Outburst flood: Difference between revisions - Wikipedia

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At the [[Wisconsin glaciation|most recent glacial maximum]], so much of the planet's water was locked up in the vast ice-sheets that formed ice domes kilometers thick, that the sea level dropped by about 120 to 130 meters. As the sheets melted, starting aboutaround 18,000 [[wiktionary:YBP|ybp]]years (16000 [[wiktionary:BCE|BCE]])ago, sea levels rose. Most of the glacial melt had occurred by aboutaround 8000 ybp8,000 (6000years BCE)ago, but the changes have not been as regular as a constant drip at the edges of the world's glaciers might suggest. In the geological past, within human experience, several great floods are widely suspected to have occurred, with varying amounts of supporting evidence.

Sea levels have changed significantly since Late [[Paleolithic]] time, and shorelines have migrated. The sea has not always steadily encroached upon the land, for the immense weight of the ice-sheets depressed the continental plates under them and caused isostatic rebound around their edges, which are still adjusting today. Averaged rates of sea-level-rise are misleading. Where sills formed dikes that protected low-lying areas, a winter storm or a sudden spurt of meltwater thousands of miles away could raise ocean levels, and the natural dike could be catastrophically eroded like a dike in the Netherlands. The ocean could fill vast basins in matters of weeks or months, in catastrophes that are unimaginable in today's world (though our grandchildren may be in for some surprises).

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In a 1981 ''Journal of Cuneiform Studies'' article, "The Earliest Tangible Evidence for Dilmun," Theresa Howard-Carter espoused her theory identifying Dilmun with [[Qurna]], an island at the Strait of Hormuz. Her scenario put the "original" mouths of the Tigris-Euphrates rivers, which she thought should be the site of the primeval Dilmun, at or even beyond the Straits of Hormuz. Mainstream archaeologists have avoided mentioning her article, for fear of its apparent [[catastrophism]], an awkward subject in geology. .

==Reflooding the Persian Gulf (after12,000 12000YBPyears ago)==

The [[Persian Gulf]] today has an average depth of only 35 meters. During the most recent glaciation, which ended about 1200012,000 years BPagp, worldwide sea levels dropped 120 to 130 meters, leaving the bed of the Persian Gulf well above [[sea level]] during the [[glacial maximum]]. It had to have been a swampy freshwater floodplain, where water was retained in all the hollows. High in the [[Taurus mountians]] glaciation will have been extensive.

The drainage of the combined glacial era [[Tigris]]-[[Euphrates]] made its way down the marshes of this proto-[[Shatt-al-Arab]] to the sill at Hormuz and disgorged far beyond, into the [[Arabian Sea]]. Reports of the exploration ship "Meteor" have confirmed that the Gulf was an entirely dry basin about 15,000 BC. Close to the steeper Iranian side a deep channel apparently marks the course of the ancient extended Shatt al-Arab. A continuous shallow shelf across the top (north) of the Gulf and down the west side (at 20 m.) suggests that this section was the last to be inundated. At the Straits of Hormuz the bathymetric profile indicates a division into two main channels which continue across the [[Bieban Shelf]] before dropping to a depth of c 400 m. in the [[Gulf of Oman]].

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During glacial times a huge peaty swampland joined Malaya, Sumatra, Java and southwestern Borneo to the Asian mainland. The present landmasses were highlands framing a vast wetlands ecosystem larger than any on earth today which is now covered by the southern part of the [[South China Sea]]. Though the area never lost its tropical to subtropical vegetation, the monsoon weather system, which is powered by the continental mass, is likely to have been more intense than it is today. At one of the "pulses" of sea level rise, the combination of violent monsoons over a single drainage basin, in a landscape that dwarfed modern Bangladesh, provide a scenario for some of the most devastating flooding humans have ever witnessed anywhere.

==The Carpenteria plain (after12,000 12000YBPto 10,000 years ago)==

During glacial times, a stretch of level plain joined [[Australia]] with [[New Guinea]] and enabled humans to walk into Australia. That plain flooded to form the [[Gulf of Carpentaria]], between ''ca''around 12,000 andto 10,000 ybpyears ago. It is significant that [[Aboriginal mythology|aboriginal Australian myth]] of the "[[dream time]]" includes a Great Flood which is not ordinarily a recognizable feature of the Australian climate and geography, except for infrequent filling of ordinarily dry lake basins (''e.g.'' [[Lake Eyre]]).

==The Black Sea (circaaround 7600YBP7,600 years ago)==

The recently disclosed and much-discussed refilling of the freshwater glacial [[Black Sea]] with water from the [[Aegean]], was described as "a violent rush of salt water into a depressed fresh-water lake in a single catastrophe that has been the inspiration for the flood mythology" (Ryan and Pitman, 1998). The marine incursion, which was caused by the rising level of the Mediterranean, occurred ''ca''around 76007,600 ybp/5600years BCEago.

* W.B. Ryan and W.C. Pitman, ''Noah's Flood: The new scientific discoveries about the event that changed history'' 1998

However, later examinations by scientists have cast doubt on this catastrophic flood theory concerning the Black sea, which show that, around 56007,600 BCE/7600years BPago, the then fresh water lake was actually emptying it's waters into the Aegean and not vice versa. [[http://lava.tamu.edu/courses/geol101/herbert/docs/BlackSeaFloodCritique.pdf]]

==The Aegean Basin==

Areas that have not been as widely discussed include the refilling of the Aegean basin itself. A look at a modern chart shows that it must have been a marshy low-lying plain dotted with lakes, where the combined waters of all the glacial-age rivers emptying from the Black Sea passed through the [[Sea of Marmara]] which was a fresh-water lake. There is a [[sill]] depth about 70 m. at the [[Dardanelles]] Strait, where the great river continued to its mouth at the edge of the [[Sea of Crete]]. Rising seas reached their present level in the Aegean aboutaround 6,000 ybpyears (4000 BCE)ago.

==Doggerland==

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In 1998, the archaeologist B.J. Coles identified as "Doggerland" the now-drowned habitable and huntable lands in the coastal plain that was formed in the [[North Sea]] when sea level dropped. Doggerland has not caught the popular imagination, but the terrain was available for settlement. Its gentle swells remain as the [[Dogger Banks]]. Paleolithic reindeer hunters roamed the land; some traces of their encampments have been identified, but the timing of the submergence has not been fixed.

*[http://doggerland.dk/PUBLIKATIONER/artikelnyt.html Doggerland website] (Danish), but the map redrawn from official Geological Surveys shows the landscape ''ca''around 14,000-15,000 ybpyears ago in the first warm (interstadial) period after the glacial maximum.

==North America==

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In North America, during glacial maximum, there were no Great Lakes as we know them, but "proglacial" (ice-frontage) lakes formed and shifted. They lay in the areas of the modern lakes, but their drainage sometimes passed south, into the Mississippi system, sometimes into the Arctic, or east into the Atlantic. The most famous of these proglacial lakes has been termed [[Lake Agassiz]]. A series of floods, as ice-dam configurations failed created a series of great floods from Lake Agassiz, resulting in massive pulses of freshwater added to the world's oceans. The [[Missoula Floods]] of [[Washington]] were also caused by breaking ice dams, resulting in the [[Columbia Scablands]].

The last of the North American proglacial lakes, north of the present Great Lakes, has been designated [[Lake Ojibway]] by geologists. It reached its largest volume aboutaround 8.5kya,500 (6500years BCE)ago, when joined with Lake Agassiz. But its outlet was blocked by the great wall of the glaciers and it drained by tributaries, into the St. Lawrence far to the south. About 8.3,300 -to 7700 kya (6300 -7,700 5700years BCE)ago, the melting ice dam over Hudson Bay's southernmost extension narrowed to the point where pressure and its buoyancy lifted it free, and the ice-dam failed catastrophically. Lake Ojibway's beach terraces show that it was 250 meters above sea level. The volume of Lake Ojibway is commonly estimated to have been about 163,000 ''cubic'' kilometers, more than enough water to cover a flattened-out Antarctica with a sheet of water ten meters deep. That volume was added to the world's oceans in a matter of months.

The detailed timing and rates of change after the onset of melting of the great ice-sheets are subjects of continuing study.

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==Tollmann's hypothetical bolide==

Compare Alexander [[Tollmann's hypothetical bolide]], a hypothesis that one or several bolides ([[meteor]]s or [[comet]]s) struck the Earth atin [[8th millennium BC|7640 BCEBC]] (+/-200), with a much smaller one at [[4th millennium BC|3150 BCEBC]] (+/-20) causing the flooding of myth.

==See also==