Deluge (prehistoric)

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. During the last deglaciation, numerous glacial lake outburst floods 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 evidence on 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.

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. 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:

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

An example is the lake overflow that caused one of the worst landslide-related disasters in history on June 10, 1786. A landslide dam on Sichuan's Dadu River, created by an earthquake ten days earlier, burst and caused a flood that extended 1400 km downstream and killed 100,000 people.

A rising sea flood, 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 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.

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