22-23 Dec 2018 eruption & tsunami of Krakatoa - updates

Krakatoa volcano (Sunda Strait, Indonesia): ash plume reported to 35-40,000 ft altitude

Sometimes strong explosive activity seems to continue at the volcano: VAAC Darwin reported an ash plume that rose to 40,000 ft (12 km) altitude this morning as well as "regular, intermittent" emissions of volcanic ash to 35,000 ft (10 km).
Seismic activity is still relatively low, but has picked up again after a phase of near absence at the end of 2018.

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Krakatoa volcano (Sunda Strait, Indonesia): fresh aerial image showing the island's dramatic change in shape

A fresh aerial image from Anak Krakatau has appeared on the very active Volcanoes and Volcanism group on facebook:
it shows in more detail what the 29 Dec satellite image already confirmed: the island of Anak Krakatau has now the shape of a 270 deg almost closed crescent, with a water-filled crater in the center where once the 330 m tall summit cone stood (which collapsed on 22 Dec causing the tsunami).
Although the image is not detailed enough, it seems (and it would be expected so after the violent phreatomagmatic activity following the collapse) that virtually all of the island has been covered and reshaped, smoothed out by fresh pyroclastic deposits. What if any of its flora and fauna has survived needs to be found out yet by visiting it.
Happy New Year to all!

update:
This processed satellite image from 28 Dec below shows the change in morphology even clearer, but includes a significant land gain after the collapse as well, which seems to have build most of the southern "arm" of the crescent. Most of the new land is probably due to accumulated material (tephra) from the intense sursteyan activity in the days after the landslide, building a new large tuff ring around the crater:

Bukti tsunami akibat flank collapse yang tak terbantahkan (dalam gambar yg lebih jelas) pic.twitter.com/Cspa7mJa87

— Daryono BMKG (@DaryonoBMKG) December 30, 2018

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• Dongeng Geologi

Krakatoa volcano (Sunda Strait, Indonesia): new satellite imagery reveal both rapid land loss and growth of Anak Krakatau in detail

A new high-resolution satellite image taken yesterday allows an update on the dramatic effects the 22 Dec collapse has had on Anak Krakatau island:
It shows that actually not all, but most of the summit cone is now missing as well as significant parts of the western and southern shores previously formed by young lava deltas. On the other side, there has been a significant enlargement of the coast on the eastern shore.
This poses several big questions (we cannot answer at this point):
1) How much of the enlargement of the coast occurred by debris flows from the collapse(s) itself (if any traveled towards the east and not only SW down into the sea); and
2) how much of it might have been added by a) the violent explosive activity during the following days, and/or b) how much of it can be attributed to sea currents washing loose material deposited into the sea back to shore ?
The latter is likely to be significant, as the coasts in the eastern parts of Anak Krakatau are very shallow, and large amounts of mud can be seen floating around Anak Krakatau in the satellit image.

3) Another question is how large was the collapse that actually led to the generation of the tsunami? Was it only one? - did the summit cone collapse in one go, or did several collapses occur? The latter idea might explain that there were two tsunami waves observed, the first being relatively small, the second one following after a few minutes (but not immediately) and much higher.

At the moment, as mentioned, we can not give answers. But at least some of them could be given by doing more research and, first of all, by detailed ground observations. We hope our team on location around our expedition leader Andi will be soon able to visit the island from close and bring us more information.

Brief review of Dec. 28 Sentinel-1 data (@sentinel_hub) for #Anak #Krakatoa #eruption #landslide. Well visible areas of land loss and gain (prior Dec. 22nd eruption/landslide). Question remains how much of the dome loss contributed to #tsunami? pic.twitter.com/HVeQSASowc

— CATnews | Andreas M. Schäfer (@CATnewsDE) December 30, 2018

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Krakatoa volcano (Sunda Strait, Indonesia): first estimates on the effect of the tsunami-triggering landslide

The activity at the volcano continues at reduced rate. Steam explosions have been more intermittent and smaller during the past 2 days, generating plumes that rose a few hundred meters only.
This trend corresponds to the seismic signals showing a marked decrease since the surge of supply of magma that led to the paroxysm during 21-22 Dec, which accumulated lots of new lava. This in turn might have been "the last straw that breaks the camel's back", i.e. the additional sudden weight on the slopes and/or the shaking induced by the continuous explosions could might have been the final trigger for the landslide to occur.
The Indonesian Center for Volcanology and Geological Disaster Mitigation (PVMBG) published a first official estimate on the effects of the landslide:
- The summit of Anak Krakatau originally 338 meters high is now reduced to only 110 meters, i.e. most of the summit cone has indeed disappeared.
- As a consequence, Anak Krakatau island is no longer visible from the mainland, as the island of Panjang (Long Island) blocks the view from Java's west coast.
- The volume of the collapse is estimated to be between 150-180 million m3, while the remaining volume is currently estimated at between 40-70 million m3.
- For this reason, unless tectonic faults in the area of the Sunda Strait are activated, the risk of other massive landslides and tsunamis is currently deemed to be low.

What's next?
However, it is likely that the current eruption continues and is only in a phase of low activity currently; if the cyclic pattern of increased supply every few weeks continues, another phase of stronger activity during January would not be surprising.

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• Perkembangan Terkini Aktivitas Gunung Anak Krakatau, PVMBG: Tinggi Gunung Tinggal 110 Meter (PVMBG press center)

Krakatoa volcano (Sunda Strait, Indonesia) activity update: Major eruption ends

After almost 6 days of an almost non-stop ash plume to 55,000ft following the devastating collapse, Darwin VAAC reports that activity has finally subsided with only very minor emissions. No updates about local conditions at the moment.

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Krakatoa volcano (Sunda Strait, Indonesia): strong explosive activity continues as sea water meets lava

The violent explosive activity caused by magma-water interaction continues at the volcano, from the area where the summit cone collapsed on 22 Dec and now exposes the vent to sea water.
A video of an overflight earlier today shows powerful steam explosions of this so-called Surtseyan activity, generating dense steam and ash clouds, often ejected as "fingers" of larger ejected rock fragments with training steam behind, as well as billowing surges that shoot horizontally above the water.
Civil Protection (BNPB) raised the alert level of the volcano from Waspada (Level II) to Siaga (Level III) on the Indonesian 1-4 scale and the exclusion zone was extended from 2 to 5 kilometers around the volcano, where people are prohibited from carrying out activities.

Kondisi Gunung Anak Krakatau yang divideokan dari KRI Teluk Cirebon 543 pads 26/12/2018, 16.33 WIB. Erupsi masih berlangsung. Tipe erupsi Gunung Anak Krakatau strombolian yang mengeluarkan abu vulkanik dan lontaran batu pijar terus menerus. Status Waspada (level 2). pic.twitter.com/ESydS9UCTc

— Sutopo Purwo Nugroho (@Sutopo_PN) December 26, 2018

Seismic activity from the volcano has been lower following the paroxysm on 21-22 Dec, following the pattern of phases of increased activity roughly every 3-4 weeks, suggesting that magma supply to the vent is currently reduced.

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• STATUS GUNUNG ANAK KRAKATAU DINAIKKAN SIAGA (LEVEL III), RADIUS BERBAHAYA DIPERLUAS MENJADI 5 KM (BNPB)

Krakatoa volcano (Sunda Strait, Indonesia): scientists had explained and warned about the tsunami risk 6 years earlier - prediction came true

Sometimes, science has already answers for tragic events, but their voice is not heard or underestimated at the time. The Indonesia tsunami disaster two days ago is such an example: What happened on 22 Dec 2018 had actually already been described and warned about for years in amazing detail:
In January 2012, scientist around volcanologist Dr Thomas Giachetti from the University of Oregon published results of numerical modelling simulating a flank collapse and associated tsunami at Anak Krakatau island and warned about the devastating effects it would have on nearby coasts:
In their paper adequately called "Tsunami hazard related to a flank collapse of Anak Krakatau Volcano, Sunda Strait, Indonesia", they "numerically simulate a sudden southwestwards destabilization of a large part of the Anak Krakatau Volcano, and the subsequent tsunami formation and propagation."

Anak Krakatau island - the intra-caldera complex called "Child of Krakatau" - has been growing rapidly growing since it first breached the surface of the sea in 1928. Since then, in less than 100 years, it built an overlapping cone during several eruptions, the latest being the one that started May 2018 and still continues. What makes the island particularly prone to gravitational flank failure is that it has been constructed close and above a steep submarine slope, the NE margin of the caldera basin left by the massive 1883 eruption.
As a consequence of this underwater topography, combined with strong sea currents, the western slope of Anak Krakatau has developed to be much steeper than the eastern. Giachetti et al. observed that as the volcano continues to grow towards the SW (which it had done particularly well visible during the recent eruption, s. attached image comparing 2007 and 2018), "landslides along its southwestern flank cannot be excluded. Such a landslide would be directed southwestwards into the 1883 caldera and would trigger waves that would propagate into the Sunda Strait, possibly affecting the Indonesian coasts".

The modeled waive heights in various location correspond amazingly well with what had been observed. At Anyer, for example, waves of up to 1.5 meters would arrive in 38 minutes. Inside the caldera, neighboring islands such as Rakata or Sertung would be hit by waves 15-30 meters in height in less than a minute after the event.

Since the local tsunami warning systems in place was built only with earthquakes as trigger in mind, no warning could be given to the people on the beaches. In addition, the flank collapse occurred at night and the resulting large ash plume (which eventually reached about 15 km altitude), and violent steam explosions as result of sudden interaction of water with magma and hot rocks could not be seen by people. When the tsunami waves arrived they caught everyone by surprise.

It should be noted that the report was written 6 years ago. Since then, the new eruption, everything described in there had even become more true: compared to the situation described in 2012, the cone of Anak Krakatau had significantly been enlarged to the SW and grown in height by estimated 30%.

Change in activity style
The eruption style changed drastically right after the landslide occurred, as it exposed the conduit and the rising magma to the water. As it no longer erupted from the now-gone summit crater, but at or below sea level in the new scar left by the landscape (which can be seen in the video below), violent steam explosions were the result. As seawater interacted with hot exposed rocks and the magma continuing to rise in the conduit, these so-called phreatomagmatic explosions ejected dense jets of steam, debris and ash, and produced a very large plume. Small surges of overheated steam can be seen traveling over the water as well, in the video by Adam Sidiq/Kumparan published on youtube:


As of now, it appears that the activity at the volcano itself has calmed down again, following the pattern since June of phases of increased magma supply occurring rhythmically about once every 4 weeks, followed by quieter phases.

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• T. Giachetti, R. Paris, K. Kelfoun and B. Ontowirjo (2012) "Tsunami hazard related to a flank collapse of Anak Krakatau Volcano, Sunda Strait, Indonesia"