Tonga boulders may be evidence of volcanic tsunami

Seven huge coral boulders standing on the western shore of Tonga’s main island Tongatapu may be evidence of a tsunami produced by a volcanic eruption, according to University of Texas researchers. The boulders, which are up to 9 metres high and weigh up to 1600 tonnes, are made of coral similar to that which makes up the offshore reefs of Tongatapu; their coral was formed about 122,000 years ago. They were clearly not formed in their present location and could not, on largely flat Tongatapu, have rolled downhill from somewhere else. The conclusion is that they were torn from the reef and hurled inland by a tsunami generated by a submarine eruption, probably from one of the volcanic centres in the northern section of the nearby Tonga-Kermadec Arc.

‘These could be the largest boulders displaced by a tsunami, worldwide’, says Matthew Hornbach of the University of Texas Institute for Geophysics. ‘We think studying erratic boulders is one way of getting better statistics on mega-tsunamis. There are a lot of places that have similar underwater volcanoes and people haven’t paid much attention to the threat’.

UPDATE: Dr Erik Klemetti at Eruptions takes a cautious look at the Tonga boulders: ‘it will be interesting to see what evidence they have for the deposit being from a volcanically-triggered event’.

UPDATE: National Geographic News has published an article on the Tonga boulders, which has been added to the list of links below.

[Found via Geology News.]

News
World’s largest tsunami debris discovered – ScienceDaily, 25 September 2008
Boulders on Tonga may have been dumped by tsunami – Reuters, 25 September 2008
Tongan coral may hold key to ancient tsunami – Radio Australia, 25 September 2008
Boulders in Tonga evidence of largest tsunami debris found – Geology News, 24 September 2008
The big boulders of Tonga – Eruptions, 25 September 2008
Ancient tsunami carried giant boulders to Tonga – National Geographic News, 30 September 2008

Information
Global Volcanism Program: Tonga Islands – volcanoes of Tonga and vicinity
New Zealand American Submarine Ring of Fire 2005 – studying submarine volcanism in the Tonga-Kermadec Arc
Bathymetry of the Tonga Trench and Forearc: a map series – article, with maps, from Marine Geophysical Researches, vol. 21, no. 5 (2000)