Ancient Megaflood Made Britain an Island, Study Says - 地球科學討論

我在國家地理網站看到的文章
很有意思喔....以前都沒想過有這麼回事
http://news.nationalgeographic.com/news/2007/07/070718-britain-flood.html
圖片
http://news.nationalgeographic.com/news/bigphotos/25633558.html

Kate Ravilious
for National Geographic News

July 18, 2007
A flood of biblical proportions cut the British Isles off from mainland
Europe sometime between 450,000 and 200,000 years ago, according to a new
study.

The research, based on three-dimensional sonar mapping of the English
Channel, provides the strongest evidence yet that a catastrophic megaflood
broke a land bridge that once connected what is now Britain and France.

"It is probably one of the largest floods ever identified," said Phillip
Gibbard, a geographer at the University of Cambridge who wasn't involved in
the study.

At its peak, the flood would have discharged water at a rate of about 264
million gallons (a million cubic meters) a second, gushing at speeds of up to
62 miles (100 kilometers) an hour, the researchers say. This is roughly
equivalent to ten times the combined flow rate of all the rivers in the
world.

In addition to making Britain an island, the authors add, the huge flood had
wide-ranging environmental consequences.

For example, the gigantic pulse of freshwater entering the Atlantic Ocean
likely caused a period of climate cooling in the Northern Hemisphere, Gibbard
said.

"The introduction of ice and freshwater into an ocean drives climate
oscillations and causes marked cooling events," he explained.

The flood also marooned many animals and plants, so those species gradually
evolved into different forms than their mainland cousins.

And humans appear to have avoided the newly made island altogether, leaving
it unoccupied for over a hundred thousand years.

Crumbled Chalk

Researchers have long known that a narrow ridge of chalk once connected Dover
in southeast England to Calais in northwest France (see a map of France
showing Calais' proximity to Britain).

During the ice ages, when sea levels were low, the ridge held back a glacial
lake from inundating a large valley between the two regions.

But during warm interglacial periods, sea levels rose and the chalk ridge was
the only link.

At some point the ridge crumbled. Theories as to why have included river or
glacial erosion, tidal scraping, and—most controversial of all—a megaflood.

Now Sanjeev Gupta of Imperial College London and colleagues say they have
found the first concrete evidence to support the megaflood theory.

A 3-D map of part of the English Channel reveals features that could only
have been created by a massive flood, the team says.

"We have identified huge scours on the seafloor and streamlined islands,"
said Gupta, whose results will appear in tomorrow's issue of the journal
Nature.

These features are very unusual, he said, and have previously been found only
in regions where megafloods are known to have occurred.

Similar features exist, for example, in the Channeled Scabland in eastern
Washington State, which was deluged when the glacial Lake Missoula burst its
banks about 12,000 years ago.

Isolated Island

Based on their analysis, Gupta and colleagues say the most likely source of
all this water was a huge glacial lake sitting in what is now the southern
North Sea off the east coast of Britain.

The water was probably held back by the chalk ridge, and a small earthquake
could have caused the first few cracks to appear.

"Chalk is not very strong, and eventually the water probably just started to
over-spill," Gupta said.

Determining exactly when the megaflood took place is difficult.

But the divergence of plant and animal species between Britain and mainland
Europe suggest that the event must have occurred sometime between 450,000 and
200,000 years ago.

"We now need to drill into the sediments to get an accurate date," Gupta
said.

The great flood could help explain why Britain remained an uninhabited region
for a large chunk of the archaeological record.

"There seems to be a large gap in the evidence for human occupation [of
Britain] during cold and warm phases from about 180,000 until about 60,000
years ago," said Nicholas Ashton, an archaeologist at the British Museum in
London.

(Related: "Humans Sped to U.K. After Ice Age, Study Says" [November 3,
2003].)

When the climate was warm, sea level between the island and the mainland was
too high for humans to cross, Ashton said.

And during the much colder ice ages, humans could have crossed, but seem to
have preferred to live in sunnier regions such as modern-day Italy and Spain.

"It wasn't until 60,000 years ago," Ashton said, "that humans—late
Neanderthals—had the technological capabilities, such as more effective
clothing and shelter, to survive the cold conditions."

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紅紅的花開滿了木棉道~長長的街好像在燃燒
沈沈的夜徘徊在木棉道~輕輕的風吹過了樹梢
木棉道我怎能忘了~那是去年夏天的高潮~木棉道我怎能忘了~那是夢裡難忘的波濤
啊 愛情就像木棉道~季節過去就謝了
愛情就像那木棉道~蟬聲綿綿斷不了

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