The tsunami that killed hundreds of people on Sumatra and Java in Indonesia late Saturday is a reminder that such destructive waves are not always caused by earthquakes.
This disaster, which struck without warning and killed at least 222 people and injured more than 800 others, appeared to have been caused by volcanic activity.
No earthquakes were recorded before the tsunami struck, the Indonesian authorities said. But there had been an eruption on the volcanic island of Anak Krakatau about half an hour before, they said, one of a series of eruptions there in recent weeks.
Tsunamis are created when large amounts of water — in the ocean, a bay or even a lake — are quickly displaced. In an earthquake, that displacement can occur when the ground moves as a fault breaks.
This was the mechanism by which a 9.1 magnitude earthquake on Dec. 26, 2004, off Aceh Province in northern Indonesia, spawned large waves that traveled across the Indian Ocean and killed 250,000 people.
Many other tsunamis have also followed earthquakes, including those in September that devastated the city of Palu on the Indonesian island of Sulawesi after a 7.5 magnitude quake.
Volcanic activity creates a tsunami differently. One possibility is an explosive eruption, or general weakening of the flanks of a volcano by hot magma passing through. Either way, part of the volcano — perhaps, in the case of Anak Krakatau, a part that is underwater — can collapse, creating a landslide that displaces water.
Another possible mechanism is the collapse of a magma chamber below the volcano as it empties during an eruption.
Volcano-related tsunamis are not uncommon. An eruption in 1792 in Japan created waves that were several hundred feet high. Landslides during the 1980 eruption of Mount St. Helens in Washington spawned large waves in a lake nearby.
Perhaps the most famous volcanic disaster in history, the 1883 eruption of Krakatau, also called Krakatoa, triggered tsunamis that killed tens of thousands of people. (Anak Krakatau island was built up in place of Krakatau, which was obliterated in the 1883 event.)
Landslides hitting water can sometimes create huge waves. The largest wave ever recorded was caused by a landslide into Lituya Bay, in southeast Alaska, in July 1958. It followed an earthquake, not an eruption, but created a wave that wiped vegetation off the hillside on the opposite side of the bay.
A United States Geological Survey geologist, measuring the scouring marks, determined that the height of the wave had been more than 1,700 feet.