Mount Vesuvius, Mount Etna and Mount St Helen’s.
Records of Krakatoa erupting stretch back to the 5th century AD, and in 1883, following several months of earth tremors, the first eruptions of the volcano began in May – steam and ash began to spew from one of Krakatoa’s smaller volcanic cones. This eruption was only a fraction of the force of those to come.
In the afternoon of Sunday the 26th of August, several explosions occurred as all three volcanic cones on Krakatoa began to erupt. Huge volumes of ash and rock were shot into the atmosphere and began to fall on the surrounding area. Two even larger explosions followed in the early hours of the morning of the 27th, causing pyroclastic flows – clouds of burning hot gas, ash and rock that rushed down the volcano and across the surface of the sea, causing tsunamis that spread around the Sunda Strait. A third huge blast occurred at 10:02 in the morning. This explosion was so violent and loud that is was heard over three thousand miles away in Mauritius, and the force generated is estimated to have been more than twenty thousand times greater than the atomic bombs dropped on Japan at the end of the Second World War. The shock waves reverberated around the world seven times. As lava was fired out of the volcano, the magma chamber beneath the islands emptied, and over half of Krakatoa Island collapsed into the sea.
The majority of the deaths from Krakatoa were as a result of the tsunami, which devastated the low-lying land around the Strait, on Sothern Sumatra and Western Java. The wave was so large that it carried one ship nearly two miles inland. Pyroclastic flows, just like those that engulfed the cities of Pompeii and Herculaneum when Vesuvius erupted in 79AD, destroyed villages up to 45km away. The death toll of the eruption was over 36 thousand people, still the largest of any volcanic eruption in recorded history.
But the effects of Krakatoa were felt around the world for years to come. Sulphur dioxide released by the blasts into the atmosphere increased the reflectivity of clouds and decreased global temperature by 0.5 degrees. Record winter snowfalls were recorded around the globe for the next four years. A report published in Nature in 2006 suggested that Krakatoa may have had a cooling effect on the oceans for over one hundred years, counteracting anthropogenic climate change for a time.
Today, all active volcanoes are monitored for any changes in activity that could indicate an imminent eruption, such as seismic activity, gas emissions, ground deformation caused by magma or gas build-up, changes in temperature of hot springs and through observations such as steam release from the crater. In most volcanic areas, some sort of activity of these kinds goes on all the time, but it is when changes and increases in activity occur that alarm bells start ringing, literally.
Globally, 500 million people live within the danger zone of a volcano, so being able to give early warnings and begin evacuations is essential. Our planet can be violent and deadly, but our knowledge has been building since Krakatoa, and hopefully any future eruptions will not bring such a catastrophic death toll.