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COSMIC IMPACTS AND TSUNAMI: THE UNDERRATED HAZARD

The following text is an extract from Edward A. Bryant's new book TSUNAMI:
THE UNDERRATED HAZARD, to be published by Cambridge University Press
(publication c. July 2001).

0 521 77244 3 Hardback £55.00/$74.95
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Description

In the past decade over ten major tsunami events have impacted on the
world's coastlines, causing devastation and loss of life. Evidence for past
great tsunami, or 'mega-tsunami', has also recently been discovered along
apparently aseismic and protected coastlines. With a large proportion of the
world's population living on the coastline, the threat from tsunami can not
be ignored. This book comprehensively describes the nature and process of
tsunami, outlines field evidence for detecting the presence of past events,
and describes particular events linked to earthquakes, volcanoes, submarine
landslides and meteorite impacts. While technical aspects are covered, much
of the text can be read by anyone with a high school education. The book
will appeal to students and researchers in geomorphology, earth and
environmental science, and emergency planning, and will also be attractive
for the general public interested in natural hazards and new developments in
science.

Chapter Contents

Preface; Acknowledgments; Part I. Tsunami as Known Hazards: 1. Introduction;
2. Tsunami dynamics; Part II. Tsunami-Formed Landscapes: 3. Signatures of
tsunami in the coastal landscape; 4. Coastal landscape evolution; Part III.
Causes of Tsunami: 5. Earthquake-generated tsunami; 6. Great landslides; 7.
Volcanic eruptions; 8. Comets and meteorites; Part IV. Modern Risk of
Tsunami: 9. Risk; 10. Epilogue; References; Index.

-------------

MORE RECENT EVIDENCE FROM LEGENDS AND MYTHS

By Edward A. Byrant

Deluge Comet Impact Event 8,200 ± 200 years ago (Kristan-Tollmann and
Tollmann, 1992)

If cosmogenically generated tsunami are so rare, certainly within the
timespan of human civilisation, then a paradox exists because evidence for
such events certainly appears often in the geological record and in human
legends. Traditionally, the difficulty in discriminating between fact and
fiction, between echoes of the real past and dreams, has discouraged
historians and scientists from making inferences about catastrophic events
from myths or deciphered records. Yet, common threads appear in many ancient
tales. Stories told by the Washo Indians of California and by the Aborigines
of South Australia portray falling stars, fire from the sky, and cataclysmic
floods unlike any modern event. Similar portrayals appear in the Gilgamesh
myth from the Middle East, in Peruvian legends, and in the Revelations of
Saint John and the Noachian flood story in the Bible. Victor Clube of Oxford
University and William Napier of the Royal Observatory of Edinburgh have
pieced together consistent patterns in ancient writings, which they
interpret as representing meteoritic showers 3,000-6,000 years ago. One of
the more disturbing accounts has been compiled from these legends by Edith
and Alexander Tollmann of the University of Vienna, who believe that a comet
circling the sun fragmented into seven large bodies that crashed into the
world's oceans 8,200 ± 200 years ago. This age is based on radiocarbon dates
from Vietnam, Australia and Europe. The impacts generated an atmospheric
fireball that globally affected society. This was followed by a nuclear
winter characterised by global cooling. More significantly, enormous tsunami
swept across coastal plains and, if the legends are to be believed,
overwashed the centre of continents. The latter phenomenon, if true, most
likely was associated with the splash from the impacts rather than with
conventional tsunami run-up. Massive floods then occurred across continents.
The event may well have an element of truth. Figure 8.9 plots the location
of the seven impact sites derived from geological evidence and legends. Two
of these sites, in the Tasman and North Seas, have been identified as having
mega-tsunami events around this time. The North Sea impact centre
corresponds with the location of the Storegga slides described in Chapter 6.
Here, the main tsunami took place 7,950 ±190 years ago. One of the better
dates comes from wood lying above tektites in a sand dune along the South
Coast of Victoria, Australia. The tektites are associated with the Tasman
Sea impact and date at 8,200 ±250 years before present. These dates place
the Deluge Comet impact event--a term used by the Tollmanns--around 6200 BC.
This event does not stand alone during the Holocene. It has been repeated in
recent times--a fact supported by Maori and Aborigine legends from New
Zealand and Australia.

MYSTIC FIRES OF TAMAATEA
(Mooley et al., 1963; Steel and Snow, 1992; McGlone and Wilmshurst, 1999)

One of the more intriguing legends associated with the Taurids is the New
Zealand Maori legend known as the Mystic Fires of Tamaatea. The legend
originates in the North Island; but, ethnographic evidence is best
chronicled in the Southland and Otago regions of the South Island, centred
on the town of Tapanui (Figure 8.10). Here there appears to be evidence for
an airburst that flatten trees similar to the Tunguska event. The remains of
fallen trees are aligned radially away from the point of explosion out to a
distance of 40-80 kilometres. Maori legends in the area tell about the
falling of the skies, raging winds and mysterious and massive firestorms
from space. The Sun was screened out causing death and decay. Maori names in
the region refer to a Tunguska-like explosion. Tapanui, itself, translates
as "the big explosion", while Waipahi means "the place of the exploding
fire". Place names such as Waitepeka, Kaka Point, and Oweka contain the
southern Maori word "ka" which means fire. Some place names put the timing
of the fires in the Southern Hemisphere winter around June at the timing of
the Taurids. A deluge then followed the widespread fires. One legend states
that the Aparima Plains west of Invercargill were flooded. Dimpling on the
plain suggests that trees were toppled landward by water from the sea, and
Maori place names such as Tainui, Tairoa and Paretai, inland from the ocean,
suggest a tsunami was involved because the affix "Tai" translates as "wave".
The Maori also attribute the demise of the Moas, as well as their culture,
to an extraterrestrial event. The extinction of the Mao is remembered as
Manu Whakatau, "the bird felled by strange fire". One Maori song refers to
the destruction of the Moa when the horns of the Moon fell down from above.
On the North Island, the disappearance of the Moa is linked to the coming of
the man/god Tamaatea who set fire to the land by dropping embers from the
sky. Remains of Moa on the South Island can be found clustered in swamps as
if these flightless birds fled en masse to avoid some catastrophe. Southern
Maori legends tell of stones falling from the sky that caused massive
firestorms that not only annihilated the Moa, but also Maori culture.

The age when these fires occurred can be determined by radiocarbon dating
wood debris from the fires. The dating evidence comes from two sources:
buried wood and carbon derived from unconformal layers in swamps and bogs
that have been interrupted as fire-induced. These dates traditionally have
been interpreted as reflecting the time of deforestation due to Maori
occupation in New Zealand. However, many of the dates come from
uninhabitable high country that was burnt on a vast scale. The distribution
of dates is plotted in Figure 8.3 and spans at least two centuries, with the
ages peaking at the beginning of the Fifteenth Century. This wide range in
dates is logical knowing that mature trees, already hundreds of years old,
burnt. The crucial point is that few ages occur after the Fifteenth Century.
The Fires of Tamaatea legend may well have a cosmogenic origin. The peak in
dates is synchronous with the highest number of meteor sightings by Chinese
and Japanese astronomers for the past two thousand years (Figure 8.3). More
importantly, the timing of the fires is also coherent with the occurrence of
mega-tsunami along the nearby coastline of Southeast Australia.

EVIDENCE FROM AUSTRALIA
(Oliver, 1988; Bryant et al., 1996)

The evidence from Australia for cosmogenic mega-tsunami is based upon the
magnitude of geomorphic features and their contemporaneously occurrence over
a wide region that includes the Tasman Sea and the East Coast of New
Zealand. This chronology coincides with the timing of legends
and the influx of comets and meteorites over the last millennium. As pointed
out in Chapter 1, Australia historically has not been affected significantly
by large tsunami. The closest sources for earthquake-generated tsunami lie
along the Tonga-New Hebrides Trenches, and the Indonesian Archipelago. An
earthquake with a surface magnitude greater than 8.3 on the Richter scale
can be generated in the Southwest Pacific every 125 years. The highest
tsunami recorded at Sydney since 1870 occurred on 10 May 1877, and had a
height of 1.07 metres. The Chilean earthquake of 1868 produced a tsunami
height of 1.0 m, while the Chilean earthquake of 22 May 1960 generated a
tsunami height of 0.85 metres. On the West Coast, the biggest tsunami run-up
measured six metres at Cape Leveque, Western Australia on 19 August 1977
following an Indonesian earthquake.

Palaeo-tsunami generated by conventional mechanisms, and larger than these
historical events are possible. The proximity of the northwest coastline to
the volcanically, and seismically active Indonesian Archipelago makes large
tsunami with run-ups of ten metres a distinct possibility.
Additionally, the East Coast lies exposed to tsunami generated by
earthquakes on seamounts in the Tasman Sea, and along the Alpine Fault
running down the West Coast of the South Island of New Zealand. This latter
fault last ruptured in the Fifteenth Century before European colonisation of
the region. Nor can volcanic activity be ruled out along the East Coast.
Active volcanoes lie in the Tonga-Kermadec Trench region north of New
Zealand. In AD 1453, a volcanic eruption in Tonga created a crater, 18 km
long, 6.5 km wide and 0.8 km deep. The volcano erupted with a force
equivalent to twenty thousand Megatons of TNT and produced a tsunami wave,
thirty metres high. Finally, local slides off the Australian continental
shelf cannot be ignored. A very large submarine landslide mentioned in
previous chapters, lies fifty kilometres offshore from the coast south of
Sydney. This slide is a prime candidate for the tsunami-deposited barrier
described in Chapter 4 along the adjacent coast (Figure 4.1).

MEGA-TSUNAMI EVIDENCE FOR A COSMOGENIC SOURCE
(Bryant and Young, 1996; Jones and Mader, 1996; Bryant et al., 1997; Bryant
and Nott, 2000)

Some of the Australian evidence for tsunami is on a scale much bigger than
could possibly be generated by the geophysical processes described in
Chapters 5-7. Very few tsunami attributed to these latter types of events
have generated anything approaching the bedrock-sculptured s-forms outlined
in Chapter 3 (Figure 3.1). For example, while the Lituya Bay landslide of
1958 generated a tsunami that surged 524 m above sea level and obtained
velocities of 210 km hr-1, it only cleared soil and glacial debris overlying
bedrock on nearby slopes. Only the Storegga slide--and this may be one of
Tollmanns' meteorite impacts--produced s-forms similar to that profusely
blanketing the rocky headlands of the New South Wales South Coast. Meteorite
impacts with the ocean can unequivocally generate the large tsunami
necessary for the formation of s-forms. Modelling results, using the SWAN
code described in Chapter 2, indicates that a six kilometre diameter
asteroid impacting into the central Pacific would produce a tsunami fifteen
metres high along the New South Wales Coast. As shown in Table 8.1, much
smaller impacts near Australia could also produce waves with this height.
Four other signatures also stand out as unique features of cosmogenic
tsunami: whirlpools bored in bedrock, imbricated boulders fronting cliffs,
mega-ripples, and overwashing of headlands up to 130 m high.

Whirlpools bored into bedrock, of the type shown in Figure 3.23, are rare.
Isolated bedrock plugs of the type shown on the frontispiece at the front of
this book are rarer still. The kolks and tornadic flow necessary to form
them have been described in detail in Chapter 3 (Figure 3.25). Suffice it to
say here that kolks involve enormous hydraulic lift forces produced by
turbulent bursting and steep pressure gradients across vortices. Tornadic
flow involves the breakdown of a wide, parent vortex with secondary vortices
developing around its circumference. The current speed around the vortex is
so high that bedrock can be bored in a matter of minutes. These types of
flow can only be produced by cosmogenic tsunami.

Imbricated and aligned boulders were also depicted in Chapters 3 and 4 as a
signature that uniquely separates the presence of tsunami from storms. While
the boulders perched on top of thirty-three metre high cliffs at Jervis Bay
are impressive evidence of the high velocity flow that only tsunami can
produce (Figure 3.11), it pales in magnitude to other boulder deposits found
in the region--namely at Gum Getters Inlet and Mermaids Inlet. At Gum
Getters Inlet, angular boulders 6-7 m in diameter have been stacked up to
thirty metres above sea level into a small indent in the cliffs (Figure
8.11). It would be tempting to attribute this debris to cliff collapse, but
for the fact that the imbricated blocks rise to thetop of the cliffs. The
deposit is all the more unusual in that the indent is virtually protected
from dominant southeast storm swell. Imbricated blocks of similar size choke
the entrances of two narrow and deep gulches at Mermaids Inlet (Figure
8.12). Some of the largest blocks, which are over five metres in length,
have not simply dropped from the cliff faces; but, have been rotated 180°
and shifted laterally in suspension flow. Not even the 26.2-m high run-up at
Riang-Kroko, following the Flores Island tsunami of 12 December 1992,
produced the magnitude and degree of organisation of these deposits (Figure
3.5). The depth of overland tsunami flow in the Jervis Bay region has been
theorised at 9.5 metres. The boulder features at Gum Getters Inlet are
suggestive of even greater flow depths of 15-20 m that only a cosmogenic
tsunami could generate.

Just as dramatic are the dunes at Crocodile Head, Jervis Bay; and at Sampson
Point, Western Australia. Both of these features were described in Chapters
3 and 4. The former lie atop eighty metre high cliffs, have a relief of
6.0-7.5 m and are spaced 160 m apart. They are akin to
undulatory-to-lingoidal giant ripples that are features of catastrophic flow
such as that observed in the scablands of Washington State. The flow over
the dunes at Crocodile Head is theorised to have been 7.5-12.0 m deep, and
to have obtained velocities of 6.9-8.1 m s-1. The Sampson Point mega-ripples
are gravelly (Figure 4.13), have a wavelength approaching one thousand
metres and an amplitude of about five metres. Flow depth here is theorised
to have been as great as twenty metres with velocities of over 13 m s-1.
More importantly, the mega-ripples occur up to five kilometres from the
coast. These mega-ripples have never been described for conventional tsunami
and could only have been produced by a cosmogenic event.

Finally, there is evidence of tsunami run-up higher and further inland than
produced by conventional processes. The largest run-up produced historically
by a volcano was ninety metres on 29 August 1741 on the West Coasts of
Oshima and Hokkaido Islands, Japan. Santorini may also have had a tsunami
wave height of ninety metres, but confirmed evidence for its run-up does not
exceed fifty metres above sea level. The largest tsunami run-up generated by
an earthquake was one hundred metres on Ambon Island, Indonesia on 17
February 1674. In recent times, an earthquake or submarine landslide off the
Sanriku Coast of Japan produced run-up of 38.2 m on 15 June 1896. The
highest palaeo-tsunami run-up identified in Australia so far is 130 m at
Steamers Beach, Jervis Bay on the crest of a chevron dune. This site has
been referred to often in this text. However, this limit is under-estimated
because the wave still had enough force not only to flow over the headland
and into Jervis Bay; but also to transport large boulders along a ramp
inside the bay. The estimated flow velocity derived from these boulders
using Equation 3.8 is 7.9 m s-1. The potential for higher run-up may have
been exceeded at Sampson Point. Here a palaeo-tsunami originating from the
Indian Ocean overran hills, sixty metres high, lying five kilometres inland.

ABORIGINAL LEGENDS OF COMETS
(Peck, 1938; Jones and Donaldson, 1989; Johnson, 1998)

This book began with a story based upon Aboriginal legends about a meteorite
impact. Many of these legends are concentrated in the southeast corner of
Australia, where some of the best signatures of large tsunami are preserved.
As with Gervasse's description of the meteor impact with the Moon on 19 June
AD 1178, the Aboriginal legend in Chapter 1 mentions that the moon rocked.
There are also similarities with the Maori legend of the Fires of Tamaatea.
In both, stars, fire and stones fell from the sky, and there was a
thunderous explosion. Further inland in New South Wales, the Paakantji
tribe, near Wilcannia on the Darling River, also tell of the sky falling. A
great thunderous ball of fire descended from the sky scattering molten rock
of many colours. As in the Maori legend of New Zealand, floods then followed
this event. The floods may have been the consequence of millions of tonnes
of seawater, vapourised by a meteorite impact with the ocean, condensing and
falling as rain. In South Australia, another legend tells of stars falling
to Earth to make the circular lagoons fringing the coast. Finally, it is
curious that when Europeans made contact with Aboriginal coastal tribes in
Western Australia, they noted that the Aborigines avoided the coast, and
made little attempt to use it for food, even though there was evidence of
past usage in the form of large, shell kitchen middens. As described in
Chapter 4, the biggest mega-tsunami to affect Australia occurred on the West
Australia Coast within the last one thousand years, before European
occupation.

Perhaps the most intriguing legend along the Southeast Coast of Australia is
the story of the eastern sky falling. Aborigines south of Sydney believed
that the sky was held up on supports, and that these gave way on the eastern
side. One version refers to the ocean as belonging to the sky. The ocean had
fallen down wiping out Aboriginal culture. Some tribes were even requested
by others to send tribute to the east to be given to the spirit people in
charge of holding up the sky, so that it could be repaired. Archaeological
evidence for tsunami and its impact on Aboriginal culture also exists along
this coast. One of the deposition signatures of tsunami mentioned in Chapter
3 was the presence of disturbed, Aboriginal kitchen middens, that form a
special case of dump deposit more than ten metres above sea level on some
rocky headlands. At Atcheson Rock, sixty kilometres south of Sydney (Figure
3.20), tsunami overwashed a 20-25 m high headland, boring whirlpools into
the sides. The wave was travelling so fast that it separated from the
headland and made contacted with the sea 100-200 m on the lee side in a bay.
Flow separation caused profuse amounts of coarse sediment to drop from the
flow under gravity, and be deposited on the lee side of the headland. On the
far side of the bay, a dump deposit contains numerous silcrete hand axes and
shaped blades that came from an Aboriginal camp at the head of the
embayment. Aborigines in this camp initially would have heard, but not seen,
the tsunami approaching. Their first indication of disaster would have been
when they looked up and saw the ocean dropping down on them from the sky, as
the tsunami wave surged over the headland.

Dating of the deposits at Atcheson Rock indicates that the meteorite-induced
tsunami occurred within the last six hundred years, rather than in some
distant Dreamtime. Archaeological research has shown that Aboriginal culture
changed dramatically along this coast about five hundred years ago. Instead
of continuing their profuse gathering of marine shells for food, Aborigines
switched to fishing. If a tsunami wave had the force to sweep over 130 m
high headlands in the region, then it would have been powerful enough to
clear all marine shells from rock platforms. The
event necessitated a change in lifestyle by Aborigines simply to survive
starvation. There is also evidence from increased usage of rock shelters,
that Aborigines moved inland around this time. While interpreted as an
indication of increasing population, it could also indicate abandonment of a
dangerous coast similar that observed in West Australia.

More physical and legendary evidence of tsunami comes from South Australia.
Here, mainland Aborigines tell about Ngurunderi who was a great, moody
ancestral figure who lived in the sky. Long ago his two wives left him, and
he came down from the sky to find them. He eventually found his wives wading
in the water between Kangaroo Island and the mainland of South Australia. He
was so angry that he decided to punish his wives. He ordered the sea to rise
up as an enormous tidal wave and drown them. Noisily, the water rushed in so
fast that it quickly drowned his wives who were turned into stone. Their
remains can be seen off the coast of Cape Jervis as rocks called the Two
Sisters. The history of Aboriginal occupation of Kangaroo Island remains
enigmatic. The island shows extensive evidence of Aboriginal occupancy; but,
when the first European, Matthew Flinders, landed on the island in 1802, it
was totally unoccupied. Mainland Aborigines call Kangaroo Island, Kanga--the
Island of the Dead. The coastline also evinces signatures of cosmogenic
tsunami. Most significant are enormous, bored whirlpools on the northern
coast of the island, where the Aboriginal legend is set. The features are
larger than those found at Atcheson Rock. In addition, there are
vortex-carved caves and massive piles of imbricated boulders, some over four
metres in diameter, near promontories. The Island of the Dead may be just
that--evidence of another, tragic, cosmogenic tsunami witnessed by
Australian Aborigines before European occupation, and then documented by the
few survivors in legend form.

CHRONOLOGICAL EVIDENCE
(Asher et al., 1994; Steel, 1995; Young et al., 1997; Estensen, 1998; Bryant
and Nott, 2000)

At present, no evidence has been found of a meteorite or comet impact linked
to the signatures of mega-tsunami along the South Coast of New South Wales.
Nor may any be found because it does not take a large meteorite impact in
the ocean to produce the size of tsunami responsible for the observed
evidence. Meteorite impacts also tend not to leave a crater on the seabed.
For example, no crater for the Eltanin Meteorite has yet been found despite
its four-kilometre diameter. However, the timing of tsunami events can be
approximated using radiocarbon dating of marine shell deposited in dump
deposits and sand layers, and attached to boulders transported by tsunami.
Radiocarbon dating is only accurate for events that are older than 460
years. At least twenty dates have been obtained from the New South Wales
South Coast. In addition, three samples related to tsunami were obtained
from Lord Howe Island situated in the Tasman Sea halfway between Australia
and New Zealand (Figure 8.10). At least ten additional dates were too young
to be plotted. Each radiocarbon age is reported as an age with an error
term. This information can be used to construct a probability distribution
of dates for that sample. An overall time series was then constructed by
summing these probabilities for all samples. For presentation purposes, this
time series has been standardised to a maximum value of one. The resulting
time series spanning the last ten thousand years is plotted in Figure 8.13,
while that for the last two thousand years is plotted in more detail in the
bottom panel of Figure 8.3.

Six separate tsunami events can be recognised over the past 8,000 years with
peaks at 7500 BC, 5000 BC, 3300 BC, 500-2000 BC, AD 500 and AD 1500. There
may be more events than this; but, until further dating, it is impossible to
know whether or not the broad sequence of dates between 500-2000 BC
represents a single event or many. This later timespan includes an impact
event in the Middle East dated around 1600 BC. Reference to fire and stones
falling from the sky appear in the Bible and other manuscripts written
around this time. The record, however, doesn't show any evidence for a
Bronze Age event around 2350 BC that is believed to have destroyed
civilisations simultaneously in Europe, the Middle East, India and China.
Nor do any of the dates cluster around the time of Tollmanns' Deluge Comet
impact event 8,200 ±200 years ago. This may be due to the poor preservation
potential of shell material this old, or to the removal of such material by
subsequent tsunami. However, thermoluminescence dating of sand layers
deposited by tsunami, on the New South Wales South Coast, indicate that a
major discontinuity in sedimentation occurred 8,700 ± 800 years ago. This
hiatus is within the timespan of Tollmanns' Deluge Comet impact event. The
New South Wales event peaking in AD 1500 appears to be the largest as it is
associated with overtopping of the headland, 130 m high, at Steamers Beach,
Jervis Bay. Because no large tsunami has been reported along the New South
Wales Coast since European settlement in 1788, the shell samples that are
too young for radiocarbon dating allude to a small, but significant, tsunami
event in the early Eighteenth Century.

The peak of the AD 1500 tsunami event corresponds with the largest number of
meteorite observations for the past two millennia (Figure 8.3). In addition,
the peak at AD 500 corresponds with a clustering of meteorite sightings that
is believed by astronomers to be one of the most
significant over this timespan in the Northern Hemisphere. Both of these
clusterings are associated with the Taurid complex. Furthermore, the event
around AD 1500 coincides with the calibrated ages for the Fires of Tamaatea
across the Tasman Sea on the South Island of New Zealand. As well, the
tsunami event at Atcheson Rock that accounts for the Aboriginal legend of
the ocean falling from the sky occurred at this time, as does the age of the
meandering backwash channels on the Shoalhaven Delta forty kilometres to the
south (Figure 4.3). Other main sightings of meteorites from the Northern
Hemisphere correspond with minor peaks in the Southeast Australian tsunami
chronology. It would appear that meteorite, rather than comet impacts,
correspond to the Australian chronology for tsunami. The two minor clusters
of meteorite activity between 1640 and 1800 may have produced cosmogenic
tsunami that account not only for evidence of a pre-European event in New
South Wales, but also for tsunami identified in Chapter 4 along the
Northwest Coasts of West Australia and Northeast Queensland. The lack of any
mega-tsunami event since AD 1788--the time of first European settlement--may
only be fortuitous. Based upon the data for the last two millennia, there is
a fifty-percent probability that such an event could occur again in the next
half century.

The events between the Fifteenth and Eighteenth Centuries preceded European
colonisation in Australia; however, they coincide with European exploration
around the continent and Dutch colonisation in Indonesia. In the Eighteenth
Century, without the means of determining longitude, merchant ships of the
Dutch East Indian Company made their way to the colonial city of Batavia in
Indonesia by sailing straight across the Indian Ocean until they sighted the
Australian coastline, and then turning north. They would have sailed by the
Northwest Coast of West Australia around the time a cosmogenic tsunami
struck that coast. Many ships in pursuit of exploration and commerce were
loss and presumed shipwrecked; but, without hard evidence, it is best to put
these losses down to storms.

Two shipwrecks in Australia stand out as unusual. The first relates to the
Mahogany ship now buried in sand dunes well above sea level at Warrnambool,
Victoria. In 1521, three Portuguese caravels under the leadership of
Cristoväo de Mendonça sailed on a secret mission from Malacca, East Indies
to explore the Australian coastline. The reason for the secrecy was the
intense competition between Spain and Portugal for world domination. Only
one of Mendonça's ships made it back. Any record of his expedition
disappeared into the secret Portuguese archives in Lisbon
where no one has seen them since. It is unlikely that they survived the
earthquake and subsequent fire of 1755. In 1836, a mahogany ship was
discovered, washed inland well above the limit of storm waves, near
Warrnambool, on an isolated part of the Victorian Coast of southern
Australia (Figure 8.10). The first Europeans known to have landed on this
coast made the discovery. Unfortunately, the stranded ship was buried in
shifting sands by 1880, never to be seen again. Intriguingly, evidence
suggests that Mendonça did reach and map the South Coast of Australia. This
evidence comes from the Dieppe maps, first published in the mid-1500s. They
show remarkably detailed coastline down the East Coast of Australia and
across the South Coast of Australia. The maps terminate at Warrnambool,
Victoria! How the Mahogany ship managed to get into the sand dunes has
remained an arcanum ever since.

The second shipwreck involves the Zuytdorp, a Dutch East India Company
merchant vessel that was part of a convoy supplying the Dutch East Indies at
regular intervals. In June-July 1712, the Zuytdorp crashed into the cliffs
off Northwest Cape, Western Australia (Figure 3.2). Debris,
including the ship's bell, was scattered amongst masses of boulders up
cliffs rising seventy metres above sea level--well above the limits of storm
waves. The ship struck the reefs at the base of the cliffs suddenly, because
all six of its anchors were found intact without having been set, as would
have been the case if the ship had been caught in a storm. Interestingly,
the top of the cliffs is covered in a dump deposit of shell, sand and
angular gravels that has been misinterpreted by many anthropologists as an
Aboriginal kitchen midden. The dates for both the
Mahogany and Zuytdorp shipwrecks fit within temporal windows for two
cosmogenic tsunami around the Australian Coast based upon radiocarbon
dating.

There is controversy about the size of tsunami that can be generated by
meteorite impacts. Also, if one examines the geological record, the
theorised distribution of tsunami wave heights, calculated using one of the
formula leaning towards higher estimates, shows that cosmogenic tsunami have
not been big enough to be a dominant force shaping the world's coastal
landscape. On the other hand, there is plenty of evidence to indicate that
some coastlines--mainly around Australia--have been affected by sufficient
depth and velocity of water to transport boulders to the tops of cliffs 33 m
high, deposit sandy bedforms on cliffs 80 m high, overwash headlands up to
130 m above sea level, and breech hills 60 m high lying five kilometres
inland. Similar evidence in the form of bedrock sculpturing can also be
found along the coastlines of New Zealand and Northeastern Scotland. Two
factors involving meteorite impacts with the ocean may account for the
discrepancy between theory and fact. For example, meteorites of varying
density and less than one kilometre in diameter can fragment and undergo
distortion before striking the ocean. If this is the case, craters ten times
larger than the radius of the original asteroid or comet may dimple the
ocean, creating a tsunami larger than could be produced by an unaltered
meteorite. Secondly, large amounts of water and heated vapour can be flung
into the ocean and tossed significant distances away from the centre of an
impact (Figure 8.4). This high velocity, air-borne splash may not only
explain the inland flooding mentioned in many comet legends, but also
account for erosion of bedrock and emplacement of dump deposits on headlands
and clifftops. Research on this aspect is in its infancy. Finally, the
threat of splash or impact-related tsunami from meteorites may be alarmist.
If coherent catastrophism is associated with the Taurid complex, then apart
from the odd random Earth-crossing meteor or comet, the next large influx of
meteorites will not occur until around the year AD 3000. The overall risk of
all types of tsunami and society's mitigation of the threat will be
discussed in the next chapter.

Figure 8.3 Incidence of comets and meteorites, and related phenomena,
between AD 0-1800. The meteorite records for China and Japan are based upon
Hasegawa (1992), while meteorite records for Europe come from Rasmussen
(1991). Peak occurrences are shaded. The Asian comet record is based upon
Hasegawa (1992). The calibrated radiocarbon dates under the Mystic Fires of
Tamaatea are from Mooley et al. (1963) for forest wood and from McGlone and
Wilmshurst (1999) for peats and bogs. The radiocarbon dates of prehistoric
tsunami events in Australia are based upon the author's published work and
other acknowledged research. See text for more details.

_____________________________________________________________________
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