Tsunamis generated by volcanic eruptions
As noted above, many landslide-generated tsunami are produced at coastal and island volcanoes. Although these landslides may be triggered by even minor eruptions, in a variety of ways, they mainly involve pre-existing volcanic rocks. Other tsunami are generated directly by explosive volcanic eruptions. It is useful to make a distinction between these and landslide-generated tsunamis because the eruptions are likely to cause significant insurance losses by other mechanisms, complicating the task of evaluating and assigning the causes of these losses.
|Paricutin Volcano, Mexico erupting in 1944|
Historical examples of tsunamis generated by volcanic eruptions
Tsunamis definitely generated by volcano lateral collapses are dealt with separately, above: the main instance where there is uncertainty about the exact process which generated tsunami waves during an eruption is that of Krakatau.
|YEAR(AD UNLESS STATED)||TSUNAMI IMPACT AREA||SOURCE VOLCANO||SOURCE TYPE||MAXIMUM RUNUP||DEATHS|
|c. 1628 BC (?)||Crete, Aegean||Thira (Santorini)||Caldera collapse||? several tens of metres||? Unknown but probably very large|
|1815||Flores, Indonesia||Tambora||Pyroclastic flows impacting sea?||? few tens of metres||? 12 000|
|1883||Cook Inlet, Alaska||Augustine||Pyroclastic flows and debris avalanches impacting sea||6 m|| - |
|1883||Java, Sumatra||Krakatau||Multiple||35 m||~33 000|
|1902||Martinique||Mt. Pelee||Mudflows||8 m?||~100?|
|1952||----||Myojin-sho, south of Japan||Submarine explosion|| ? ||~30 (crew of sunken research vessel)|
Submarine explosive eruptions
These occur at volcanoes with their summits at depths, depending upon the concentration and composition of gases dissolved in the magmas within them, of up to several hundreds of metres below sea level. Explosive fragmentation of erupting magmas is commonly boosted by rapid mixing of the hot material with the surrounding water, which explodes into superheated steam. As the expanding mass of gases and lava or rock fragments rises to the surface it displaces water and creates waves; further waves may be created if the erupted material bursts above sea level and then falls back. A notable example of a loss associated with a submarine explosive eruption is the sinking of the research ship Kaiyo Maru during the 1952 eruption of Myojin-Sho volcano off Japan; this volcano continues to erupt at intervals and has a permanent maritime exclusion zone placed around it. Another submarine volcano that has attracted considerable attention both in this context and in respect of a possible submarine landslide, is Kick 'em Jenny off Grenada in the Lesser Antilles.
Where eruptions occur at sea level they are also commonly highly explosive, as water can gain access to the vent through permeable rocks around it. Explosions at surface level, on land or at sea, produce base surges: high-speed volcanic ash clouds which flow radially outwards over the surface. Where these surge clouds flow over water they generate tsunami wave trains that continue to move outwards even after the surges themselves have decelerated and have the potential to cause damage over a much wider area. Volcanoes in crater lakes, such as Taal in the Philippines, have produced particularly damaging tsunami of this type.
Submarine pyroclastic flows
Many eruptions, both subaerial and submarine, produce dense flows composed of mixtures of hot gas, pumice and rock and lava fragments. The term "glowing avalanche", although it strictly applies to only one type of these flows, is a useful description of all of them. While some have densities less than that of water, as with base surges, others have an overall density greater than water and flow over the sea bed, displacing the overlying water column upwards and sideways to produce tsunami waves. Mixing at the boundary of the flow and the water also produces steam explosions and further wave generation. As the flows cool they typically evolve into sediment flows similar to those produced by collapses of delta fronts or continental slope sediments and may continue to produce weaker tsunami waves until they stop altogether.
Lahars are a type of mudflow, but generated by mixing of freshly-erupted volcanic material with water (often originally present on the volcanoes as ice or in water - saturated soil). These mixtures of hot volcanic fragments, heated water, and other material ranging from blocks of un-melted ice to trees, are extremely mobile and dangerous on land and also have a significant potential for tsunami generation when they enter water. A mudflow from Mt. Pelee volcano (Martinique, West Indies) on 6 May 1902 generated a substantial tsunami which wrecked the waterfront of the city of St. Pierre (subsequently destroyed by a more violent eruption of the volcano, two days later). A notable prehistoric lahar, which erupted from Mount Rainier (Washington State, USA) some 5000 years ago, forms the land on which the city of Tacoma is now built. Repetition of such an event would, besides the obvious effect on Tacoma itself, also set up tsunami within Puget Sound and present a significant hazard to the port facilities and waterfront of Seattle.
Large eruptions commonly involve subsidence of the volcanic cone into the emptying magma reservoir beneath, producing giant craters known as central subsidence calderas. If the rims of these craters are at or below sea level, flow of water into the crater can produce tsunami directly if the subsidence is rapid enough; or, more probably, lead to magma - water mixing and a variety of explosive phenomena as detailed above. All of these can produce tsunami.
Large explosive eruptions have produced some of the most famous and catastrophic tsunami, of which perhaps the most widely known are those associated with the Krakatau eruption of 1883 and the eruption of Santorini in Greece in about 1470 BC. Both of these eruptions involved the formation of water - filled central subsidence calderas and also generated series of extremely destructive tsunami, but considerable uncertainty exists about the precise mechanism of generation of the tsunami. Rapid flooding of the calderas, base surges and large-volume submarine pyroclastic flows have all been implicated; and in the case of the fourth tsunami generated at Krakatau, right at the end of the main explosive phase of the eruption, the cause may well have been the collapse of a pre - existing volcanic cone into the newly formed caldera. Although such a major tsunamigenic eruption in the near future is likely to be much better documented than either of these eruptions significant uncertainty is likely to exist in the precise mechanism of wave generation.
© 2000 Natural Environment Research Council, Coventry University and University College London