Giant Avalanches and Mega-tsunami (not)

N coast of E. Moloka‘i

Dana (1890) “Thus such precipices are rather the rule in the Hawaiian group; and if seashoreerosion is not the origin, - as many facts from the islands of the Pacific appear to show, -fractures and subsidence must be.

WHAT CAUSED THE HUGE PALI? FAULTING? MARINE EROSION?VALLEY EROSION? SOME COMBINATION OF THESE?

Formation of a cliff (that might be, or has been, mistaken for a fault or avalanche scarp) by coalescence of amphitheater-headed valleys

(Modified from Stearns & Vaksvik 1935)

“Topographic evidence indicates thepresence of two large submarinelandslides on the slope of the Hawaiianridge northeast of Oahu. One slide ismore than 150 km long and moved on aslope with an overall gradient of about2 degrees.” (Moore 1964)

“However, recent detailed work on the topo-graphy of the region…does not support thishypothesis. The undersea hills are very largeas compared with the bumps on known land-slides, and some of them have forms charac-teristic of volcanic cones. The Tuscaloosa(also called Tuscarora) Seamount, for example,has a height of about 6,000 feet above thesurrounding sea floor, and its general form…suggests that it is probably a guyot…” (Macdonald and Abbot 1970)

(Moore 1964)

(Moore et al. 1989)

GLORIA side-scansonar data (smoothsurfaces show up as dark,rough surfaces show upas bright)

(Moore et al. 1989)

17 large landslides in principal Hawai‘i region

Represents ~6 million yrs (= 1 every 350,000 yrs)

Two Kinds: slumps (prolonged, progressive formation) and catastrophic debris avalanches

Off-shore failure accelerates on-shore erosion

Debris avalanches are likely to produce huge tsunami locally, butWould they be Pacific-wide?

Wailau Slide

Wai‘anae Slump

Ka‘ena Slump

Nu‘uanuDebrisAvalanche

(slide by John Sinton)

after Moore (1964)

obvious cliffs

actual avalanchescarps

Profiles through E. O‘ahu showing landslide blocks (~3x vertical exaggeration)

E. O‘ahu - restored

Diagrams by Moore and Clague [2002], slide by John Sinton

Wai‘anae

Landslide headwall

potential giant avalancheslide-plane?

A deposit of coral and basalt boulders resting ~conformablyon basalt flows, ~100 m above sea level, S. coast of Lana‘i-

Moore and Moore (1988): the Hulopoe Gravel was deposited by giant waves (megatsunami) generated by large submarine landslides.

Moore and Moore (1988) also speculated that other high-level deposits elsewhere in Hawai‘i might also be explained with this mechanism

(slide by John Sinton)

Rubin et al. (2000) dated carbonates from the deposit:

-Age correlates with stratigraphic height

-None of the clasts indicate a 105 ka “event”

(slide by John Sinton)

The deposit is too old to have been produced from the Alika Slide

The presence of internal stratigraphy indicates it is not the product of a singe event (or wave)

These results favor deposition from rising and falling sea level over a period of more than 100,000 years (scenario B).

Rubin et al. [2000]

(slide by John Sinton)

Felton et al. [2000] recognized up to 14 different beds in the unit, and 8 different disconformities in the 9-m thick type section.

Three of the disconformities are associated with truncated paleosols

The deposition of the Hulopoe Gravel was not continuous.

At least one bed is clearly alluvial

(slide by John Sinton)

The boulders are sortedby age: Different sea levels?Multiple tsunami?Enough time between eventsTo develop soil?

~1 m

Photo by Peter Mouginis-Mark, HIGP/SOEST

Ward SN (2001) Landslide Tsunami. J Geophys Res 106: 11201-11215

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Ward SN, Day S (2001) Cumbre Vieja Volcano – Potential collapse and tsunami at La Palma, Canary Islands. Geophys Res Lett 28:3397-3400

http://geology.com/news/images/canary-islands-la-palma.jpg

Ward & Day (2001)

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http://geology.com/news/2005/09/atlantic-ocean-tsunami-threat.html

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http://www.drgeorgepc.com/TsunamiMegaEvaluation.html

A web version of this is at

http://www.tsunamisociety.org/PressReleases.html

(Lamb et al. 2007)

Giant avalanches as a mechanism to give a head start to large amphitheater-headed valleys: KohalaThe giant avalanche speeds up erosion and thereby enhances stream piracy.

(Lamb et al. 2007)

(Lamb et al. 2007)

CREATION OF A SUBMERGED TERRACE REPRESENTING THE END OF THE THOLEIITE SHIELD

Clague & Moore (2002)

Coastline when tholeiiteshield stopped erupting(~1.5 million years ago)

Coastal terrace:wave-cut platform +reef, from pre-Kalaupapa low sea-levelTime (~420,000 or~350,000 years ago)

Avalanche scar (shortly beforethe end of tholeiite shield-building)

Kalaupapa (rejuvenation)~350,000 years old