It has been a few weeks since the massive collapse of the Anak Krakatau cone that slid into the sea, generating the deadly tsunami that swept along both sides of the Sunda Strait in Indonesia. We’ve finally been able to see what occurred during that landslide and sure enough, most of the cone that was Anak Krakatau is gone (see below). In its place is, well, not much but open space that has seen seawater fill in. This volatile mix of water and erupting magma has meant that Anak Krakatau has been churning out tall steam-and-ash plumes that, at times, towered >10 kilometers (>30,000 feet) over the volcano. Luckily, the collapse of the cone will temporarily reduce the threat of another tsunami but as the cone builds back up over the years-to-decades, that threat will return because volcanic collapses and landslides are actually fairly common in the geologic record.
Here are a few examples of other massive landslides and collapses. Some are the trigger for an eruption, like what occurred at Mount St. Helens in 1980. As magma filled in the volcano prior to May 1980, the slopes were over-steepened and became unstable. When an earthquake struck on the morning of May 20, that over-steepened slope slid away, releasing the pressure on the magma. Much like popping the cork on a bottle of champagne, that magma quickly formed bubbles to make the explosion that happened seconds after the landslide.
Others occurred well after the eruption and don’t seem to coincidence with any eruption, such as Chaos Jumbles at Chaos Crags in California.The Crags themselves are domes that formed about 1,100 years ago as sticky rhyodacite lava oozed and exploded onto the surface in northern California next to Lassen Peak. The collapse that formed Chaos Jumbles only occurred 340 years ago, well after the eruptions that formed Chaos Crags.
These scarps that are created by the landslide/collapse can fill up over time as new eruptions repair the wound, so we might only have the debris that was left behind as evidence that a landslide even happened. A landslide occurred on the slopes of Mt. Shasta in California somewhere between 300,000-380,000 years ago and it was ~10 times larger than the collapse during the 1980 eruption of St. Helens. However, since then Shasta has repaired its scarp so the evidence of this collapse was noticed until we saw the debris after the 1980 St. Helens eruptions.
If you’re looking for the largest volcanic avalanche that we know of, look at Mt. Meru in Tanzania. The eastern side of the volcano collapsed ~9,000 years ago. Meru produced a landslide deposit that ran out 49 kilometers and contained over 20 cubic kilometers of debris! So, this collapse at Anak Krakatau was possibly ~100 times smaller than this collapse of Meru. This isn’t too belittle what happened at the end of December, but the devastation was caused by only ~0.15 cubic kilometers of material sliding off into the ocean … so you can imagine what might happen if a larger volcano collapsed.
There are estimates that over half of all volcanoes and volcanic domes in Japan have experienced a collapse that generated a landslide/avalanche. That’s just how common they can be in volcanic areas. They tend to be unexpected because many times, they aren’t caused by an eruption (but can trigger them, sometimes), so understanding which slopes might be most susceptible to such collapses is immensely important for volcanic hazard mapping and mitigation.