What is a volcanic tsunami, and why did one sneak up on Indonesia?

Hundreds are dead, with more missing, following a tsunami that took the Indonesian coast by surprise on Dec. 22. 

But this was no typical tsunami. Rather, it was a volcanic tsunami, and understanding what that means may help explain why this particular manifestation of nature’s power was so devastating. 

The tsunami that struck between the Sumatra and Java islands was triggered not by an earthquake — as is typically the case with tsunamis — but instead by an active volcano in the area known as Anak Krakatau. The volcano reportedly partially collapsed, triggering a landslide underwater, which then in turn kicked off the tsunami, said Dwikorita Karnawati, who leads Indonesia’s meteorological agency, according to multiple news reports.

The sequence of events didn’t trigger a tsunami alert, catching the country by surprise. 

“[It] did not rise to the level of triggering [a tsunami] alert,” University of Southern California’s Tsunami Research Center director Costas Synolakis told NBC News. “So from that point of view, the Tsunami Warning Centers were essentially useless.”

The 305-meter volcano responsible for the landslide, reports Al Jazeera, rose from the sea starting in the 1920s at the site of a deadly 1883 eruption that left over 30,000 dead. 

It was only a few months ago in late September that another tsunami struck the Indonesia coast. That tsunami followed a magnitude 7.5 earthquake, and resulted in 6-foot waves. 

As for Saturday’s tragedy, Northwestern University earth science professor Emile Okal told NBC News that volcanoes are constantly in motion and that landslides are one essentially unavoidable outcome of that fact. 

“A volcano is something that is a living thing,” explained Okal. “Eventually it’s going to have a landslide, and if it’s underwater, going to displace water and make a wave.”

Tragically, for those on the Indonesia coast affected by Saturday’s tsunami, this fact reverberates both into the past and the future. Officials warn that more volcanic tsunamis may be coming, with CNN reporting that Anak Krakatau is still erupting and could experience more landslides. 

But for now, rescue workers are focused on today, and that means finding those people still missing following the uniquely dangerous volcanic tsunami.