For the second time this year, an unexpected tsunami hit the coast of Indonesia resulting in hundreds of deaths. This tsunami may have been generated by an eruption of Anak Krakatau in the Sunda Strait, with Indonesian officials speculating that an eruption that occurred late last night may have triggered undersea landslides. So far, at least 220 people are known to have died , over 800 were injured and multitudes are missing. Many of the deaths were in Pandelang, located at the end of bays that could have amplified the height of the waves. The tsunami waves were as high as 3 meters. You can see some of the destruction from these waves in the video and images below. UPDATE: The death toll is now over 400 and local authorities are asking people to stay off the beaches until midweek.

Whatever happened at Anak Krakatau to cause the tsunami, it appears that the volcano has experienced a major eruption as well. The Darwin VAAC report for the volcano indicate ash reaching as high as 17 kilometers (55,000 feet). The Himawari-8 weather satellite caught the plume from the eruption — you can see it in the false color loop:

UPDATE: Here’s another GIF showing last night’s eruptive plume from Anak Krakatau:

There are unconfirmed reports that the tsunami was generated by more than half of the existing Anak Krakatau cone collapsing into the sea. No significant earthquakes were recorded in the area last night, so the eruption of Anak Krakatau may be the likeliest source of the tsunami. There are some video that claim to be from after the eruption showing that does look like a dissected cone for the volcano:

UPDATE: Here’s another video from today of Anak Krakatau showing the strong “rooster tail” plumes being produced by magma interacting with seawater. It really does look like the island has changed significantly:

UPDATE: These images from Sentinel-1 show evidence of a collapse of Anak Krakatau along with potential waves generated by the collapse:

UPDATE: Apparently, this exact scenario was hypothesized in a 2012 paper by Giachetti and others. Hat tip to Jonathan Amos for that find.

UPDATE: Apparently the current eruption of Anak Krakatau has produced more volcanic lightning than possibly any eruption observed. Over 30,000 strikes since the eruption 2 days ago:

Sector collapse is one of the major ways that volcanic eruptions can produce tsunamis. An eruption of Unzen in 1792 caused part of the volcano to collapse into the sea, killing over 15,000. Krakatau, the volcano that preceded Anak Krakatau, famously produced a massive eruption and tsunami in 1883. Unlike this event, the 1883 tsunami that killed 36,000 was produced by a caldera collapse, where the entire volcano collapsed into itself forming a bowl that was filled by the sea (below). Anak Krakatau has been built over the past 125 years within that 1883 caldera.

ector collapse is one of the major ways that volcanic eruptions can produce tsunamis. An eruption of Unzen in 1792 caused part of the volcano to collapse into the sea, killing over 15,000. Krakatau, the volcano that preceded Anak Krakatau, famously produced a massive eruption and tsunami in 1883. Unlike this event, the 1883 tsunami that killed 36,000 was produced by a caldera collapse, where the entire volcano collapsed into itself forming a bowl that was filled by the sea (below). Anak Krakatau has been built over the past 125 years within that 1883 caldera.

Sentinel-2 image of Anak Krakatau erupting in September 2018. The three outer islands outline the shape of Krakatau prior to the 1883 eruption. NASA Earth Observatory.

Sentinel-2 image of Anak Krakatau erupting in September 2018. The three outer islands outline the shape of Krakatau prior to the 1883 eruption. NASA Earth Observatory.

As Simon Carn speculates, part of the volcano has grown rapidly over this year and Anak Krakatau has had an active fall, with frequent strombolian eruptions and lava flows – see the images above taken in September. One scenario [SPECULATION] is that a collapse of part of the volcano would allow seawater to interact with the erupting magma, causing the explosive eruption seen last night [SPECULATION] UPDATE: Simon Carn sees characteristics in the eruption that suggest a lot of magma and seawater interaction. Whatever the case, it may be a while before the full reason how the tsunami was generated.

You can watch some of the eruptions going on at Anak Krakatau only a few hours before the tsunami hit, taken by Øystein L. Andersen.

Øystein and his family escaped the tsunami (just barely) and he’s been updating with images of the aftermath of the tsunami.

I’ll add more information about this tragedy when possible.