Evidence for a mega-tsunami from flank collapse of Mauna Loa
the late 1980s and early 1990s the U.S. Geological Survey carried out extensive
sonar surveys in the waters surrounding the Hawaiian islands. A surprise outcome of this project was the discovery of vast
hummocky debris deposits extending for many tens of kilometres from shore.
These deposits are interpreted to be the products of slow-moving slumps
and fast-moving debris avalanches derived from the periodic collapse of parts of
the Hawaii volcanoes. The largest
avalanches have volumes in the order of 5000 km3, making them the
largest known slope failures on earth (Decker and Decker, 1998).
widely accepted that avalanches of this type could have generated very large
local tsunamis. The possible scale
of those tsunamis is graphically illustrated by the recent identification and
interpretation of ancient tsunami deposits on the Kohala volcano (McMurtry et
sedimentary beds in question, some of which are situated just above the existing
shoreline, were initially interpreted to be beach deposits from a former marine
highstand (Stearns and Macdonald, 1946). Subsequent
work has shown that Hawaii has been consistently subsiding at a rate of about
2.6 mm/y for at least the past 475 ka (475,000 y).
Subsidence at this rate is not consistent with marine highstand deposits.
McMurtry et al. have described
geological evidence that the beds were formed through tsunami-type depositional
processes. They have also dated the deposits.
They have also dated the deposits.
deposits occur sporadically within a 7-km long section of the Kohala
coast, at elevations ranging up to 60 m above sea level.
They comprise angular to subrounded basalt boulders and cobbles supported
in a matrix of weathered cemented carbonate consisting of broken marine shells
and rare coral clasts. Reverse
grading is present in some instances. These deposits lack the organic framework structure typical
of reefs, and the seaward dipping fabric of beach deposits.
They overly a paleosoil, indicating deposition on land, but the abundance
of marine materials is inconsistent with fluvial or debris flow deposition.
carbonate-bearing deposits have been dated using U-series methods. the
results give ages of between 102
and 110 ka (+ 10 ka), although these
values are interpreted to represent a minimum age, since U-series ages can be
decreased by weathering.
on an age of 110 ka, and the observed subsidence rate of 2.6 mm/y, the Kohala
tsunami deposits were deposited at a time when relative sea level on this part
of Hawaii was approximately 290 m below current sea level. This means that
some of the observed deposits were at least 350 m above sea level at the time of their
deposition. The mega-tsunami event
that produced these deposits is assumed by McMurtry et al. to be the massive Alika 2 debris
avalanche (see picture below), which has been dated at 112 + 15 ka.
If an event of this type were to occur today the consequences for the present-day Hawaiian islands would be devastating. It appears unlikely, however, that distant parts of the Pacific rim would be severely impacted. As described by Pararas-Carayannis (2002), a tsunami generated by a landslide, even a massive slide like Alika 2, would have a significantly shorter wavelength and period than a tsunami generated by a large subduction-zone earthquake. Such short-period waves attenuate much more significantly over great distances than long-period earthquake-generated waves, and it is unlikely that catastrophic wave run-ups would affect mainland areas on either side of the Pacific.
R and Decker, B, 1998, Volcanoes, Freeman and Co., New
York, 321 p.
McMurtry, G, Fryer, G, Tappin D,
Wilkinson, I, Williams, M, Fietzke, J, Garbe-Shoenberg, D and Watts, P, 2004, Megatsunami
deposits on Kohala volcano, Hawaii, from flank collapse of Mauna Loa, Geology,
V. 32, p. 741-744. (Sept. 2004)
Pararas-Carayannis, G, 2002, Evaluation
of the threat of mega tsunamis from postulated slope failures of island
stratovolcanoes on La Palma, Canary Islands, and on the Island of Hawaii,
Science of Tsunami Hazards, Vol 20,
Stearns, H and Macdonald, G, 1946. Geology and Ground-Water Resources of the Island of Hawaii, Hawaii Division of Hydrography. Bull. 9, 363 p.
Steven Earle, 2004. Malaspina University-College, Geology Department, Return to Earth Science News