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vineri, 31 august 2012

From Flood Myth to Martian Megafloods

“No one with an eye for land forms can cross eastern Washington in daylight without encountering and being impressed by the “scabland.” Like great scars marring the otherwise fair face to the plateau are these elongated tracts of bare, black rock carved into mazes of buttes and canyons. Everybody on the plateau knows scabland…[]…The popular name is a metaphor. The scablands are wounds only partially healed – great wounds in the epidermis of soil with which Nature protects the underlying rock…[]…The region is unique: let the observer take wings of the morning to the uttermost parts of the earth: he will nowhere find its likeness.”

So wrote geologist J. Harlen Bretz (1882-1981) in 1928, as the origin of the mentioned channeled scablands of the Columbia Plateau were still a mystery.

In 1838 Reverend Samuel Parker explained the scablands as the fossil valleys of the Columbia River and in 1882 Lieutenant T.W. Symons (who lead a team of topographers into the region) proposed that the Columbia River changed direction in a remote past, blocked by a large dam of ice, nowadays disappeared. The fluvial or glacial origin of the scablands was supported by various evidence preserved in the landscape – deep incised gorges and eroded bedrock, cliffs polished by running water, hanging valleys and widespread erratic boulders.

However the extant of the scablands could only be explained by a very large flood event. Bretz proposed in 1919 and subsequent years (1923 paper – 1925 paper) that the water came from the interior of the North American continent and followed the path of the Late Pleistocene Columbia River, however significantly eroding and deepening the former riverbed. Bretz imagined two scenarios: After the Ice Age the Laurentian Ice Shield suddenly collapsed, releasing large quantities of meltwater, or a volcanic eruption triggered a catastrophic “jokulloup” (jokulhlaup). There was however no evidence for a Late Pleistocene/early Holocene volcanic activity in the area of the Canadian Shield and therefore the first scenario was considered more plausible.

This hypothesis arouse much interest and skepticism in the scientific community and in January 1927 the meeting of the Geological Society of Washington discussed the “Channeled Scabland and the Spokane Flood“. The main criticism focused on the hypothetical short interval involved in the formation of the scablands and the estimated amount of water required, not explainable by the proposed mechanisms.

Some geological observations published by geologist Joseph T. Pardee (1871-1960) years earlier helped to solve this dilemma. Pardee had already mapped the outlines of a gigantic glacial lake extending from Washington to Idaho and Montana, which he named Lake Missoula (in fact there were various lakes, the two largest being Lake Missoula and Lake Columbia/Spokane). Meltwater of the Laurentian Ice Shield accumulated maybe for centuries in these lakes, until the dam of ice broke releasing the stored water during an almost biblical flood.

Bretz continued to collect geologic evidence for these megafloods, but only after 1960-1970 the flood hypothesis became finally accepted by the scientific community. In the same decade new evidence for megafloods – even larger than the catastrophic floods forming the scablands – came from an unexpected place.

Today Mars is a dry, cold desert and water exists only in solid form in the polar ice caps or hidden below the surface of gigantic rock glaciers. In 1964 the space probe Mariner 4 send to earth the first images of the surface – later probes provided even more and detailed images of the Martian landscape. Surprisingly many features found on Mars resembled the landscapes formed by flowing water – like canyons, riverbeds, deltas, waterfalls and even ancient terraces, which on earth form along the shores of a lake.

Various hypotheses to explain the former presence of such large quantities of water on Mars exist. One idea suggests that during the slow cooling of the planet billions of years ago liquid water became entrapped in the forming permafrost. From time to time these pockets of water were squeezed out from the underground, forming gigantic, however temporary rivers. Alternative hypotheses suggest episodic volcanic activity, also in the recent past. During these events volcanoes provided large quantities of carbon dioxide, which triggered a greenhouse climate, lasting however only some millions of years. Finally climatic models suggest that the orbital variations of Mars can induce ice ages and warmer interglacial episodes. In these periods the steady increase of the solar insolation releases large quantities of carbon dioxide from the permafrost, the resulting greenhouse effects accelerated the warming effect.

However to test these various hypothesis the images taken from orbit of the planet were not sufficient, it was necessary to investigate the rocks of Mars.

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