Increasing eruption at Anak Krakatau ... ?

Anak Krakatau erupting in 2007.

There is a report out tonight that activity might be increasing at Anak Krakatau in Indonesia, however, it is unclear whether it is actually increasing or not. People are evacuating the coastal area of the Sunda Strait to Bandarlampung in fear that a major eruption of the volcano is on the horizon. However, the Volcanology and Geological Disaster Mitigation Center (PVMBG) in Indonesia has not changed the alert status at Anak Krakatau and it remains at 2 of 4 (~Orange Status). The volcano has been erupting almost constantly for the last few years, producing strombolian eruptions and explosions but local residents fears that a perceived increase in eruptions over the last few days could herald something larger.

It is hard to tell exactly what is going on at Anak Krakatau beyond descriptions like these from local fishermen:

"So far it has never emitted such loud blasts and we have been observing that it has often discharged molten lava."

This sort of activity is not unheard of at Anak Krakatau, and the last eruptive cycle that was thought to have ended in August of 2008 produced lava and pyroclastic flows. There are also reports from the Indonesia Weather Service and Qantas pilots of ash as high as 800 m above the vent. Again, nothing too shocking, but apparently the activity has increased enough to cause people in the neighboring regions to become terrified of a potential larger eruption.

This is the crux of the problem when it comes to volcanic mitigation in a populated area: perception versus reality. Anak Krakatau may or may not be experiencing an uptick in activity - from the report, it would seem that it is, at the very least, seeing a small increase - but the people living along the Sunda Strait believe that it is and this belief is driving their behavior. There are no indications (well, none that have been released to the public) that Anak Krakatau might be headed toward a larger eruption, but the memory of previous eruptions and the fear of new activity is still there. Muhammad Hendrasto, the chief of Anak Krakatau's monitoring team, is calling on resident to not panic (good advice). Until more information comes out, it might be time to keep an eye on the child of Krakatau.

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What is the height of Anak Krakatau now compared to before the 1883 eruption?

From what I could find, Krakatau was ~450 m / 1465 ft tall (above sea level) prior to the cataclysmic eruption in 1883. Anak Krakatau is now listed as 813 m / 2,700 ft tall, so it is actually large (heightwise) than the previous incarnation of Krakatau.

When you consider that Anak Krakatau grew from the caldera (what apart and what depth I don't know, but likely deep) and did not break the surface until the 1920s, the amount of growth is stunning.

Is that 813 m the height asl? If so, the true height will be about 1100 m, given that the caldera was about 300 m deep after the 1883 eruption. Would that be enough for a sector collapse, the type of activity likeliest to threaten populated areas?

Yup, that is 813 m above sea level, so plenty of material for to generate a tsunami if the volcano were to collapse catastrophically ... not that we have any evidence that it might do so any time soon.

Found some more information on Anak Krakatau on PBS's website. Their information conflicts with the other information posted here. They say that the volcano is about 300 m asl., as of the mid 90'. Also, some good pictures of eruptions as well as drawing of where the old caldera are located compared to the current volcano. The old ones were much larger then Anak Krakatau.

The 813 m height I listed was from the GVP page for Krakatau, so it could be that the 813 m value is for a location that is not Anak Krakatau. Wikipedia mentions Since the 1950s, Anak Krakatau has grown at an average rate of five inches (13 cm) per week, however, no reference is listed so I would consider this dubious. In any case, Anak Krakatau is still a sizable volcano considering it has only only for ~120 years, although its footprint is much smaller than the predecessor Krakatau.

Possible solution to height mystery: if GVP list the highest point of Krakatau as a whole, that would be the summit of Rakata rather than the summit of Anak Krakatau. Sound reasonable?

Mike - I'd agree with that guess. I think the height of Anak Krakatau is somewhere in the low hundreds of meters from what I can gather elsewhere.

How would have thought trying to determine the height of a mound of rock would foster such discussion.

Hooray for GeoRef- the following is the abstract for a 2006 presentation by Hoffman-Rothe et al.:
"Krakatau volcano, in Indonesia, showed its destructive vigor when it exploded in 1883 [Self and Rampino, 1981]. The eruption and subsequent tsunami caused more than 35,000 casualties along the coasts of the Sunda Strait. In 1928, the âchildâ of Krakatau, Anak Krakatau, emerged from the sea at the same location as its predecessor and has since grown to a height of 315 meters. The volcano exhibits frequent activityâon average one large eruption every four yearsâyet again posing risk for the coastal population of Java and Sumatra and for the economically important shipping routes through the Sunda Strait."

So 315 meters (assume that's asl) as of 2006.


Ref: Hoffmann-Rothe, Arne; Ibs-von Seht, Malte; Kniess, Rudolf; Klinge, Klaus; Reichert, Christian; Purbawinata, Mas Atje; Patria, Cahya (2006) Monitoring Anak Krakatau Volcano in Indonesia, Eos, Transactions, American Geophysical Union, vol. 87, no. 51, pp.581, 585-586

I saw a TV show in the past few months of either Tambora or Krakatau (I forget - but I am assuming it was of Krakatau).

One thing about the show was that the eruption in the 1800's was proceeded by a lot of noise and bangs.

Now the noise is unprecedented compared to what? To their history at the volcano - or just in the current cycle of unrest?
Now, do the people living in the area who are scared of the loud cannon-like booms currently emanating from Krakatau who say, "So far it has never emitted such loud blasts" - are they comparing the noise to just this current eruptive cycle? Or are they comparing the increase in noise to earlier unrest over the last several decades?

If the current noise at Krakatau is truly unprecedented for this population - I would certainly agree that it is probably time for them to go. They are probably jumping the gun and leaving early - but maybe they should leave just so they can sleep better and have less stress. Hearing cannon-like booms throughout the night while trying to sleep - isn't very comforting as they reflect on the history of this volcano.

By Thomas Donlon (not verified) on 21 Apr 2009 #permalink

I would certainly agree that it is probably time for them to go

There are 35,000 people in the past who probably wish they had thought that way too.

When I visited Krakatau in 1996, occasional large explosions woke me up and shook the hotel building on the mainland, some 60 km away. So I don't think loud explosions are all that unusual there, although perhaps for the past few years this has not been happening.

The height of 813 m (above sea level) of the Krakatau complex applies to the summit of the remainder of old Rakata volcano, which was the highest peak also before the 1883 explosion. The cliff now cutting that peak in half is the rim of the 1883 caldera. It is in the center of the caldera that Anak Krakatau is building up, and I recall that the activtiy in the 1990s built its cone from around 200 m to more than 300 m (above sea level). That fits well with the 1996 figure of 315 m. No further growth has occurred during the 2007-2008 activity, because this occurred from a new vent below the summit. So if there is a new period of major activity starting now, it might start growing once more. In any case I don't think this new activity is a precursor to some 1883-type disaster, it's too soon after that, caldera-forming eruptions do not characteristically occur at such short intervals. I guess Anak Krakatau will grow much bigger before it explodes again 1883 style, possibly hundreds if not thousands of years from now.

When I visited Krakatau in 1996, occasional large explosions woke me up and shook the hotel building on the mainland, some 60 km away.

Mike, thanks for the personal story. I had no idea people would stay in a hotel 36 miles away and hear the volcano they were about to visit.

I guess Anak Krakatau will grow much bigger before it explodes again 1883 style, possibly hundreds if not thousands of years from now.

Boris, thanks for the explanation. It's sometimes hard for me to grasp the timelines on a 4.5 billion year old planet.

Patrick, it would've been far preferable to camp on one of the neighboring islands near the volcano, but I was only able to arrange a day trip by boat from the mainland at that time, unfortunately. Next time!

Boris Behncke,

I enjoyed reading your informative comment on Anak Krakatau.

And if it be unlikely that another caldera forming eruption happens so soon - and you gave good reasons to think it won't - now those nearby will wrestle with the possible threat of a less energetic eruption.

If you ever in the future get a sense that an old volcano coming back to life is more dangerous than generally thought - I'd certainly respect it too if you can share your thoughts on such a resurgent volcano.

With so many volcanoes becoming active, it is hard to know which ones (if any) deserve greater scrutiny.

By Thomas Donlon (not verified) on 22 Apr 2009 #permalink

Just stumbled upon a few recent photos of Anak Krakatau erupting, which have been posted at the Flickr photo site. They show that the active vent lies in one corner of the large pit that opened during the 2007-2008 activity, and new ejecta have led to the growth of a new cone within that pit, filling much of it. But see for yourself.

11 April:

21 April:

Hi All,
I just found this blog. I have been 'studying' Anak Krakatau for many years now, having had a childhood fascination for the volcano, and then being fortunate enough to have been posted to Indonesia to work for a few years. I return regularly to Jakarta with my Indonesian wife, and visit Krakatau at least 3 times annually. Last visit was in February this year. Last year saw a renewed period of eruption, following the opening of a new vent on the South West side of the island in October 2007. When you see the size of the vent, it helps understand why the locals talked about very loud bangs being heard on the mainland of Java. It ust have been a very big explosion when the vent first opened up. Eruptions are coming from a lower position than the cone at the top. Explosions can be deafening, especially if you are on the island (where the subterranean tremors are also very frightening!), or close to it (I usually charter a boat and spend a few nights there at a time). From my reckoning, the volcano is now at least 860 metres above sea level, when compared to the remains of Rakata (which provides an excellent viewing location for the current vent eruptions on Krakatau), which was the highest peak of the original Krakatau island before the 1883 explosions (estimated to be around 450 metres above sea level). My friends from the area around Anyer tell me that the volcano is going through a renewed period of activity right now, and because of this I am planning a visit again for July this year. I have some of my earlier pictures on Google Earth, but these were taken during periods of inactivity. I intend to upload some of my more recent pics soon. I also have them on Picasa (which is where Google pick them up from).
I look forward to sharing your future discussions with you!

If Anak Krakatua has a major eruption, would the coast of Thailand be at risk for Tsunami? If so, which side of the penninsula would be hit hardest? I'm sorry if this isn't the place to ask. . .

By jan pichel (not verified) on 03 Jun 2009 #permalink

Given Anak Krakatau's recent history, I'd guess that a major eruption with tsunami is pretty unlikely; but if there was one, the tsunami would probably affect roughly the same areas as in 1883, that is, the shores of Java and Sumatra, bounding the Sunda Strait. If the 1883 collapse didn't cause major tsunami on the Thai coast, it's unlikely that a collapse of the much smaller present cone would do so

I agree with Mike. I don't think Thailand would be affected, especially given the protection provided by Sumatera and Java islands. This is, of course apart from the consequences that would be suffered the world over, from events such as those caused by the ejected materials circling in the stratosphere for years. The earlier catastrophic event in 535 AD, which was much larger (although local details are not as clear), caused much more global damage and is being attributed as the cause for many major global changes. Read 'Catastrophe' by David Keys (hard to get a hold of now though), which alludes to many of the significant changes to world order around that time as being the result of this earlier Krakatau eruption. A good read for all Krakatau fans!

To be fair, it's not entirely certain yet that the worldwide effects in 535AD were from Krakatau, there are several other 'suspects' including a major caldera-collapse event at Rabaul and (possibly?) a VEI6 from Bona/Churchill in eastern Alaska

Yes Mike, I concede your point.
I guess I am somewhat biased in my opinions :)

Getting impatient to return to Krakatau now ...just over 6 weeks to go :)

Rakata is about 800 m high and Anak is about 400 m high. You can see this is photoes from the Verlaten - both seem to have about the same hight with Rakata twise the distance away.
Best regards!

Jan, yes... as would the entire Indian ocean and perhaps S. America. It could also blow great chunks of ice loose in Antarctica. I have a friend who goes there durng their Winter months to look for viruses in the ice samples. Its so cold in the winter that waves will come in under the ice and then hit the beach, break through the fissures and rise like rockets. Due to the temps they freeze instantly and they are a cool blue like you have never seen before. This though would be a tsunami and anything is possible I would say.

Erik/Boris, whats your read on the angle of repose on this thing? Thats a shaky looking slope and if this is getting ready to go they need to think landslide tsunami first and then possibly a Pinian eruption one. Werent there like 3 or 4 major tsunamis in the 1800s from this thing.

Also isnt it at a near height that it was when it blew last time? I read somewhere that it was at about 600 when the countdown to eruption started and MADE it to about 800 just before it blew. Thats a crapload of rock that could drop into the sea and create one helluva tsunami and that would be only round one.

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