Researchers have studied satellite images of the mountain to determine how much rock and ash had tumbled into the sea, and estimate it has lost more than two-thirds of its height and volume over the course of the last week. They believe much of this happened in one movement, which if try would explain the colossal waves of up the five metres which flooded the coastline of nearby Java and Sumatra, bringing with them colossal chaos, destruction and tragic loss of life. Indonesia’s disaster agency puts the official death toll at 431, with another 20 or so missing.
In addition, more than 40,000 people have been displaced from their homes, having been told to keep away from coastal areas for fear of more tsunamis.
The country’s Centre of Volcanology and Geological Hazard Mitigation (PVMBG) looked at photographs from various radar satellites, including the European Union’s Sentinel-1 constellation, plus Germany’s TerraSAR-X.
Radar satellites are able to offer a clear view of the ground irrespective of whether it is day or night, and can also see through clouds.
In this way they have been able to take initial measurements indicating Anak Krakatau’s volcanic cone, which previously stood 340 metres high, is now just 110 metres.
In addition, the volcano has lost a vast amount of its volume – 150-170 million cubic metres of material has gone, with just 40-70 million cubic metres still in place.
Further tests are planned in a bid to determine how much was lost in the initial event on December 22, compared with the days which followed.
Dr Mohammad Heidarzadeh, Assistant Professor of Coastal Engineering at London’s Brunel University, tweeted after the news of the tsunami emerged: “Many volcanic eruptions cannot generate tsunami.
“The Krakatoa volcanic mass should have been very large; probably a volume of a few kilometers cubed.
“Volcanic eruption can generate small earth shaking which are called “volcano-induced earthquakes”.
“The challenge of volcanic tsunamis is that they are silent killers with no warnings unlike tectonic tsunamis which are usually preceded by earthquake shaking as warnings.”
The website Volcano Discovery added: “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 December, which accumulated lots of new lava.
“This in turn might have been “the last straw that breaks the camel’s back”, ie 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.”
A three-mile safety exclusion zone is currently in force around Anak Krakatau, while flights has also been rerouted.
The possibility of the volcano’s cone collapsing had been a concern for scientists well before Saturday’s event.
They even modelled the possibility in a 2012 study, correctly identifying the mountain’s western flank as the one most likely to fail. The study predicted the actual wave heights and flooding with remarkable accuracy.