Volcanic tsunamis can be produced by pyroclastic flows (fast-moving curents of hot gas and rock) and/or the collapse of a volcano during a volcanic eruption. Volcano collapse can also occur without an accompanying eruption.
The 1883 eruption of Krakatau volcano in Indonesia unleashed a series of devastating tsunamis that resulted in the loss of tens of thousands of lives.
Tsunamis can be generated in volcanic settings both during an eruption and in periods of inactivity. Hot fast moving bodies of gas and rock (known as pyroclastic flows) can travel over the ocean, pushing the water outwards and creating a tsunami. Volcanic slopes are unstable and prone to landslide activity. Volcanic landslides both above and below the ocean's surface can displace the water column and generate a tsunami.
The 1883 Krakatau eruption in Indonesia caused tens of thousands of deaths, including 77 about 800 kilometres away from the eruption. The effect of the tsunami was reported up to 10 kilometres inland and one large ship was raised 10m above sea level and carried 3 kilometres inland.
As the tsunami moves across the open ocean it is almost undetectable on the ocean surface.
In this example, the tsunami waves are only about half a metre high but have a wavelength of 200 kilometres. Travelling at speeds of up to eight or nine hundred kilometres an hour (the speed of a commercial passenger jet), it will take each wave about 15 minutes to pass a slow moving ship.
As the tsunami leaves the deep water of the open ocean and approaches the shallower waters near the coast, it slows down and may grow in height depending on the shape of the seafloor.
A tsunami that is unnoticeable by ships at sea may grow to be several metres or more in height near the coast.
Our example tsunami is now 1.5 metres high with a wavelength of 100 kilometres and is moving at about 400 kilometres an hour.
Depending on whether the first part of a tsunami to reach the shore is a crest or a trough, it may appear as a rapidly rising or falling tide, and in some cases the tsunami may appear as a series of breaking waves. People near the beach may also hear a roaring sound, like an approaching train.
In this example, the first crest of our tsunami arrives without warning and inundates the beach and low lying land causing extensive damage.
After the first wave, the water will draw back and then the second and third waves will repeat the process at 15 to 20 minute intervals. The first wave may not be the biggest. Reefs and offshore islands may help to protect the coast from the devastating effect of a tsunami.
Volcanic eruption generated tsunami animation