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Roger Weller, geology instructor
by Natallee Zufelt
Krakatau: The Beast from the East
Krakatau is a world famous volcano. Perhaps it is best known due to the 1883 eruption that killed over 36,000 people, sunk more than half of the island, and holds the record for the loudest noise in history. Even today the great volcano lives on as Anak Krakatau, or ‘Child of Krakatoa’, continues to grow and erupt.
Geography and Geology of Krakatau
Krakatau, also known as Krakatoa, is a volcanic island located in Indonesia in the Sundra Strait on the convergent plate between the Indo-Australian Plate and the Eurasian Plate. It is part of the Ring of Fire which is ring of volcanoes that somewhat outline the Pacific Ocean. It only seems natural that a volcano this great would be located in such a place, as Indonesia does have over 150 volcanoes, making it the country with the most active volcanoes. Krakatau sits right between the larger Indonesian islands of Java and Sumatra.
This picture shows what the convergent plate the Krakatau lies on looks like. Photo Credit: EarthScope
Krakatau is an interesting volcano in that it is formed from a large caldera beneath the ocean. There are three small islands that have formed from the caldera; Verlaten, Lang, and Krakatau. On the 12 square mile wide island of Krakatau there were three volcanic peaks or points before the 1883 eruption. Those were Perboewatan, Danan, and Rakata. After the 1883 disaster only Rakata remains.
satellite image shows what present day Anak Krakatau looks like. In the lower
south west are you can see Rakata. Photo Credit: Google Maps
Krakatau ia a stratovolcano. Stratovolcanoes are composite volcanoes. They can be very explosive when they erupt due to the dissolved gases that build up in the volcano and high silica content in the magma. This helps explain why the 1883 eruption was so bad. The lava that erupted out of Krakatau was rhyolite. It seems that composite volcanoes have notoriously grand eruptions. Mt. St. Helens is a great example of this type of volcano and the destruction they can cause. Of course, any type of volcano is dangerous.
This 1888 lithograph shows what the 1883 eruption might have looked like.
Photo Credit: EarthScope
The Eruption of 1883
After years of smaller eruptions and activity, the time for Krakatau to make its mark on history had come. In the fall of 1883 the eruption began. It started early the Sunday morning of August 26. Apparently some ocean water found its way into the magma chamber of Perboewatan. To add to that the magma was rhyolite, which is very high in silica and viscous. The mixture was deadly. Huge ash clouds and plumes of gases began to escape from the volcano as far high as 25 km above the island. The eruption continued on into the next day, August 27, when the volcano finally exploded. As this happened, it literally cause 2/3 of the island to sink or collapse into the underlying caldera. Perboewatan and Danan were gone. The sound of this could be heard nearly 3,000 miles away and is the loudest sound ever heard. To give an idea of how big the eruption was think of this. The biggest bomb made, Tsar Bomba, is equivalent to 50 megatons of TNT. Krakatau’s eruption was equivalent to about 200 megatons of TNT. That is 400% more powerful.
This map show what Krakatau Island looked before and after the 1883 eruption. Photo Credit: USGS
The Tsunamis and Pyroclastic Flows
While a giant eruption was impressive, Krakatau would not stop there. Those blasts and the collapse of the island caused many tsunamis to form. These tsunamis reached heights around 40 m tall. They were big and they were moving fast. It was mostly the tsunamis that killed the whopping 36,417 people that perished. This makes the Krakatau eruption the record holder for the most deaths caused by a volcano. Pyroclastic flows also played a part in the destruction and deaths being responsible for around 10% of the total count. These fiery flows could have reached temperatures of 700 degrees Celsius. They too moved swiftly and probably took out those that survived the tsunami wave.
The effects of the eruption did not end with the tsunamis and pyroclastic flows. Tephra, ash and dust material had propelled into the atmosphere and stayed in there for over the next 3 years. This caused sunsets to appear in very vivid red, orange, and yellow colors and even at times green. It also cause the global temperature to drop 1.2 degrees Celsius. Also, for years pumice floated on the sea in varying sizes along with corpses. It is incredible to see the ripple effects one eruption can have.
This photo shows an eruption
on Anak Krakatau. Photo Credit: Indonesia Matters
The "Child of Krakatoa"
The eruption of 1883 was not enough to put Krakatau to rest forever. With time, it would be back. In 1927 a new volcanic island was forming. It would be called Anak Krakatau. Today the volcano is 300 m high and grows about 5 m every year. It is fairly active and often releases plumes. Interestingly, the lava flows of Anak Krakatau are basalt while the magma was originally rhyolite in Krakatau. This is due to basaltic magma traveling to the rhyolite magma chamber and mixing there. Anak Krakatau does have the potential to grow into a beast like its predecessor. With it the legacy of Krakatau lives on and serves as a reminder of just how powerful and devastating volcanoes can be.
This photo shows an eruption on Anak Krakatau. Photo Credit: Indonesia Matters http://www.indonesiamatters.com/13295/sumatra-travel/krakatoa4/
Australian Government Bureau of Meteorology. “The eruption of Krakatoa, August 27, 1883.” Retrieved April 17, 2015 (http://www.bom.gov.au/tsunami/history/1883.shtml)
Earlham College. 2004. “Krakatoa.” Retrieved April 17, 2015 (http://legacy.earlham.edu/~bubbmi/krakatoa.htm)
EarthScope. 2014. “A Look Back at Krakatau, Indonesia.” Retrieved April 17, 2015 (http://www.earthscope.org/science/geo-events/a-look-back-at-krakatau-indonesia)
Oregon State University. “Describe the 1883 eruption of Krakatau.” Retrieved April 17, 2015 (http://volcano.oregonstate.edu/describe-1883-eruption-krakatau)
Roscoe, R. “Krakatau.” Retrieved April 17, 2015 (http://www.photovolcanica.com/VolcanoInfo/Krakatau/Krakatau.html)
San Diego State University. “How Volcanoes Work.” Retrieved April 17, 2015 (http://www.geology.sdsu.edu/how_volcanoes_work/Krakatau.html)
US Geological Survey. 2003. “Mixing Magmas at Krakatau.” Retrieved April 18, 2015 (http://hvo.wr.usgs.gov/volcanowatch/archive/2003/03_05_22.html)