ANYER, Indonesia—The volcano that unleashed a tsunami and killed more than 400 people has collapsed to about a third of its previous height and is ejecting superheated magma into the sea, magnifying the power of the eruptions and sending ash more than a mile into the air.
The volcano, called Anak Krakatau, partially collapsed in a violent spasm on Dec. 22, causing a massive landslide that sent high waves barreling toward both shores of the Sunda Strait, the narrow passage between the islands of Java and Sumatra. The volcano’s cone is now about 360 feet above sea level, Indonesian officials said Saturday, reduced from more than 1,000 feet before the collapse.
Volcanoes erupt or collapse when a buildup of pressure inside becomes too much for the cone to sustain. Anak Krakatau has been erupting since June and scientists say it is impossible to tell what it will do next, but it could erupt violently or the stress on the rock could force another collapse and send a landslide into the strait, triggering another tsunami.
How a Volcano Triggered a Tsunami
Indonesia’s Anak Krakatau volcano killed hundreds of people when part of it collapsed and caused a tsunami Dec. 22. The volcano has grown on the ruins of its infamous parent, Krakatau (also known as Krakatoa), which destroyed itself in an 1883 eruption.
Note: The illustration is not to scale.
Sources: International Tsunami Information Center; Indonesia's volcanology agency
The earlier collapse pushed Anak Krakatau’s active vent underwater, creating a constant reaction with the sea that instantly turns water to steam. As it expands, the steam blows apart the molten rock.
“The big difference is unlike prior to the collapse, the erupting magma is being exposed directly to seawater, so the eruptions are more explosive and actually creating its own ‘volcano thunderstorm’ from all the steam, ash and volcanic gases,” said Erik Klemetti, a volcano expert at Denison University in Granville, Ohio. “It is inherently unstable as it’s just made of layers of lava and ash and debris.”
On Friday, it was sending ash more than a mile skyward in eruptions as frequently as every minute, officials said. Anak Krakatau has been barely visible from shore for the past week, partly due to storms. But even when the sky clears, the plume of ash generated by the underwater reaction, known as a pyroclastic cloud, smothers the silhouette. After dark, lightning strikes caused by energy released from the eruption can be seen every few seconds.
Perhaps most likely, scientists say, the volcano will erupt slowly and grow foot by foot, piling fresh lava on top of old. Over time, the cone will again push higher above the surface. Officials said Saturday the volcano’s smaller size makes the chance of another large collapse unlikely.
Anak Krakatau, which means Child of Krakatau, has been bulking up for a century upon the ruins of its much larger ancestor, best-known as Krakatoa, a volcano that loomed like a giant over the strait until it blew itself into oblivion in 1883.
The force of that eruption sent tsunami waves as high as 130 feet into the shorelines, killing 36,000 people, and blew so much ash into the atmosphere that the planet’s temperature cooled for years.
Though only a few fragments of Krakatoa are visible today, the forces at work under the surface never stopped. Indonesia is home to 127 active volcanoes and many hundreds of inactive ones, the product of tectonic activity on the Ring of Fire, a rim around the Pacific where several of the plates that form the earth’s crust meet each other.
Beneath Indonesia, plates are colliding, forcing one beneath the other and in the process melting the plates and sending molten magma toward the surface. Heated rock seeks a place to rise.
One of those places is at Anak Krakatau. While it isn’t clear why volcanoes emerge in one location over another, the liquid rock tends to follow a path of least resistance “like oil through water,” said Janine Krippner, a volcanologist at Concord University in Athens, West Virginia.
Satellite data and updates from Indonesian authorities indicate a 64-hectare (160-acre) chunk that slid into the sea with the force of a 3.4-magnitude earthquake.
The constant eruption and belching of ash has prevented a full assessment. Authorities have diverted aircraft and ordered surface vessels to stay at least 3 miles (five kilometers) away.
“It’s too early to say whether the seismic activity is slowing,’’’ said Kristianto, a senior volcanologist at Indonesia’s volcanology agency.
Volcanic ash—a mix of rock, ash, glass and steam—can damage and even stop aircraft engines and destroy electronics. It also causes respiratory problems.
Authorities say that 40,000 people of the 350,000 living in the area were displaced. Many are seeking refuge in shelters on higher ground in case another strikes. The government is promising to improve early warning systems, though past attempts in this nation of 18,000 islands have fared poorly.
Even the best sensors, typically designed to detect movements of water that could indicate an incoming tsunami, may not be very helpful. The tsunami unleashed by Anak Krakatau took just 24 minutes to reach land, at its nearest point about 25 miles away.
“No tsunami warning system is designed for that short of a range,” said Ms. Krippner.
Other protections could work better, experts said. Stretches of coastline where resort owners had built high retaining walls were much less damaged. Regulations to prohibit building close to beaches could mitigate the damage. Officials acknowledge that fishing families, for example, would likely resist wholesale relocation.
Experts say that while the science has come a long way, there are still many unknowns about why and when activity takes place, partly due to the limitations of volcanic sensory equipment.
“Anak Krakatau is a very young volcano,’’ said Robin George Andrews, a volcanologist. “That means there’s not much data on how it behaves, so forecasting its future is tricky and concrete predictions—like with any volcano, each of them unique—are impossible.”
—I Made Sentana contributed to this article.
Write to Jake Maxwell Watts at jake.watts@wsj.com
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