Outburst flood

In the relatively recent geological past, several great floods are widely suspected to have occurred, with varying amounts of supporting evidence, usually as a result of the last Ice Age ending.

At the 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 around 18,000 years ago sea levels rose. Most of the glacial melt had occurred by around 8,000 years ago, but the changes have not been as regular as a constant drip at the edges of the world's glaciers might suggest.

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. Also, parts of Scandinavia are rising isostatically for this reason, by up to centimeter a year in some places; it rises as fast as mantle rock can flow in under it, and that mantle rock must come from somewhere around, including from under the Netherlands, which are slowly sinking as a result.

These floods happened in various ways, which can be categorised into 5 types:-

1. Very flat land being steadily flooded over a long time as the sea rises, sometimes fast enough to be easily noticed in a human's lifetime.
2. The same, plus the occasional stormflood submerging land and washing the loose soil and subsoil away, leaving the flooded land too deep to be reclaimed. This is more noticeable if the people try to defend their land with dikes, for example in the Netherlands and in the Solent.
3. The rising sea overflowing a natural sill and entering an enclosed basin. The sill may then erode away catastrophically, 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. Some people argue that these events may have sparked the flood myths found in many cultures.
4. Big glacier-dammed lakes bursting as their ice dams melt until they suddenly collapse.
5. Other causes, for example megatsunamis.

Several examples where such rapid encroachment of the sea occurred are provoking geologists' and archaeologists' investigations.

The Black Sea (around 7,600 years ago)

Main article: Black Sea deluge theory

Black Sea today (light blue) and in 5600 BC (dark blue) according to Ryan's and Pitman's theories

This is type 3. 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 around 7,600 years ago. It remains an active subject of debate among archaeologists, with subsequent evidence discovered to both support and discredit the existence of the flood, while the theory that it formed the basis for later flood myths is subjective and unprovable. Although this theory has to withstand pressure from other scientists, German researchers maintain the concept of a catastrophic flood around 5,500 BC and present the theory that this event also is the basis for Plato's Atlantis account.

The Caspian and Black Seas (around 16,000 years ago)

This is type 1. An alternative theory proposed by Andrey Tchepalyga of the Russian Academy of Sciences dates the flooding of the Black Sea basin to an earlier time and from a different cause. According to Tchepalyga, global warming beginning from about 16,000 BP caused the melting of the Scandinavia Ice Sheet, resulting in massive river discharge that flowed into the Caspian Sea, raising it to as much as 50m above normal present-day levels. The rise was extremely rapid and the Caspian basin could not contain all the floodwater, which flowed through the Kuma-Manych Depression and Kerch Strait into the ancient Black Sea basin. By the end of the Pleistocene this would have raised the level of the Black Sea by some 60–70m to about 20m below its present-day level, and flooding large areas that were formerly available for settlement or hunting. Tchepalyga suggests this may have formed the basis for legends of the great Deluge.

The lower Tigris-Euphrates Valley, reflooding the Persian Gulf (12,000 years ago)

This is type 1. When sea levels were low, the combined Tigris-Euphrates river flowed through a wide flat marshy landscape. The Persian Gulf today has an average depth of only 35 m. During the most recent glaciation, which ended 12,000 years ago, worldwide sea levels dropped 120 to 130 m, 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 Mountains 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 Strait of Hormuz 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; the deeper parts of these channels may be due to delta deposits at the edge of the deep ocean collapsing in a succession of big underwater landslides, causing underwater erosion by the resulting turbidity currents.

There is a theory that there was also a Black-Sea-type sill collapse at the Strait of Hormuz at the outlet of the Persian Gulf, so converting this case into type 3.

In a 1981 Journal of Cuneiform Studies article, "The Tangible Evidence for the Earliest 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. Theresa Howard-Carter also wrote: "It is more likely that the original Gulf inhabitants lived along the banks of the lower or extended Shatt al-Arab, ranging some 800 km across the dry Gulf bed. We can thus postulate that the pre-Sumerian cultures had more than ample time to be born and flourish in a riverine setting, encouraged by the agricultural potential and the blessings of a temperate climate. The fact that the body of proof for the existence of these societies must now lie at the bottom of the Gulf furnishes at least a temporary excuse for the archaeologist's failure to produce evidence for their material culture."

In our time, mangrove edge habitat and coral reefs characterize the Persian Gulf. Mangroves recolonize easily from established mangrove fringe colonies elsewhere in the Arabian Sea. Artificial reefs are being established today along the coast of Iran. But if the Persian Gulf filled so recently, then how have the reefs re-established? The present-day natural reef developments in the Persian Gulf, corals grow on hardground substrates but have not yet formed the massive calcium carbonate structures familiar from, say, Australia's Great Barrier Reef.

The article Dive conditions described by Eric Bjornstrom found in 1999 in Dubai coral-encrusted sand barrier islands situated 32 km off the coast of the Saudi city of Jubail. There lies a chain of five coral cays, barely above the tide. They appear to be formations called diapirs in which a mobile core containing minerals of low density such as salt, deforms under pressure. The core pushes upwards, deforming overlying rock to form a dome. An ancient diapir at Enorama formed an island in shallow seas, buoyed up by salt. There are similar examples today in the Persian Gulf.

None of these theories, however, are to be confused with more mainstream, confirmed evidence of relatively recent extended local flooding in this part of the world. Excavations in Iraq, for example, have shown evidence of a flood at Shuruppak around 2900-2750 BCE which extended nearly as far as the city of Kish (whose king, Etana, supposedly founded the first Sumerian dynasty after the Deluge).

Great Sunda wetlands, Indonesia

This is type 1. During glacial times a huge peaty swampland joined the Malay peninsula, 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 (12,000 to 10,000 years ago)

This is type 1. 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 around 12,000 to 10,000 years ago. 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 Aegean Basin

Areas that have not been as widely discussed include the refilling of the Aegean basin. A look at a modern chart shows that it is a kilometer or more deep in some places and that it has a sill along the line of the Peloponnese - Crete - Rhodes - southwest Turkey. However, that sill is very deep, and it may have caused a sill-overflow effect at refloodings of the empty Mediterranean in the Miocene, if that sill existed then, but not later. The Aegean Basin formed very slowly by the crust thinning because of stretching due to tectonic events.

Doggerland and a Channel flood

This is type 1. 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. The region was watered by the glacial River Rhine, into which flowed the River Thames as a tributary; the combined river flowed into the North Sea, permitting access to Britain by large mammals and humans.

During an earlier glacial maximum, the combined rivers had been blocked to the north by an ice dam; they filled a vast lake with freshwater glacial melt on the bed of what is now the North Sea. A gently upfolding chalk ridge linking the Weald of Kent and Artois, perhaps some thirty meters higher than the current sea level, contained the glacial lake at the Strait of Dover. At a certain time, and apparently more than once, the barrier failed or was overtopped, loosing a catastrophic flood that permanently separated Britain from the continent of Europe; a sonar study of the sea bed of the English Channel published in Nature, July 2007, revealed the discovery of unmistakable marks of a megaflood on the English Channel seabed: deeply-eroded channels and braided features have left the remnants of streamlined islands among deeply gouged channels. where the collapse occurred.

North America

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 was Lake Agassiz. A series of floods, as ice-dam configurations failed (type 4) 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 Channeled Scablands.

Lake Bonneville burst catastrophically due to its water overflowing and washing away a sill composed of two opposing alluvial fans which had blocked a gorge.

The last of the North American proglacial lakes, north of the present Great Lakes, has been designated Glacial Lake Ojibway by geologists. It reached its largest volume around 8,500 years 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 8,300 to 7,700 years 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 m above sea level. The volume of Lake Ojibway is commonly estimated to have been about 163,000 cubic kilometres, more than enough water to cover a flattened-out Antarctica with a sheet of water 10 m 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.

There is also a strong possibility that a global climatic change in recent geological time brought about some large deluge. Another theory, although one not widely supported, suggests some of the major floods may have been caused by plate tectonics, the drifting apart of continents. Evidence is mounting from ice-cores in Greenland that the switch from a glacial to an inter-glacial period can occur over just a few months, rather than over the centuries that earlier research suggested.

The refilling of the Mediterranean

An earlier catastrophe, too far back to be within human memory, occurred at the most recent re-flooding of the Mediterranean Sea's dry basin, dated by general consensus about 5 million years ago, before the emergence of modern humans.

The basin had previously become a desert once again, the most recent desiccation in a series, as deep cores in the seabed have revealed a series of several layers of salt, separated by loess deposits, after continental movement had closed the Strait of Gibraltar, an event variously placed at 8 million or 5.5 million years ago. The Mediterranean did not dry out during the most recent glacial maximum.

Tollmann's hypothetical bolide

Compare Alexander Tollmann's hypothetical bolide, a hypothesis that one or several bolides (meteors or comets) struck the Earth in 7640 BC (+/-200), with a much smaller one at 3150 BC (+/-20) causing the flooding of myth.

External links

• Kurt Lambeck, "Sea-level change and shore-line evolution in Aegean Greece since Upper Palaeolithic time" (abstract).

See also

• Megatsunami
• Flood (mythology)