At the GSA Annual Meeting this week, Matthew Hornbach from the University of Texas presented a rather interesting study that has just been undertaken on the island of Tongatapu in Tonga. Tongatapu is located in the South Pacific with a number of active volcanoes nearby (Fig 1).
Fig. 1: Google Earth image of the island of Tongatapu showing the location of three unnamed submarine volcanoes to the west.
Hornbach found that on Tongatapu there are seven coral boulders located up to 400 m from the current coastline (Fig. 2). The largest of these boulders has a mass of about 1200 m. The boulders have clearly been lifted from the coral that fringes the island - Science News reports that for the largest boulder the point in the reef from where it has been removed is clear.
Fig. 2: Science News image of one of the boulders on the island of Tongatapu.
Now, this island is flat, meaning that the boulders cannot have rolled there. Hence, Hornbach has concluded that these boulders were emplaced by a huge tsunami, which they have provisionally dated to have occurred in the Holocene (i.e. in the last 10,000 years). This is potentially the largest tsunami deposit so far recorded - the wave must have had a run up of >40 m vertical distance. Hornback believes that the source of this tsunami was a submarine volcano located to the west of the island (Fig. 1). They report that bathymetric surveys of this feature show that it rises to to the water surface - some reports suggest within 65 m. This large volcano apparently has a section missing from the east side, which Hornbach interprets as being the result of a flank collapse - i.e. a giant landslide in which the side of the volcano collapses. Flank collapses undoubtedly can cause very large, localised tsunamis.
As an aside, it is interesting to note that the major population centres are on the sheltered north and east side of the islands (Fig. 3) . I wonder if this is a coincidence?