Some Christians believe Noah's flood was a local event rather than a global flood.
What physical or geological evidence has been given by such Christians to support their position?
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There are two variants of the "local flood" view. One variant is that Genesis 6-9 refers to an actual flood that was large-scale but not global, and the other is that the Noah story is fiction, but derived from the memory of a large flood in the past.
Both variants point first to evidence found in the geological column. In particular:
1) All of the early contributors to geology were Christians who believed in a global flood before they began studying rock columns. Buffon, Hutton, Lyell, Agassiz, and Kelvin all became convinced after seeing the rock layers that a single global flood could not have produced them.
2) Some formations, such as the Grand Canyon, include layers of desert sandstone in between layers of marine sediments. Yet YECs claim the entire formation was laid down in one global flood. Grey Neyman of Old Earth Ministries explains the problem:
The young earth scientist would have to explain how the water receded, then the sandstone formed, then the water came back and deposited the other layers. However, in the Biblical Flood account, the waters rose, then fell. There were no cyclic water levels, nor was there a massive amount of time during the flood for a desert environment to create a 315-foot thick rock layer.
3) Other areas, such as the Gulf of Mexico, have thick layers of salt. Salt deposits are created when water evaporates. Gregg Davidson and Ken Wolgemuth, writing for BioLogos, explain why this is a problem for flood geology:
One might argue that the waters from the Flood could have evaporated to leave behind the salt deposits we see today, but there is a serious problem. The thousands of feet of sediment on top of the salt is also said to be from the Flood, meaning the flood waters cannot have evaporated to produce the salt and still be present and violent enough to transport thousands of feet of sediment to the same location. In other words, a single flood cannot be called upon to explain both the salt and the overlying sediment.
4) Among the deposits are layers of igneous rock, formed by volcanic eruptions. When volcanoes erupt, they release greenhouse gases. The number of eruptions necessary to produce all the igneous layers found in the geological column, if they all happened within the same year, would produce enough CO2 to kill Noah and his family. Glenn Morton, writing for Old Earth Ministries, has the details:
How does this relate to the present atmosphere? Currently we are approaching 400 parts per million (ppm) CO2 in the atmosphere, yet the YEC scenario would produce an atmosphere that had AS A MINIMUM a CO2 level of 58,615 parts per million. Scientists are worried about a 600 ppm CO2 world next century, the YEC post flood world would create such a hot climate that all life would be destroyed. Yet amazingly, Creationists like Austin, Baumgardner, Wise, Snelling, Vardiman, Humphreys and Oard think that the post flood world would be glacially cold.
5) Industrial geologists looking for oil deposits use temperature differences in boreholes to determine the age of the sediment they're drilling through. It's kind of complicated, but Willy Fjeldskaar, writing for Age of Rocks, explains:
The temperature of the sediments is caused by the heat flow from the inner regions of the Earth. If you put a kettle with water on a hot plate, it will take some time for the water to be heated. A certain amount of time is also needed to heat sediments from the time they were deposited until today. The amount of time needed depends on the flow of heat from the inner regions of the Earth and on the thermal properties of the sediments. Both the heat flow and the thermal properties are relatively well known.
As the heat radiates up from the earth's core, the rocks in lower layers retain a certain percentage. The longer the rocks have been buried, the more heat they will have accumulated. In the short time span proposed by YECs, there's simply not enough time for the sediments to have reached the temperatures geologists find in the boreholes:
We have seen that it is not possible to explain the measured temperature in these sediments on the assumption that the sediments are only some thousands of years old. Using measured properties of the sediments and conventional values for the Earth’s heat flow, we realize that the sediments must be considerably older than 1 million years.
6) The total amount of water on and in the earth is not nearly enough to produce a global flood. Hugh Ross of Reasons to Believe explains the problem:
The four different Hebrew verbs used in Genesis 8:1-8 to describe the receding of the flood waters indicate that these waters returned to their original sources. In other words, the waters of the flood are still to be found within the aquifers and troposphere and oceans of planet Earth. Since the total water content of the earth is only 22 percent of what would be needed for a global flood, it appears that the Genesis flood could not have been global.
That's a sample of the evidence against a global flood. Some Christians take the idea of a local flood further, attempting to find evidence of a large local flood in the geological record.
Greg Neyman of Old Earth Ministries lays out the parameters:
The site of the flood would have to meet three requirements. First, it would have to be capable of containing the waters of the flood. In order to do this, we need a basin, with no outlet to the sea. If there were an outlet, the water would simply run out of the area.
Second, the flood would have to fit the parameters mentioned in the Bible. The source of the waters is not in question. The only point that matters here is that Noah believed that the world was flooded, and that all the mountains were covered with water.
Neyman calculates that Noah, standing on the deck of the ark, would have been able to see a mountain 95 miles away. That gives us a ballpark estimate of how much area the flood would have to include to cover the highest mountains.
The final requirement is this: Does the proposed location agree with the geography mentioned in the Genesis account? It would have to flood the areas populated by mankind. We don't have many clues as to the extent of the geographic area. However, it would appear to include the area around the Garden of Eden, and east of the Garden.
There is some evidence of massive flooding in the Middle East in the distant past. One conjecture which has received a lot of attention recently is the Black Sea deluge hypothesis. Geologists have found some evidence of a massive flood following an inrush of water from the Mediterranean about 7600 years ago, causing the Black Sea to overflow.
There's some evidence that glacial melting caused the Caspian Sea to flood the surrounding area somewhere between 9000 and 17,000 years ago.
Hugh Ross favors the Black Sea hypothesis, while Neyman favors the Caspian. Critics (including YECs) point out that there is no clear evidence that either flood could have produced enough water to cover the mountains and wipe out all of human civilization.
Here's an interesting article I came across which discusses several ancient myths and likely corresponding floods including the Black Sea event: Biblical-Type Floods Are Real, and They're Absolutely Enormous.
Were early farmers in the area forced to flee as their world disappeared underwater? Archaeologists found the rising waters coincided with the onset of the initial migration of farming cultures into Europe and the floodplains of Mesopotamia. Wherever they came from, the first farmers arrived in southern Mesopotamia shortly after the filling of the Black Sea. Did they bring the story of a great flood that destroyed their world?
Which further links to: Geologists Link Black Sea Deluge To Farming's Rise.
An international team of geologists and oceanographers has reconstructed the history of this catastrophic flood from data gathered by a Russian research ship in 1993. Seismic soundings and sediment cores revealed traces of the sea's former shorelines, showing an abrupt 500-foot rise in water levels. Radiocarbon dating of the transition from freshwater to marine organisms in the cores put the time of the event about 7,500 years ago, or 5500 B.C.
The Black Sea deluge hypothesis suggests the Black Sea may have suddenly and violently flooded due to it being below sea level at one point, thus possibly providing material for the story of Atrahasis, Gilgamesh, and the latter story of Noah.
Naturally if you lived in a little hut on the shores of the Black sea when this may have happened it would not be completely irrational for you believe the whole world has flooded.
Archaeologists argue over whether it happened or not.