Discovered: world's largest tsunami debris

September 25, 2008

Boulder, CO, USA - A line of massive boulders on the western shore of Tonga may be evidence of the most powerful volcano-triggered tsunami found to date. Up to 9 meters (30 feet) high and weighing up to 1.6 million kilograms (3.5 million pounds), the seven coral boulders are located 100 to 400 meters (300 to 1,300 feet) from the coast. The house-sized boulders were likely flung ashore by a wave rivaling the 1883 Krakatau tsunami, which is estimated to have towered 35 meters (115 feet) high.

"These could be the largest boulders displaced by a tsunami, worldwide," says Matthew Hornbach of the University of Texas Institute for Geophysics. "Krakatau's tsunami was probably not a one-off event." Hornbach and his colleagues will discuss these findings on Sunday, 5 October 2008, at the Joint Annual Meeting of the Geological Society of America (GSA), Soil Science Society of America (SSSA), American Society of Agronomy (ASA), Crop Science Society of America (CSSA), and the Gulf Coast Association of Geological Societies (GCAGS), in Houston, Texas, USA.

Called erratic boulders, these giant coral rocks did not form at their present location on Tongatapu, Tonga's main island. Because the island is flat, the boulders could not have rolled downhill from elsewhere. The boulders are made of the same reef material found just offshore, which is quite distinct from the island's volcanic soil. In fact, satellite photos show a clear break in the reef opposite one of the biggest boulders. And some of the boulders' coral animals are oriented upside down or sideways instead of toward the sun, as they are on the reef.

Hornbach says the Tongatapu boulders may have reached dry land within the past few thousand years. Though their corals formed roughly 122,000 years ago, they are capped by a sparse layer of soil. And the thick volcanic soils that cover most of western Tongatapu are quite thin near the boulders. This suggests the area was scoured clean by waves in the recent past. Finally, there is no limestone pedestal at the base of the boulders, which should have formed as rain dissolved the coral if the boulders were much older.

Many tsunamis, like the one that struck the Indian Ocean in 2004, are caused by earthquakes. But the boulders' location makes an underwater eruption or submarine slide a more likely culprit. A chain of sunken volcanoes lies just 30 kilometers (20 miles) west of Tongatapu. An explosion or the collapse of the side of a volcano such as that seen at the famous Krakatau eruption in 1883 could trigger a tremendous tsunami.

Another possibility is that a storm surge could have brought the boulders ashore. But that scenario isn't likely. No storms on record have moved rocks this big. Another possibility is that a monster undersea landslide caused the tsunami. But Hornbach's analyses of adjacent seafloor topography point to a volcanic flank collapse as the most probable source of such a wave.

"We think studying erratic boulders is one way of getting better statistics on mega-tsunamis," Hornbach says. "There are a lot of places that have similar underwater volcanoes and people haven't paid much attention to the threat." The researchers have already received reports of more erratic boulders from islands around the Pacific. Future study could indicate how frequently these monster waves occur and which areas are at risk for future tsunamis.

The boulders are such an unusual part of the Tongan landscape that tales of their origins appear in local folklore. According to one legend, the god Maui hurled the boulders ashore in an attempt to kill a giant man-eating fowl.

And though many other Pacific islanders follow the custom of heading uphill after earthquakes, Tongans have no such teachings. Such lore may be useless for near-shore volcanically-generated tsunamis, which arrive too quickly for people to evacuate. Instead, most of Tongatapu's settlements are huddled together on the northern side of the island--away from the brunt of the tsunami threat.
-end-
**WHEN & WHERE**

Sunday, 5 October 2008, 8:00 AM-4:45 PM (authors scheduled from 3:00-4:45 PM)

View abstract, paper 149-8: "Unraveling the Source of Large Erratic Boulders on Tonga: Implications for Geohazards and Mega-Tsunamis" at http://gsa.confex.com/gsa/2008AM/finalprogram/abstract_149849.htm
George R. Brown Convention Center: Exhibit Hall E (poster, booth 202)

**CONTACT INFORMATION**

For on-site assistance during the 2008 Joint Annual Meeting, 5-9 October, contact Christa Stratton or Sara Uttech in the Newsroom, George R. Brown Convention Center, Room 350B, +1-713-853-8329.

After the meeting, contact:

Matthew Hornbach, Jackson School of Geoscience
The University of Texas Institute for Geophysics, Austin, Texas
Phone: +1-512-636-5030 (cell)
Email: matth@ig.utexas.edu

**IMAGES AVAILABLE**

For more information on the 2008 Joint Meeting visit www.acsmeetings.org.

www.geosociety.org

Geological Society of America

Related Tsunami Articles from Brightsurf:

Landslide along Alaskan fjord could trigger tsunami
Scientists noted that the slope on Barry Arm fjord on Prince William Sound in southeastern Alaska slid some 120 meters from 2010 to 2017, a slow-moving landslide caused by glacial melt that could trigger a devastating tsunami.

Scientists improve model of landslide-induced tsunami
MIPT researchers Leopold Lobkovsky and Raissa Mazova, and their young colleagues from Nizhny Novgorod State Technical University have created a model of landslide-induced tsunamis that accounts for the initial location of the landslide body.

Rethinking tsunami defense
Careful engineering of low, plant-covered hills along shorelines can mitigate tsunami risks with less disruption of coastal life and lower costs compared to seawalls.

'Tsunami' on a silicon chip: a world first for light waves
A collaboration between the University of Sydney Nano Institute and Singapore University of Technology and Design has for the first time manipulated a light wave, or photonic information, on a silicon chip that retains its overall 'shape'.

Tsunami signals to measure glacier calving in Greenland
Scientists have employed a new method utilizing tsunami signals to calculate the calving magnitude of an ocean-terminating glacier in northwestern Greenland, uncovering correlations between calving flux and environmental factors such as air temperature, ice speed, and ocean tides.

Salish seafloor mapping identifies earthquake and tsunami risks
The central Salish Sea of the Pacific Northwest is bounded by two active fault zones that could trigger rockfalls and slumps of sediment that might lead to tsunamis, according to a presentation at the 2019 SSA Annual Meeting.

Heading towards a tsunami of light
Researchers at Chalmers University of Technology and the University of Gothenburg, Sweden, have proposed a way to create a completely new source of radiation.

Paradigm shift needed for designing tsunami-resistant bridges
Researchers argue in a new study that a paradigm shift is needed for assessing bridges' tsunami risk.

How large can a tsunami be in the Caribbean?
The 2004 Indian Ocean tsunami has researchers reevaluating whether a magnitude 9.0 megathrust earthquake and resulting tsunami might also be a likely risk for the Caribbean region, seismologists reported at the SSA 2018 Annual Meeting.

Preparing for the 'silver tsunami'
Case Western Reserve University law professor suggests how to address nation's looming health-care and economic crisis caused by surging baby-boom population.

Read More: Tsunami News and Tsunami Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.