Outburst flood

In geomorphology, an outburst flood, which is a type of megaflood, is a high magnitude, low frequency catastrophic flood involving the sudden release of water.^[1][2] During the last deglaciation, numerous glacial lake outburst floods (GLOF) were caused by the collapse of either ice sheets or glaciers that formed the dams of proglacial lakes. Examples of older outburst floods are known from the geological past of the Earth and inferred from geomorphological evidences in Mars. Landslides, lahars, and volcanic dams can also block rivers and create lakes, which trigger such floods when the rock or earthen barrier collapses or is eroded. Lakes also form behind glacial moraines, which can collapse and create outburst floods.^[3]

Definition and classification

Megafloods are paleofloods (past floods) that involved rates of water flow larger than those in the historical record. They are studied through the sedimentary deposits and the erosional and constructional landforms that individual megafloods have created. Floods that are known to us through historical descriptions are mostly related to meteorological events, such as heavy rains, rapid melting of snowpacks, or combination of these. In the geological past of the Earth, however, geological research has shown that much larger events have occurred.^[3] In the case of outburst floods, such floods are typically linked to the collapse of the barrier forming a lake. They fall in the following classification according to the mechanism responsible:

• Collapse of glacier dams that impound proglacial lakes (Missoula Floods).
• Rapid erosion, melting of ice sheets (Jokulhlaups).
• Collapse of earthen barriers (landslides or glacial moraines).
• Collapse of volcanic dams created by lava flows, lahars, or pyroclastic flows.
• Overtopping
  • Lake overtopping (e.g., Lake Bonneville).
  • Ocean spilling over a dividing ridge into a landlocked basin (e.g., Zanclean flood, Black Sea flood).^[1] A smaller scale example would be the Pantai Remis landslide.

Outburst flood examples

Examples where evidence for large ancient water flows has been documented or is under scrutiny include:

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

A rising sea flood, recently disclosed^{[clarification needed]} 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)^{[citation needed]}. 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 geologists, 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 not proven.

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

See also: West Siberian Glacial Lake

A 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 50 metres (160 ft) above normal present-day levels. The rise was extremely rapid and the Caspian basin could not contain all the floodwater, which flowed from the northwest coastline of the Caspian Sea, through the Kuma-Manych Depression and Kerch Strait, over the current eastern coastline of the Sea of Azov 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 to 70 metres (200 to 230 ft) 20 metres (66 ft) 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.^[4]

The English Channel Flood 400,000 BP

Originally there was an isthmus across the Strait of Dover. During an earlier glacial maximum, the exit from the North Sea was blocked to the north by an ice dam, and the water flowing out of rivers backed up into 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 30 metres (100 feet) 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^[5] or was overtopped, loosing a catastrophic flood that permanently diverted the Rhine into the English Channel and replacing the "Isthmus of Dover" watershed by a much lower watershed running from East Anglia east then southeast to the Hook of Holland and (as at modern sea level) separated Britain from the continent of Europe; a sonar study of the sea bed of the English Channel published in Nature, July 2007,^[6] 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.^[7]

Glacial lake outburst floods in North America (15,000 to 8,000 years ago)

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, 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 state were also caused by breaking ice dams, resulting in the Channeled Scablands.

Lake Bonneville burst catastrophically in the Bonneville Flood, due to its water overflowing and washing away a sill composed of two opposing alluvial fans which had blocked a gorge. Lake Bonneville was not a glacial lake, but post-glacial climate change determined the lake level and its overflow.

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 Ottawa and St. Lawrence Rivers 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 metres (820 ft) 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 metres (33 ft) 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. 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 Sea (5.3 million years ago)

Main article: Zanclean flood

A catastrophic flood refilled the Mediterranean Sea 5.3 million years ago, at the beginning of the Zanclean age that ended the Messinian salinity crisis.^[8] The flood occurred when Atlantic waters found their way through the Strait of Gibraltar into the desiccated Mediterranean basin, following the Messinian salinity crisis during which it repeatedly became dry and re-flooded, dated by general consensus to before the emergence of modern humans.^[9]

The Mediterranean did not dry out during the most recent glacial maximum. Sea Level during glacial periods within the Pleistocene is estimated to have dropped only about 110 to 120 metres (361 to 394 ft).^[10][11] In contrast, the depth of the Strait of Gibraltar where the Atlantic Ocean enters ranges between 300 and 900 metres (980 and 3,000 ft).^[12]

See also

• Altai flood
• Deluge myth
• Missoula Floods (pleistocene megaflood)
  • Glacial Lake Missoula
  • J Harlen Bretz
• Outflow channels
• Zanclean flood

References

1. ^ ^{a b} O’Connor, J.E. and Beebee, R.A., 2009, Floods from natural rock-material dams, in: Burr, D., Carling, P., and Baker, V. editors, Megafloods on Earth and Mars: Cambridge University Press.
2. ^ Goudie, A., 2004, Encyclopedia of Geomorphology. Routledge. London, England. ISBN 0-415-27298-X
3. ^ ^{a b} Burr, D.M., Baker, V.R., Carling, P.A. (Eds), 2009. Megaflooding on Earth and Mars. Cambridge University Press. 319 pp.
4. ^ Tchepalyga, Andrey (2003-11-04). "Late glacial great flood in the Black Sea and Caspian Sea (abstract)". Abstracts with Programs. 35-6. The Geological Society of America 2003 Seattle Annual Meeting. Seattle, Washington. pp. 460. http://gsa.confex.com/gsa/2003AM/finalprogram/abstract_63243.htm. Retrieved 2007-07-24. 
5. ^ The area is subject to earth movements: see the Dover Straits earthquake of 1580
6. ^ Sanjeev Gupta et al. in Nature 448 (2007), pp 342-345.
7. ^ BBC News, "Megaflood' made 'Island Britain'"; News at Nature, "Geological evidence supports theory of surge down the English Channel."
8. ^ Garcia-Castellanos, D., et al., (2009). Catastrophic flood of the Mediterranian after the Messinian Salinity Crisis. Nature, 462, 778-782.
9. ^ Hsu, K.J., 1983, The Mediterranean Was a Desert, Princeton University Press, Princeton, New Jersey.
10. ^ Lambeck, K., 1996, Sea-level change and shore-line evolution in Aegean Greece since Upper Palaeolithic time. Antiquity. v. 70, no. 269, pp. 588-611.
11. ^ Lambeck, K., 2005, Sea-level change in the Mediterranean Sea since the LGM: model predictions for tectonically stable areas. Quaternary Science Reviews. v. 24, no. 18-19, pp. 1969–1988.
12. ^ .See Robinson, Allan Richard and Paola Malanotte-Rizzoli, Ocean Processes in Climate Dynamics: Global and Mediterranean Examples. Springer, 1994, p. 307, ISBN 0792326245.

External links

• Alexei N. Rudoy. Glacier-dammed lakes and geological work of glacial superfloods in the Late Pleistocene, Southern Siberia, Altai Mountains
• Catastrophic floods in the English Channel - Inundations