In the straits between the islands of Java and Sumatra lies a serial killer. Here,
where the crust of the earth is weak, violent forces below have erupted
time and time again, leaving death and destruction in their wake. A century and a half ago,
it erupted with such violence, that it may have been the loudest sound ever heard.
It is a volcano with a name that has become infamous. It is Krakatoa.
Indonesia is a giant archipelago consisting of large, well-known islands, and thousands
of smaller ones. Situated at the boundary between two tectonic plates, it is a part of the Pacific
Ring of Fire, with the country home to 130 active volcanoes, more than any other nation.
The majority of these are on the large islands of Sumatra and Java, since these lie direct atop
the subduction zone where the Australian plate is being pushed below the Sunda plate.
Plate tectonic subduction is responsible for the majority of the world’s volcanoes,
as the trapped water within the buried plate leads to softening and lightening of the rock,
resulting in it moving to the surface as magma and ultimately erupting in the form of volcanoes.
The Sunda Strait between these two islands is where we find our killer, Krakatoa.
Due to the historical violence of this area, the geography of our perpetrator today is only
a snapshot of an ever-changing scene. Essentially the location is occupied by a large stratovolcano
(meaning a relatively steep sided volcano composed of multiple layers, or strata, of ash, lava etc.).
This volcano at present is broken into four islands, and together these form a giant caldera.
A caldera is a sunken piece of the earth’s crust where once a magma
chamber existed beneath a volcano, but eruptions have drained it of its volume,
resulting in the central part of the crust above collapsing, but leaving the edges intact.
The word “Krakatoa” is but one of many assigned to this area, with alternate spellings and so on
being the result of the multiple languages of the European colonial powers in this area reaching
different conclusions from the local dialect. Krakatau is now regarded as the correct form,
but since Krakatoa is the most familiar to us, I’ll continue to use that name.
This area has erupted regularly throughout history,
with various sources hinting at eruptions as early as 416AD, and continuing every century or so until
more reliable accounts were recorded with the arrival of European colonial powers in the 1600s.
In 1681, a number of Dutch travellers recorded devastation on the island there at the time
while passing through the Sunda Strait. A century later in 1780, British Royal Navy
vessels stopped on the island and reported habitation and lush vegetation once again.
Another century on, however, and we come to the eruption of 1883, which has established this
volcano as Infamous Geography beyond perhaps any save Vesuvius of Italy.
In the years leading up to the eruption, seismic activity in the area had been increasing,
but by May of 1883, steam was regularly being vented from one of the mountain’s three cones,
while ash was now regularly being hurled into the atmosphere, blanketing the surrounding region.
Earth tremors were regular, being felt as far as Batavia, modern day Jakarta, 160km away.
On the 25th of August, the eruptions dramatically intensified. By 2pm on the 26th, a large black
ash cloud had risen 27km high, and regular explosions were being heard every few minutes.
Pumice as large as 10cm (4-inches) across was landing on the many ships that used the straits
to pass through. A small tsunami was experienced on the surrounding shores around 8pm that evening.
The morning of 27th of August was the time of infamy for Krakatoa, however. On that morning
four enormous explosions occurred. These were timed at 5:30am, 6:44am, 10:02am and 10:41am.
Each had their own accompanying consequences of violence, but the atmospheric pressure waves
from the suddenness and scale of the detonations alone puts this eruption into the record books.
The third of these is believed to be the loudest sound heard in historic times.
It was heard across 10% of the surface of the earth, most notably recorded in Perth,
Western Australia, 3,110km (1,930 miles) to the south, and all the way across the Indian Ocean
on the island of Rodriguez near Mauritius, 4,800km (3,000 miles) distant, where the sound
was mistaken for cannon fire from a nearby ship. The atmospheric pressure wave was recorded on
devices on the other side of the world, in Europe and the United States. Closer to the eruption,
and in Batavia, 160km (100 miles) away, the sound was calculated to have been 180dB, equivalent to
standing next to a rocket during launch. Sailors in the Sunda Strait, within 64km (40 miles)
had their eardrums ruptured and anyone within 16km (10 miles) would have been permanently deafened.
The sound at the centre of the detonation itself has been calculated to have been an unimaginable
310dB. Bear in mind that anything over 100dB is considered a loud sound, and consider also
that the decibel scale is logarithmic. So the sound energy at the centre of the eruption would
have been 100 billion billion times greater than being next to a chainsaw in operation.
Such figures are perhaps not surprising when, looked at another way, the energy released in
these detonations has been estimated to be an equivalent of 200 megatons of TNT,
four times greater than the Soviet Tsar Bomba, the largest ever thermonuclear bomb explosion.
The eruptions of that day ejected about 25km³ (6 cu. Miles) of rock, blowing the mountain apart.
Thrust into the air to the edge of space at 80km in altitude as a plume of ash,
rocks and hot gas, the heavier fractions of this superheated column then sank back to the earth and
sea as a pyroclastic flow, reaching as far as the village of Ketimbang some 64km (40 miles) distant,
killing hundreds there. Closer to the eruption, the island of Sebesi and its
3,000 inhabitants were covered in the flow, with no survivors. But most significantly,
the force of this collapsing column of hot gas, ash and rock, on hitting the ocean, displaced
several cubic kilometers of water there, resulting in a massive series of tsunami.
The tsunami that destroyed the town of Merak 70km distant, was estimated to have been at
least 40m high. It was these tsunami that were the most significant in terms of loss of life.
The official death toll by the Dutch authorities for the eruption was 36,417,
with possibly as many as 34,000 of those caused by the tsunami. Some estimates, however,
put the death toll as high as 120,000. But the official death toll ranks the Krakatoa eruption
as the second deadliest in recorded history. But it wasn’t just the immediate locality or
region that was affected. With so much ash and sulphur dioxide ejected into the atmosphere,
global temperatures were cooled by as much as 0.4°C over the following year in addition
to dramatic sunsets in Europe and America recorded by the numerous artists of the day.
All very apocalyptic, and although Krakatoa is probably the most well-known eruption of
recent times, it pales into insignificance in comparison to other eruptions of the historical
and geological record in that region. Since the 1980s, Vulcanologists have measured the
violence of an eruption using the Volcanic Explosivity Index, or VEI. Because natural
phenomenon occur across orders of magnitude in scale, and in the same fashion of the Richter
Scale measuring the violence of an earthquake, VEI is logarithmic, meaning that a VEI 7 eruption
is 10 times more violent than a VEI 6, a hundred times more violent than a VEI 5, and so on. What
determines “violence” in this context is a composite of several measurable factors,
including total volume of ejected material and cloud column height. Since the vast majority of
eruptions are in the geological and historical record, these parameters are often estimated,
and so VEI can be subjective, and should only be used as quick comparative assessment.
So where did the Krakatoa eruption of 1883 rate on the VEI scale? Well, it’s a “6”, which is classed
as “Colossal”, and is the same as the Mount Pinatubo eruption in the Philippines in 1991,
but an order of magnitude more violent than the VEI 5 eruption of Mount St Helens in 1980,
and Vesuvius of 79AD, which were merely “Cataclysmic”.
And yet Krakatoa is not the largest eruption to have been recorded.
Early that century, and a few hundred miles east, within the same archipelago, another volcano
exploded, but this time at a VEI of 7, an order of magnitude more powerful. This was Mount Tambora,
and its super-colossal 1815 eruption, ejecting 40km³ (10 cu. mi) of rock,
is the largest in recorded history, and probably the largest of the last 10,000 years. This
eruption killed 70-100,000 people directly, and dramatically affected earth’s climate over the
years following, including the “year without summer” in Europe of 1816, that led to crop
failures, disease and possibly hundreds of thousands of deaths on that continent alone.
A few hundred miles west of Krakatoa, however, and we find evidence of an even larger eruption,
the VEI 8 mega-colossal eruption of Toba, about 74,000 years ago. This eruption was so violent
that it has been proposed that it almost wiped out the human race at that time, with evidence of a
bottleneck in the gene-pool of humans through study of mitochondrial DNA around that time.
So the archipelago of Indonesia is undoubtedly the most infamous in terms of vulcanism throughout
recent history. But beyond the tragedy, one should note that there is a positive side to
all this seismic activity. It is no accident that Java has such a staggering population
for its size - 147 million people on an island just 1,000km east-west and 150km north-south.
Home to so many volcanoes Java’s soil is enriched through deposits of mineral rich ash,
that has acted as a natural fertiliser in the thousands of years that humans have lived here.
And so where such volcanoes at times toll such great human tragedy, they also bring life.
But going back to Krakatoa… was that the end of the story? Actually no. Although the island
itself was blown apart, and the emptied magma chamber led to the collapsed caldera of four
different islands that we see today, a new island has emerged at the centre of the old volcano.
Known as Anak-Krakatoa, or “son of Krakatoa”, this island had grown to as much as 338m (1,100ft) in
height since its emergence in 1927 until it erupted in 2018, leading to a major collapse
of its cone that triggered a tsunami, resulting in 437 deaths, and thousands injured. At the time of
this video, the island is undergoing yet another eruption. And so the cycle continues, as it will,
perhaps for millions of years to come. The island will grow again, and perhaps in two or
three centuries from now we will see yet another cataclysm unfold in the ongoing story of Krakatoa.
And that’s it for this video. Please like and share if you enjoyed it or found it useful,
and please let me know your thoughts in the comments, especially if you’ve travelled
to this region, or live there, and if I missed out anything you feel is important.
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