The news on Christmas Eve was a tsunami hitting the Java shoreline facing Anak Krakatau. As of this writing, there are at least 437 dead with over 9,000 injured in parts of Java and Sumatra facing the Sunda Strait.
The tsunami was caused by a flank collapse of the cone on the island that cut the height of the island from 338 m to 110 m. The flank collapse took place after a particularly vigorous eruption and lava flow entry into the sea. The collapse took place at 9:03 L, after dark. Tsunami height for the first wave was initially reported as a meter above sea level. Estimates of later waves range 2 – 5 m above sea level.
There is a terrifying video of the Boy Band called Seventeen playing a set as the water engulfs the stage from the back side. At least three band members and the wife of the lead guitarist were killed or are still missing.
The collapse coincided with a substantial eruption putting a plume 17 km into the atmosphere. Base surges / pyroclastic flows, while present, did not travel far from the island. On the other hand, the collapse did expose the main conduit to sea water, triggering a Surtseyan eruption for several days afterwards. The plume following the collapse shrouded the eruption from distant viewers.
Nearly a week later, once the eruptive action died down a bit, the remaining island looked relatively flat, tephra covered, and changed in above-water shape, looking not unlike Bogoslof in the Aleutians following its cycle of eruptions a couple years ago.
While this particular collapse was not specifically predicted, there is plenty of scientific research out there supporting the danger of this volcano in this location, especially following a growth spurt in the size and shape of the cone over the last couple years.
Krakatau is a particularly deadly volcanic vent, with perhaps two caldera-forming eruptions in the last 1,600 years. This version of the above-water cone was not thought to be all that dangerous yet. It is possible that flank collapses could be part of the normal life cycle of andesitic – basaltic volcanic islands in subduction zones and this one was the first one to be closely studied.
Volcanic tsunami wave height modeled for a 0.20 km3 flank collapse of Anak Krakatau. Image courtesy Giachetti, et al, 2012. http://sp.lyellcollection.org/content/361/1/79
Dec. 22 Flank Collapse and Tsunami
International photographer Øystein Lund Andersen published a first-hand account of activity before, during and after the flank collapse and tsunami. He took over 150 photos and over 40 minutes of video of the eruption leading up to the flank collapse. All are for commercial sale. He graciously gave permission to use a few of his photos in this post. The following summary is from his online blog: http://www.oysteinlundandersen.com/krakatau-volcano-witnessing-the-eruption-tsunami-22december2018/?fbclid=IwAR2U1vWzvB5bk5W0g0OMrai9BQqTjmnJNj_i4T2aqIoz9HoBbHttJh-tkTM
This was the third time Øystein observed the volcano eruption since this cycle started in June 2018. He was staying at a beach resort hotel in Anyer – Carita. First photos were shot at 1429 L, some 47 km from the volcano. They show perhaps a kilometer high eruptive plume. Sounds from the eruption were heard. This was not a common occurrence, and he used it to partly gauge the intensity of the eruption. Around 1700 L the eruption seemed to increase in intensity with a corresponding increase in height of the ash plume and the frequency of sounds heard. At this point a white plume started to rise from the seashore of the Southern flank of the volcano. He described this as a new lava flow reaching the sea rather than a new vent opening. The location of the lava flow was similar to that of lava flows down that same slope earlier in the year.
After sunset, activity was still high and seemed to peak around 1830. The shore level plume was mostly white but started to show grey from time to time. The majority of erupted material seemed to land on the S / SW flank of the volcano. Glow from the sea level plume also increased in intensity and lit the main ash column nicely. The eruption became more ash rich around 1900 L.
The photographer took a break from observing for dinner, although he and his family were able to hear the sounds of the continuing eruption during the meal. Between 2030 L and 2100 L the eruptions sounds stopped. He set up his camera again shortly after 2100 L and noted that the location of the volcano was pitch black, completely obscured by ash. In reality, this was around the time of the cone collapse.
The initial wave arrived around 2127 L. He estimated its height was 1 – 1.5 m. The second wave was at least 3 – 4 m high and struck around 2131 L. as he was waking his family to evacuate to higher ground. Throughout the remainder of the night, the volcano continued to erupt, though the sound was different.
It can be assumed that the plume on the western flank, near sea level, was not lava flow entry into the sea. Rather, it was probably a second vent opening on the western side of the cone near sea level. You can see it clearly from Øystein’s photos taken on Dec. 22. In interviews after the eruption survivors claimed that the largest explosion did not originate from the summit, but from the side of the main cone, likely observed as the second plume in photos. The cone collapsed shortly after that explosion.
Øystein had translated interviews with fishermen who were on the water within line of sight of that explosion. They echoed the report that the largest explosion came through the side of Anak Krakatau, causing the first collapse and wave. This wave destroyed their boat. The second collapse of the east side of the cone and wave took place a few moments after the first blast and heated the seawater around them. Video of one of the interviews (in Indonesian): https://t.co/YHY7En9lZx
An article in Nature by Thomas Gaichetti described a paper by him and his colleagues in 2012 analyzing hazards from Anak Krakatau. The researchers concluded that it was difficult to forecast when and if it would collapse. But if it did, the characteristics of the tsunamis produced may be somewhat predictable.
Their 2012 paper, http://sp.lyellcollection.org/content/361/1/79 “Tsunami hazard related to a flank collapse of Anak Krakatau volcano, Sunda Strait, Indonesia”. The worst case flank collapse modeled was 0.28 km3, just a bit larger than the estimated 0.20 km3 of cone that collapsed into the caldera in December.
In this case, a large, rapidly sliding mass struck the water. As Anak Krakatau is perched on the rim of the 1883 caldera, the cone material slid down the slope of the caldera rim onto its floor. These were the first deadly tsunamis produced by Anak Krakatau following the catastrophic tsunamis of the 1883 eruption. There was a more recent 1 – 2 m tsunami that hit neighboring Rakata Island in Oct. 1981.
Activity following the eruption was vigorous. There were nearly 11,000 lightning strokes detected in and around the eruptive plume on Dec. 23, and over 60,000 events detected by Dec. 24. The plume of the Dec. 22 eruption was measured at almost 17 km high.
The island was shrouded in an ash cloud for six days as the eruption transitioned from an explosion plume into a Surtseyan style as the activity tailed off. The Surtseyan activity included base surges as the eruption progressed.
By the time the eruption subsided to the point where the rearranged island was visible, it became clear that it had been resurfaced with tephras, ashfall, and pyroclastic flow deposits. It also became clear that the conduit was now below water level and being busily rebuilt to above sea level by eruptive deposits. Neighboring islands were also covered with ash and tephras, growing the above water size of them a bit.
Due to the persistent ash plume after the eruption, radar images gave the first indication that the SE half of the cone was gone. The Straits Times reported two cracks appeared on or before Jan. 2. They were deep, emitted “smoke”, and extended beneath the water. https://www.straitstimes.com/asia/se-asia/indonesia-on-tsunami-alert-after-new-cracks-appear-on-anak-krakatau-volcano
Anak Krakatau first breached the ocean surface in 1928. It is sited just off the steep NE wall of the caldera basin created during the 1883 eruption. It is located where the main vent of the 1883 eruption is thought to have been located, midway between former volcanoes Danan and Perbuatan. It took three attempts for the new cone to permanently appear above the waves in 1930.
It built a 152 m hyaloclastic tuff ring in 1959. This tuff ring hosted a lake and looks a bit like a small somma when filled by the newer cone. This ring is now the highest portion of the remaining island. The eruption style was Surtseyan in 1928 – 1930. It shifted to Vulcanian until 1960 when it shifted to Strombolian building a 200 m cone by 1981. The eruptive conduit moved SW in 1981 and started erupting more evolved, differentiated volcanic products (acid andesites, dacites) than previous basalts and andesites. The eruptive center also changed during the 2018 eruptive cycle from the top of the cone down the slope a bit. The final shift was short-lived, probably opened a second vent on the western side of the cone, which, according to eyewitness accounts, was the source of the initial large blast on Dec.22 that destabilized part of the cone. That collapse was quickly followed by the collapse of the eastern side of the cone.
Video from 2010 eruption by Earth Uncut TV:
The submerged western slope of the volcano is much steeper than the eastern slope at over 28 degrees. The angle of that slope has not appreciably changed over the last 90 years, meaning that subsequent eruptions have not substantially filled the neighboring caldera. This steep underwater slope led investigators to suspecting that there was a possibility of landslides along the SW flank and subsequent tsunamis. Those fears were realized on Dec. 22.
The basement structure under the edifice is dominated by compression above a 4 km depth and by extension at deeper than 4 km. There are two low velocity zones identified. The deepest is at 22 – 32 km and is interpreted as a very wide magma body associated with the extensional nature of the strait. The upper zone is at less than 7 km deep and thought to be a number of irregular pockets of magma. Their volume is equivalent of that thought to be associated with the 1883 eruption. They are also connected in some way with the deeper chamber.
The petrology of lavas erupted by Anak Krakatau suggests multi-stage evolution of magmas in a shallow system. The current vent could be fed by at least two different shallow magma pockets through a common conduit.
The present day Anak Krakatau may represent the initial stages in another cycle that would end with yet another catastrophic rhyolitic eruption. On the other hand, it could be the end of the cycle, as the magmas that triggered the 1883 eruption finally reach the surface. One researcher who suggested cyclic activity at Krakatau believes the system evolved to andesitic – dacite by 1981. Since then, the system has erupted less evolved basaltic andesites. Hopefully, this means that the system needs more time to evolve to another rhyodacitic caldera-forming eruption.
Since its formation, Anak Krakatau has erupted at least 45 times since it first appeared above the water in Dec. 1927. Eruption intensity was generally VEI 2 with a VEI 3 in 1932.
One note on the date of its formation. It was first discovered on Dec. 29, 1927 by local fishermen. The vent was above and below the surface multiple times before it permanently breached the surface in 1930. Some papers list its formation date as 1927 and others as 1928. I have used both dates in this piece.
Activity in 2018 was vigorous with multiple Strombolian eruptions and lava flows ejected. The cone had grown rapidly during the last couple years.
Anak Krakatau is the active vent remaining from the August 1883 climactic eruption of Krakatau. That caldera forming eruption in turn destroyed the island built following the previous caldera forming eruption. The latter was either 416 AD or at the end of what could be as many as three previous cycles. The collapse of the 1883 caldera appears to have been guided by the remnants of an earlier caldera. Actual creation of that caldera has not been precisely dated.
There is a line of structural weakness in the upper crust underlying the entire system. This line passes through older and current vents of the volcano and could have guided the development of volcanic activity and the 1883 caldera formation.
The oldest proto-Krakatau that we know about was destroyed around 60,000 years ago, the postulated end of the first of up to four possible caldera forming events. The initial volcano was a large stratovolcano made of andesite that erupted dacite pumice. The caldera forming event, whenever it took place, left the islands of Rakata, Sertung, Panjang and Poolshe Hoad.
The Javanese Book of Kings records what would be the 416 AD eruption at a time when the Sunda Strait did not exist: It created a glowing fire out of the sky, thunderous noise, eventually breaking into pieces and sinking into the earth. This caused tsunamis that inundated the land, drowning the inhabitants and sweeping away their possessions. When the water subsided, Java and Sumatra were newly separated by ocean.
Whether that eruption was Krakatau or not remains to be seen, as it is difficult to determine massive eruptions in tropical parts of the world. Rain will remove soft volcanic ash and pyroclastic flow remnants. The other problem with dating historic Krakatau eruptions are large eruptions of neighboring volcanoes. VOGRIPA classifies the 416 AD eruption as a VEI 4.3 with around 0.10 km3 of rhyolite produced. Given the Book of Kings narrative, it appears a volcano occupying the newly inundated Sunda Strait suffered a flank collapse rather than a caldera formation eruption. This is thought to have been Krakatau.
Climate change (cooling) and the beginning of the Dark Ages has been tied to a pair of massive eruptions in 535 and 540 AD. Krakatau was suspected of being one of these eruptions for many years. There is nothing about Krakatau that we know today that supports this speculation.
Between 416 and 1883, the Rakata cone grew to around 800 m. There were numerous dikes through the cone, exposed when the caldera collapsed. This period also had vulcanian and strombolian activity and grew the Danan and Perbuwatan volcanoes. There were pyroclastic fall, surge and flow deposits constructing the 1883 island. Eruptive products were basalts, andesites, dacite pumices.
The 1883 caldera forming eruption came from a zoned rhyodacite magma chamber and produced banded rhyodacite pumice, obsidian, and basaltic andesites.
There are several books on the 1883 eruption. The definitive one appears to be a 1984 one entitled “Krakatau 1883, The Volcanic Eruption and Its Effects” by Simkin and Fiske. https://www.amazon.com/Krakatau-1883-Volcanic-Eruption-Effects/dp/0874748410
This section will only summarize that eruption.
At the time of the eruption, the island of Krakatau was situated in an older caldera. It had three cones aligned in a roughly NNW trending line. From north to south, there were Perboewatan, Danan and Rakata. Part of the Rakata cone is the only remaining portion of the pre-1883 volcanic island. Verlaten Island is close to the NW and Lang Island is close to the NE. Both mark portions of what may be the ancestral island destroyed in a caldera-forming eruption in 416 AD (unlikely) or at the end of an earlier eruptive cycle.
The island had been dormant for 200 years. The first report of activity was an ash plume perhaps 11 km above the island on May 20, 1883. Sounds were heard on Java and Sumatra up to 130 km away. Noise from the initial eruption was not heard closer to the island. Activity died down after a few days.
It resumed and on June 19 and became paroxysmal by Aug. 26. The main eruption began on Aug. 26, as Krakatau sent a Plinian ash plume at least 35 km into the air. This plume may have reached 50 – 80 km during the latter stages of the eruption before it collapsed. This portion of the eruption also started creating high waves on either side of Sunda Strait along with significant ashfall and darkness.
There were four massive explosions on Aug. 27 over the course of 4.5 hours. The last one at 1002 L was so loud it was heard as distant gunfire (artillery) thousands of kilometers away in Sri Lanka and Perth, Australia. These explosions took place in the final stages of the eruption. Barometers worldwide measured multiple passages of the final shock wave afterwards.
The first explosion ruptured the magma chamber and exposed the magma chamber to seawater, the eruption then became phreatoplinian / superplinian. This was one of the causes of the loud explosions. During this sequence, the eruptive column collapsed, sending pyroclastic flows tens of kilometers across the Strait and kilometers onto the shoreline of neighboring Java and Sumatra. The pyroclastic flows generally preceded the tsunamis
The collapse of the caldera and pyroclastic flows created a series of tsunamis up to 40 m. These killed over 36,000 on the neighboring islands. 90% of the deaths during this eruption were due to the tsunamis. The rest were due to the massive ashfall and pyroclastic flows hitting the shoreline.
Ashfall blacked out the sun for three days in the region near the Strait. It also created pumice rafts that clogged shipping into and out of the Strait.
Only a third of the island survived the eruption, this was the southernmost portion where the Rakata vent was cut in half by the caldera collapse. The caldera ended up being some 7 km in diameter with a flat floor at 250 m depth and steep walls. The eruption ejected over 48 km3 of dacite, over 21 km3 of which were tephras, in a 6.7 VEI eruption. Small eruptions continued in the following months until Feb. 1884.
It also seemed to impact global temperatures a bit, generally cooling by a degree C or so until the aerosols injected into the stratosphere were removed a few years after the eruption.
The Krakatau volcanic complex is part of the Sunda arc, a 3,000 km long line of volcanoes stretching from Sumatra to the Banda Sea. Most of the volcanic activity is driven by the subduction of the Indo-Australian Plate beneath the Eurasian Plate.
The Sunda Strait is the dividing line between Java Trench frontal subduction and the Sumatra Trench frontal subduction. The Sumatra Fault system travels close to the western edge of Sumatra, extends into the Sunda Strait, and mostly ends. There are other faults that begin on the eastern portion of the Sunda Strait and extend into Java to the east.
There is no small argument about the existence of a rotation of Sumatra relative to Java starting some 2 Ma. If it exists, this rotation opened the Strait before 2 Ma. An alternate explanation is the movement of the SW Sumatra Block NW along the Sumatra Fault. Either way, the extension regime and multiple lineaments appear to have allowed melts / magmas to occupy some portion of the newly opening volume. As long as the extension continues, whatever the reason, the longer the vigorous volcanic activity will continue.
Krakatau sits at the intersection of two graben zones, a north – south active, shallow seismic belt, which coincide with a 200 km volcanic lineament. The ongoing subduction offshore drives compression in the Strait while the rotation / NW block movement drives the extension regime within it. The extensional zone in the Sunda Strait widens considerably to the south. As there is insufficient melt to maintain the seafloor of the newly spreading basing, it is subsiding as it opens, particularly to the south on the Indian Ocean / subduction side of the Sunda Strait.
The Strait is a rapidly subsiding basin that has at least a caldera and two ignimbrite tuffs dated 1.0 and 0.1 Ma on its far western shore. Ignimbrites appear to be generated by remelting crustal rocks. The chemical composition of Krakatau has a wider range than normal island arc volcanoes.
This flank collapse was a surprising event, remarkably complex and violent. The Krakatau vent continues to be one of the more deadly volcanic systems on the planet. Even though a new tsunami warning system is presently being implemented Given the dense population on either side of the Sunda Strait, future loss of life due to volcanically caused tsunamis should not be unexpected.