A large chunk of a flank of the volcanic Anak Krakatau island slipped into the ocean and triggered a tsunami that hit Indonesian shores, killing hundreds of people, officials and scientists said on Monday.
At least 281 people were killed, hundreds injured and many buildings were heavily damaged when the tsunami struck, almost without warning, along the rim of the Sunda Strait, between Java and Sumatra islands, late on Saturday.
Anak Krakatau had been spewing ash and lava for months before a 64-hectare (0.64 square km) section of the southwest side of the volcano collapsed, an Indonesian official said.
"This caused an underwater landslide and eventually caused the tsunami," said Dwikorita Karnawati, head of the meteorological agency, adding that the waves hit shorelines 24 minutes later.
Images captured by the European Space Agency's Sentinel-1 satellite showed that a large portion on the southern flank of the volcano slid off into the ocean, scientists said.
"When that land pushes into the ocean ... it displaces the ocean surface causing the vertical displacement that causes the tsunami," Sam Taylor-Offord, a seismologist at GNS Science in Wellington, said of underwater landslides.
The timing of the tsunami, over the Christmas holiday season, revived memories of the Indian Ocean tsunami triggered by an earthquake on Dec. 26, 2004, which killed 226,000 people in 14 countries, including more than 120,000 in Indonesia.
Taylor-Offord said the eruption and "high noise environment" may be why the landslide was not recorded seismically.
The fact the tsunami was triggered by a volcano, and not by an earthquake, may be the reason why no tsunami warning was triggered, scientists said.
Coastal residents reported not seeing or feeling any warning signs, such as an earthquake or receding water along the shore, before waves up to 3 metres (10 feet) high surged in.
Jose Borrero, coastal engineering expert specializing in tsunami hazards at eCoast Marine Consulting, said landslide-generated volcanic tsunami were more of a mystery than tsunami generated by earthquakes, which are better studied.
There are so many different variables involved with the landslide-generated tsunami and a "sweet spot" of exactly the right speed and volume of rocks slipping into and sea and down submerged slopes to generate a wave.
"In Indonesia, we've all been waiting for another big earthquake-tsunami and then boom, here we have a volcanic landslide one," said Borrero.
"I've seen a few bits of imagery that suggest there's some sort of slant collapse that may extend underwater but none of this will be confirmed until there can be an offshore survey where they go and map the seafloor."
Anak Krakatau or "child of Krakatau" emerged from the Krakatau volcano, which in 1888 erupted with such force the blast was heard all the way in Perth, said Mika McKinnon, a geophysicist based in Vancouver, Canada.
Further eruptions have continued from the massive crater left behind.
McKinnon said volcanoes are weak, sloppy heaps of loosely bound rocks all slanted downhill and they slip off all the time.
If this happens to be a large portion, then it would displace enough water to trigger a tsunami.
There are no early warnings systems that can detect such landslide-driven tsunami.
Anak Krakatau is so close to shore there would not have been enough time to react and evacuate the population.
"It's hard to identify landslide-triggered tsunami, especially quickly enough to issue useful warnings," said McKinnon.
"A similar event at Anak Krakatau might trigger another one, or it might not. Maybe a month later, or year from now. We will never know," she said.