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Huge erratic boulders in Tonga deposited by a prehistoric tsunami

Cliff Frohlich, Matthew J. Hornbach, Frederick W. Taylor, Chuan-Chou Shen, 'Apai Moala, Allan E. Morton and Jens Kruger
Huge erratic boulders in Tonga deposited by a prehistoric tsunami
Geology (Boulder) (February 2009) 37 (2): 131-134


Along some coastlines there are erratic boulders apparently emplaced by tsunamis or cyclonic storms; evaluating their origin and time of emplacement places constraints on the frequency, severity, and location of coastal hazards. Seven such large coral limestone boulders are present near Fahefa village on Tongatapu Island, southwest Pacific, apparently emplaced by a prehistoric tsunami. These boulders are 10-20 m above sea level and above any possible source, and all are 100-400 m from the present shoreline. Coral (super 230) Th ages indicate that the limestone formed during the last interglacial sea-level highstand, ca. 120-130 ka. The largest boulder is approximately 20 times more massive than any reported boulders emplaced by historically documented storms and may be the largest known tsunami or storm erratic worldwide situated above its source. We performed computer simulations to assess whether tsunamis produced by earthquakes, undersea landslides, or volcanoes could emplace the boulders. The simulations indicate that either volcanic flank collapse along the Tofua arc approximately 30-40 km to the southwest or undersea landslides on the submarine slopes of Tongatapu could be responsible. Either could explain why these boulders are not widespread on Tongatapu, and instead occur in a localized group along the western coast. This study demonstrates that small (<1 km (super 3) ) submarine slope failures sometimes generate locally large tsunamis. The Fahefa boulders are in a well-studied and well-populated area, yet were unknown to the scientific community until recently; this suggests that systematic searches elsewhere for erratic boulders and other tsunami deposits might provide new information for assessing the size and extent of prehistoric tsunamis.

ISSN: 0091-7613
EISSN: 1943-2682
Serial Title: Geology (Boulder)
Serial Volume: 37
Serial Issue: 2
Title: Huge erratic boulders in Tonga deposited by a prehistoric tsunami
Affiliation: University of Texas at Austin, Institute for Geophysics, Austin, TX, United States
Pages: 131-134
Published: 200902
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 26
Accession Number: 2009-031181
Categories: Quaternary geology
Document Type: Serial
Bibliographic Level: Analytic
Annotation: With GSA Data Repository Item 2009036
Illustration Description: illus. incl. sketch map
S21°15'00" - S21°10'00", W175°19'60" - W175°19'60"
Secondary Affiliation: National Taiwan University, TWN, TaiwanMinistry of Lands, Survey, and Natural Resources, TON, TongaSouth Pacific Islands Applied Geoscience Commission, FJI, Fiji
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 200917
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