a boulder depends on various assumptions about the
dimensions and hydrodynamic properties of the boulder
(e. g., see Nott and Bryant, 2003; Noormets et al., 2004).
However, a very crude ‘rule of thumb’ is that
h w >
r b
r w
h b ;
(3)
i.e., the height of a wave necessary to displace a boulder
of given density and height is approximately proportional
to the product of the boulder’s relative density and its linear dimension. This is often an adequate estimate as generally knowledge of the boulder’s hydrodynamic
parameters and density r b (see Spiske et al., 2008) is
highly uncertain.
In practice, the interaction of waves and shorelines is
highly nonlinear and depends critically on details of the
nearshore bathymetry, and thus computer modeling is
required to obtain substantially more accurate results
(Mader, 2004). Of practical importance is the observation
that boulders entrained in a tsunami tend to saltate, tumbling end-over-end rather than sliding, and thus attention
to the dimensions and orientations of megablocks may
provide information about the direction of the flow that
emplaced them (Imamura et al., 2008).
Summary
Megablocks are erratic coral boulders, some with dimensions as great as 10 m or more, found in and landward of
active reef zones. In locations where they can only have
been wave-emplaced, they provide important information
about the frequency and intensity of rare, very large
storms and tsunamis. However, at present identifying the
specific characteristics that constrain the features of the
waves that emplaced them, or that confirm their origin to
be a storm or tsunami, is still a subject of active research.
Bibliography
Bourrouilh-Le Jan, F. G., and Talandier, J., 1985. Sedimentation et
fracturation de haute energie en miliue recifal: Tsunamis,
ouragans et cyclones et leurs effets sur la sedimentologie et la
geomorphologie d’un atoll: Motu et Hoa, a Rangiroa, Tuamotu,
Pacifique SE. Marine Geology, 67, 263–333.
Darwin, C., 1842. On the distribution of erratic boulders and on the
contemporaneous unstratified deposits of South America, Transactions Geological Society London, 2nd ser. 6, 415–431.
Dunnavan, G. M., and Diercks, J. W., 1980. An analysis of super
typhoon Tip (October 1979). Monthly Weather Review, 108,
1915–1923.
Fine, I. V., Rabinovich, A. B., Bornhold, B. D., Thomson, R. E., and
Kulikov, E. A., 2005. The Grand Banks landslide-generated tsunami of November 18, 1929: preliminary analysis and numerical
modeling. Marine Geology, 215, 45–57, doi:10.1016/
j.margeo.2004.11.007.
Frohlich, C., Hornbach, M. J., Taylor, F. W., Shen, C.-C., Moala, A.,
Morton, A. E., and Kruger, J., 2009. Huge erratic boulders
in Tonga deposited by a prehistoric tsynami. Geology, 37,
131–134, doi:10.1130/G25277A.1.
Goto, K., Chavanich, S. A., Imamura, F., Kuntasap, P., Matsui,
Minoura, K., Sugawara, D., and Yanagisawa, H. 2007. Distribution, origin and transport process of boulders deposited by the
2004 Indian Ocean tsunami at Pakarang Cape, Thailand.
Sedimentary Geology, 202, 821–837, doi:10.1016/j.sedgeo.
2007.09.004.
Hearty, J. P., 1997. Boulder deposits during the last interglaciation
on north Eleuthera Island, Bahamas. Quaternary Research, 48,
326–338.
Heinrich, P. H., Piatanesi, A., and Hebert, H., 2001. Numerical
modeling of tsunami generation and propagation from submarine slumps: The 1998 Papua New Guinea event. Geophysical
Journal International, 145, 97–111, doi:10.1111/j.1365–
246X.2001.00336.x.
Imamura, F., Goto, K., and Ohkubo, S., 2008. A numerical model
for the transport of a boulder by tsunami. Journal of Geophysical
Research, 113, C0008, doi:10.1029/2007JC004170.
Kaplan, M. R., Coronato, A., Rabassa, J. O., Kubik, P. W., and Freeman, S. P. H. T., 2007. Cosmogenic nuclide measurements in
southernmost South America and implications for landscape
change. Geomorphology, 87, 284–301, doi:10.1016/j.geomorph.
2006.10.005.
Kelletat, D., Scheffers, S. R., and Scheffers, A., 2007. Field signatures of the SE-Asian mega-tsunami along the west coast of
Thailand compared to Holocene paleo-tsunami from the Atlantic
region, Pure and Applied Geophysics, 164, 413–431,
doi:10.1007/s00024–006–0171–6.
Mader, C. L., 2004. Numerical Modeling of Water Waves (2nd edition). CRC Press.
Mader, C. L., and Gittings, M. L., 2002. Modeling the 1958
Lituya Bay mega-tsunami, II, Science of Tsunami Hazards, 20,
241–245.
Matsukura, Y., Maekado, A., Aoki, H., Kogure, T., and Kitano, Y.,
2007. Surface lowering rates of uplifted limestone terraces estimated from the height of pedestals on a subtropical island of
Japan. Earth
Surface
Processes
and Landforms,
32, 1110–1115, doi:10.1002/esp.1510.
Morton, R. A., Gelfenbaum, G., and Jaffe, B. E., 2006. Physical
criteria for distinguishing sandy tsunami and storm deposits
using modern examples, Sedimentary Geology, 200, 184–207,
doi:10.1016/j.sedgeo.2007.01.003.
Nakamura, M., 2006. Source fault model of the 1771 Yaeyama tsunami, southern Ryukyu Islands, Japan, inferred from numerical
simulation. Pure and Applied Geophysics, 163, 41–54,
doi:10.1007/s00024–005–0007–9.
Nishizumi, K., Kohl, C. P., Arnold, J. R., Dorn, R., Klein, I., Fink,
D., Middleton, R., and Lai, D., 1993. Role of in situ cosmogenic
nuclides
10 Be and
26 Al in the study of diverse geomorphic processes, Earth Surface Processes and Landforms, 18, 407–425,
doi:10.1002/esp.3290180504.
Noormets, R., Felton, E. A., and Crook, K. A. W., 2002. Sedimentology of rocky shoreslines: 2. Shoreline megaclasts on the north
shore of Oahu, Hawaii - origins and history: Sedimentary Geology, 150, 31–45, doi:10.1016/S0037–0738(01)00266–4.
Noormets, R., Crook, K. A. W., and Felton, E. A., 2004, Sedimentology of rocky shorelines: 3. Hydrodynamics of megaclast emplacement and transport on a shore platform: Oahu, Hawaii: Sedimentary
Geology, v. 172, p. 41–65, doi:10.1016/j.sedgeo.2004.07.006.
Nott, J., 2004. The tsunami hypothesis – Comparisons of the field
evidence against the effects, on the western Australian coast, of
some of the most powerful storms on Earth. Marine Geology,
208, 1–12, doi:10.1016/j.margeo.2004.04.023.
Nott, J., and Bryant, E., 2003. Extreme marine inundations (tsunamis?) of coastal western Australia. The Journal of Geology,
111, 691–706, doi:10.1086/378485.
Parks Camp, J., and Montgomery, M. T., 2001. Hurricane maximum
intensity, past and present. Monthly Weather Review, 129,
1704–1717.
Satake, K., and Tanioka, Y, 2003. The July 1998 Papua New Guinea
earthquake: Mechanism and quantification of unusual tsunami
682
MEGABLOCKS
dimensions and hydrodynamic properties of the boulder
(e. g., see Nott and Bryant, 2003; Noormets et al., 2004).
However, a very crude ‘rule of thumb’ is that
h w >
r b
r w
h b ;
(3)
i.e., the height of a wave necessary to displace a boulder
of given density and height is approximately proportional
to the product of the boulder’s relative density and its linear dimension. This is often an adequate estimate as generally knowledge of the boulder’s hydrodynamic
parameters and density r b (see Spiske et al., 2008) is
highly uncertain.
In practice, the interaction of waves and shorelines is
highly nonlinear and depends critically on details of the
nearshore bathymetry, and thus computer modeling is
required to obtain substantially more accurate results
(Mader, 2004). Of practical importance is the observation
that boulders entrained in a tsunami tend to saltate, tumbling end-over-end rather than sliding, and thus attention
to the dimensions and orientations of megablocks may
provide information about the direction of the flow that
emplaced them (Imamura et al., 2008).
Summary
Megablocks are erratic coral boulders, some with dimensions as great as 10 m or more, found in and landward of
active reef zones. In locations where they can only have
been wave-emplaced, they provide important information
about the frequency and intensity of rare, very large
storms and tsunamis. However, at present identifying the
specific characteristics that constrain the features of the
waves that emplaced them, or that confirm their origin to
be a storm or tsunami, is still a subject of active research.
Bibliography
Bourrouilh-Le Jan, F. G., and Talandier, J., 1985. Sedimentation et
fracturation de haute energie en miliue recifal: Tsunamis,
ouragans et cyclones et leurs effets sur la sedimentologie et la
geomorphologie d’un atoll: Motu et Hoa, a Rangiroa, Tuamotu,
Pacifique SE. Marine Geology, 67, 263–333.
Darwin, C., 1842. On the distribution of erratic boulders and on the
contemporaneous unstratified deposits of South America, Transactions Geological Society London, 2nd ser. 6, 415–431.
Dunnavan, G. M., and Diercks, J. W., 1980. An analysis of super
typhoon Tip (October 1979). Monthly Weather Review, 108,
1915–1923.
Fine, I. V., Rabinovich, A. B., Bornhold, B. D., Thomson, R. E., and
Kulikov, E. A., 2005. The Grand Banks landslide-generated tsunami of November 18, 1929: preliminary analysis and numerical
modeling. Marine Geology, 215, 45–57, doi:10.1016/
j.margeo.2004.11.007.
Frohlich, C., Hornbach, M. J., Taylor, F. W., Shen, C.-C., Moala, A.,
Morton, A. E., and Kruger, J., 2009. Huge erratic boulders
in Tonga deposited by a prehistoric tsynami. Geology, 37,
131–134, doi:10.1130/G25277A.1.
Goto, K., Chavanich, S. A., Imamura, F., Kuntasap, P., Matsui,
Minoura, K., Sugawara, D., and Yanagisawa, H. 2007. Distribution, origin and transport process of boulders deposited by the
2004 Indian Ocean tsunami at Pakarang Cape, Thailand.
Sedimentary Geology, 202, 821–837, doi:10.1016/j.sedgeo.
2007.09.004.
Hearty, J. P., 1997. Boulder deposits during the last interglaciation
on north Eleuthera Island, Bahamas. Quaternary Research, 48,
326–338.
Heinrich, P. H., Piatanesi, A., and Hebert, H., 2001. Numerical
modeling of tsunami generation and propagation from submarine slumps: The 1998 Papua New Guinea event. Geophysical
Journal International, 145, 97–111, doi:10.1111/j.1365–
246X.2001.00336.x.
Imamura, F., Goto, K., and Ohkubo, S., 2008. A numerical model
for the transport of a boulder by tsunami. Journal of Geophysical
Research, 113, C0008, doi:10.1029/2007JC004170.
Kaplan, M. R., Coronato, A., Rabassa, J. O., Kubik, P. W., and Freeman, S. P. H. T., 2007. Cosmogenic nuclide measurements in
southernmost South America and implications for landscape
change. Geomorphology, 87, 284–301, doi:10.1016/j.geomorph.
2006.10.005.
Kelletat, D., Scheffers, S. R., and Scheffers, A., 2007. Field signatures of the SE-Asian mega-tsunami along the west coast of
Thailand compared to Holocene paleo-tsunami from the Atlantic
region, Pure and Applied Geophysics, 164, 413–431,
doi:10.1007/s00024–006–0171–6.
Mader, C. L., 2004. Numerical Modeling of Water Waves (2nd edition). CRC Press.
Mader, C. L., and Gittings, M. L., 2002. Modeling the 1958
Lituya Bay mega-tsunami, II, Science of Tsunami Hazards, 20,
241–245.
Matsukura, Y., Maekado, A., Aoki, H., Kogure, T., and Kitano, Y.,
2007. Surface lowering rates of uplifted limestone terraces estimated from the height of pedestals on a subtropical island of
Japan. Earth
Surface
Processes
and Landforms,
32, 1110–1115, doi:10.1002/esp.1510.
Morton, R. A., Gelfenbaum, G., and Jaffe, B. E., 2006. Physical
criteria for distinguishing sandy tsunami and storm deposits
using modern examples, Sedimentary Geology, 200, 184–207,
doi:10.1016/j.sedgeo.2007.01.003.
Nakamura, M., 2006. Source fault model of the 1771 Yaeyama tsunami, southern Ryukyu Islands, Japan, inferred from numerical
simulation. Pure and Applied Geophysics, 163, 41–54,
doi:10.1007/s00024–005–0007–9.
Nishizumi, K., Kohl, C. P., Arnold, J. R., Dorn, R., Klein, I., Fink,
D., Middleton, R., and Lai, D., 1993. Role of in situ cosmogenic
nuclides
10 Be and
26 Al in the study of diverse geomorphic processes, Earth Surface Processes and Landforms, 18, 407–425,
doi:10.1002/esp.3290180504.
Noormets, R., Felton, E. A., and Crook, K. A. W., 2002. Sedimentology of rocky shoreslines: 2. Shoreline megaclasts on the north
shore of Oahu, Hawaii - origins and history: Sedimentary Geology, 150, 31–45, doi:10.1016/S0037–0738(01)00266–4.
Noormets, R., Crook, K. A. W., and Felton, E. A., 2004, Sedimentology of rocky shorelines: 3. Hydrodynamics of megaclast emplacement and transport on a shore platform: Oahu, Hawaii: Sedimentary
Geology, v. 172, p. 41–65, doi:10.1016/j.sedgeo.2004.07.006.
Nott, J., 2004. The tsunami hypothesis – Comparisons of the field
evidence against the effects, on the western Australian coast, of
some of the most powerful storms on Earth. Marine Geology,
208, 1–12, doi:10.1016/j.margeo.2004.04.023.
Nott, J., and Bryant, E., 2003. Extreme marine inundations (tsunamis?) of coastal western Australia. The Journal of Geology,
111, 691–706, doi:10.1086/378485.
Parks Camp, J., and Montgomery, M. T., 2001. Hurricane maximum
intensity, past and present. Monthly Weather Review, 129,
1704–1717.
Satake, K., and Tanioka, Y, 2003. The July 1998 Papua New Guinea
earthquake: Mechanism and quantification of unusual tsunami
682
MEGABLOCKS