Surfing The Wierd

Monday, December 3, 2007

Ancient Catastrophes - Science and Myth

*http://abob.libs.uga.edu/bobk/ccc/ce053101.html

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 0 521 77599 4 Paperback £19.95/$27.95 For more details and how to order, please visit the CUP website at http://uk.cambridge.org/earthsciences/catalogue/052177599X/ >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> 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. >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> _____________________________________________________________________ REFERENCES Asher, D.J., Clube, S.V.M., Napier, W.M. and Steel, D.I. 1994. Coherent catastrophism. Vistas in Astronomy v. 38 pp. 1-27. Bryant, E. and Nott, J. 2000. Geological indicators of large tsunami in Australia. Natural Hazards (in press). Bryant, E. and Young, R.W. 1996. Bedrock-sculpturing by tsunami, South Coast New South Wales, Australia. Journal Geology v. 104 pp. 565-582. Bryant, E., Young, R.W. and Price, D. 1996. Tsunami as a major control on coastal evolution, Southeastern Australia. Journal of Coastal Research v. 12 pp. 831-840. Bryant, E., Young, R.W., Price, D., Wheeler, D. and Pease, M.I. 1997. The impact of tsunami on the coastline of Jervis Bay, southeastern Australia. Physical Geography v. 18 pp. 441-460. Estensen, M. 1998. Discovery: The quest for the Great South Land. Allen and Unwin, Sydney, 286p. Hasegawa, I. 1992. Historical variation in the meteor flux as found in Chinese and Japanese chronicles. Celestial Mechanics and Dynamical Astronomy v. 54 pp.129-142. Johnson, D. 1998. Night Skies of Aboriginal Australia: A noctuary. Oceania Monograph No. 47, University of Sydney, Sydney, 147p. Jones, A.T. and Mader, C.L. 1996. Wave erosion on the southeastern coast of Australia: tsunami propagation modelling. Australian Journal Earth Sciences v. 43 pp. 479-483. Jones, D. and Donaldson, K. 1989. The story of the falling star. Aboriginal Studies Press, Canberra, 94p. Kristan-Tollmann, E. and Tollmann, A. 1992. Der Sintflut-Impakt (The Flood impact). Mitteilungen Der Österreichischen Geographischen Gesellschaft v. 84 pp. 1-63. McGlone, M.S. and Wilmshurst, J.M. 1999. Dating initial Maori environmental impact in New Zealand. Quaternary International v. 59 pp. 5-16. Mooley, B.P.J., Burrows, C.J., Cox, J.E., Johnston, J.A. and Wardle, P. 1963. Distribution of subfossil forest remains Eastern South Island, New Zealand. New Zealand Journal of Botany v. 1 pp.68-77. Oliver, J. 1988. Natural hazards In Jeans, D.N. (Ed.) Australia: a geography. Sydney University Press. pp. 283-314. Peck, C.W. 1938. Australian legends. Lothian, Melbourne, 234p. Rasmussen, K.L. 1991. Historical accretionary events from 800 BC to AD 1750: Evidence for Planetary rings around the Earth? Quarterly Journal of the Royal Astronomical Society v. 32 pp. 25-34. Steel, D. 1995. Rogue Asteroids and Doomsday Comets. Wiley, New York, 308p. Steel, D. and Snow, P. 1992. The Tapanui region of New Zealand: Site of a 'Tunguska' around 800 years ago? In Harris, A. and Bowell, E. (Eds.) Asteroids, Comets, Meteors 1991. Lunar and Planetary Institute, Houston, pp. 569-572. Young, R., Bryant E., Price D.M.,, Dilek, S.Y., and Wheeler, D.J. 1997. Chronology of Holocene tsunamis on the southeastern coast of Australia. Transactions Japanese Geomorphological Union v. 18 pp. 1-19 Copyright 2001, Cambridge University Press


Labels:

0 Comments:

Post a Comment

Subscribe to Post Comments [Atom]



Links to this post:

Create a Link

<< Home