エーゲ海で大噴火

Ｎａｔｉｏｎａｌ　Ｇｅｏｇｒａｐｈｉｃ　より

ギリシャ、エーゲ海に浮かぶセラ島近海で大規模な海底地質調査が行われたそうです。ギリシャとアメリカの調査隊がナショナル・ジオグラフィックの協力を得て、サントリニ火山のあるセラ島周辺の海底地形を調べました。その結果、火山による地層が３０ｍの深さもあり、３０ｋｍ離れた地点でも深い堆積が確認されたそうです。西暦１８８３年に噴火したインドネシアのクラカタオとの対比などで相当の津波が予想され、周辺の人々に影響を与えたことでしょう。ギリシャ神話の神々の戦いやいわゆるアトランティスの伝説などはこの事件に由来しているのではないかと言われています。エーゲ海で約３６００年前に起きたサントリニ火山島での噴火がクレタ島で栄えたミノア文明の滅亡のきっかけを作ったと考えられています。

“Atlantis” Eruption Twice as Big as Previously Believed, Study Suggests
Richard A. Lovett
for National Geographic News

August 23, 2006

A volcanic eruption that may have inspired the myth of Atlantis was up to twice as large as previously believed, according to an international team of scientists.

The eruption occurred 3,600 years ago on the Santorini archipelago, whose largest island is Thera. Santorini is located in the Aegean Sea about 125 miles (200 kilometers) southeast of modern-day Greece

The massive explosion may have destroyed the Minoan civilization based on nearby Crete.

Writing in this week’s issue of the journal Eos, a team of Greek and U.S. researchers estimate that the volcano released 14 cubic miles (60 cubic kilometers) of magma—six times more than the infamous 1883 eruption of Krakatau (Krakatoa).

Only one eruption in human history is believed to have been larger: an 1815 explosion of Tambora, in Indonesia, which released 24 cubic miles (100 cubic kilometers) of magma.

(Related story: “‘Lost Kingdom’ Discovered on Volcanic Island in Indonesia [February 27, 2006].)

The researchers, partially funded by the National Geographic Society, obtained the new data by conducting the first seismic survey of the seabed near Santorini. (National Geographic News is part of the National Geographic Society.)

Previously, scientists had been forced to guess the size of the eruption based on ash deposits found in Turkey, Crete, Egypt, and the Black Sea.

A Hundred Feet Thick

Using techniques similar to those employed by oil companies to search for offshore deposits, the research team found a ring of volcanic deposits extending all the way around the Santorini archipelago.

The deposits averaged 100 feet (30 meters) thick and extended about 19 miles (30 kilometers) in all directions, says Haraldur Sigurdsson, a volcanologist at the University of Rhode Island in Kingston, who led the research.

During the eruption, the material that formed the deposits would have plunged into the sea as pyroclastic flows—hot, fast-moving mixtures of gas, ash, and molten rock. As these hit the water, they would have kicked up massive tsunamis.

“In a very similar setting, [the milder] Krakatau produced 100-foot [30-meter] tsunami waves,” Sigurdsson said.

Other pyroclastic flows would have been comprised of pumice—a frothy rock so light it floats.

These flows, known as overwater flows, would have zoomed across the sea in scalding waves of debris, eventually hitting land many miles away.

An overwater flow from Krakatau killed more than a thousand people on the coast of Sumatra, 25 miles away from the site of the eruption.

The devastation caused by Santorini—once a single island—would have been far worse.

“We have to scale the effects of both the tsunami and overwater pyroclastic flows to the Santorini eruption,” Sigurdsson said.

His team, he adds, will soon begin studies in Crete and western Turkey looking for the remnants from such flows.

Jelle Zeilinga de Boer, an emeritus professor of geology at Wesleyan University in Middletown, Connecticut, notes that the strength of the eruption also depends on its duration.

“We don’t know whether this came out in one flow or a number,” he said.

There is some archaeological evidence, he adds, that people returned to the devastated area and started rebuilding, only to be blasted anew by the next round of activity.

Massive Destruction

Whether it occurred in one large blast or in a series of smaller events, the eruption produced massive devastation.

In his book Volcanoes in Human History, de Boer links the eruption to the demise of the Minoan civilization.

The seafaring Minoan culture was based on Crete, which is only a few dozen miles from Thera. At the time of the eruption, they dominated that part of the ancient Mediterranean.

When Thera erupted, the Minoans would have been clobbered by tsunamis, overwater pyroclastic flows, and fires from oil lamps knocked over by the eruption’s shockwave.

Famine, plague, and a destruction of the Minoans’ shipping economy would also have followed, de Boer says.

The eruption may also have had an enormous impact on Mediterranean mythology.

“I have no doubt that every myth is based on some event, and so is the myth of Atlantis,” the University of Rhode Island’s Sigurdsson said. “An event of this magnitude must have left its imprint.”

Sigurdsson also sees traces of Santorini in a Greek poem called the Theogony, composed by Hesiod about 800 years after the eruption.

The poem describes an epic battle between giants and the Greek gods and includes imagery of a great battle far out at sea.

Hesiod must have picked up the story as folklore handed down from survivors close enough to see the event but not close enough to know what happened, Siggurdsson says.

“He uses all the terminology one would use in describing an eruption,” he said. “The people who lived close enough to see that it was a volcano were all killed. [The rest] could only describe it in supernatural terms.”

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引用元：http://news.nationalgeographic.com/news/2006/08/060823-thera-volcano.html