湯加火山噴發重塑了太平洋海底

2週前
湯加火山噴發重塑了太平洋海底

"海洋地質學家和Niwa項目主任凱文-麥凱博士說:"你可以把它看作是'獵槍式爆炸',直達天空。"他告訴BBC新聞:"其中一些物質甚至超出了平流層,進入了中間層(57公里的高度)--這是人類歷史上最高的噴發柱記錄。

這種運輸採取了熱碎石密度流的形式,這是翻滾的、炙熱的岩石的雪崩。在水中,它們的灼熱會將它們包裹在一個無摩擦的蒸汽墊中,它們可以簡單地在上面以非常高的速度奔跑和運行。

這就解釋了,例如,連接湯加和全球互聯網的海底電纜的損失。儘管在亨加-湯加以南50公里處,在海底的一座大山之外,這條數據鏈的一大部分被切斷了。

"在你有這些水流的地方,今天沒有任何東西生活在那裡。它就像一個離火山70公里的沙漠,"Mackay博士說。"然而,令人驚訝的是,就在火山邊緣下,在避開這些密度流的地方,你確實發現了生命。你發現了海綿體。他們躲過了一顆子彈。"

Niwa研究小組說,基本上有四種方式使水移位以產生這些海嘯:通過密度流將水推開;通過噴發的爆炸力也推到水;作為破火山口地面急劇塌陷的結果(它下降了700米);以及通過大氣爆炸的壓力波作用於海面。

一個很好的例子是襲擊湯加主島湯加塔普的最大波浪,位於亨加-湯加以南65公里處。這發生在第一次重大噴發爆炸後45分鐘。一道數米高的水牆衝過卡諾庫波盧半島,在這個過程中摧毀了海灘度假村。

"對於當地的大浪--為了正確理解它們,我相信你也必須有這種大氣耦合,"她解釋說。"我們有一個巨大的壓力異常,它本身就會產生海嘯。所以,當你已經有了波浪,那麼你只是在給它們增加能量。"

第二階段,在山的正上方,交給了英國機器人船USV Maxlimer。由Sea-Kit國際公司從16000公里外的英國Tollesbury的控制室操作,這艘沒有船員的船能夠識別正在進行的火山活動,儘管是相對低調的。該船通過追蹤火山口中持續存在的玻璃狀火山灰層回到它的源頭--水下約200米處的一個新噴口錐。


"You can think of it as 'a shotgun blast' straight up into the sky," said marine geologist and Niwa project director Dr Kevin Mackay. "Some of that material went beyond even the stratosphere into the mesosphere (57km in altitude) - the highest recorded eruption column in human history," he told BBC News.

This transport took the form of pyroclastic density currents, which are avalanches of tumbling, scorching rock. In water, their searing heat would have enveloped them in a frictionless steam cushion on which they could simply run and run at very high speed.

This explains, for example, the loss of the submarine cable connecting Tonga to the global internet. A large section was cut out of this data link despite lying 50km to the south of Hunga-Tonga and beyond a large hill on the seafloor.

"Where you had these flows, there is nothing living there today. It's like a desert 70km from the volcano," Dr Mackay said. "And yet, amazingly, just under the rim of the volcano, in places that avoided these density currents, you do find life. You find sponges. They dodged a bullet."

The Niwa team says there were essentially four ways water was displaced to generate these tsunamis: by the density flows pushing the water out of the way; through the explosive force of the eruption also pushing on the water; as a result of the dramatic collapse of the caldera floor (it's dropped by 700m); and by pressure waves from the atmospheric blast acting on the sea surface.

A good example is the biggest wave to hit Tonga's main island, Tongatapu, 65km to the south of Hunga-Tonga. This occurred just over 45 minutes after the first major eruptive blast. A wall of water several metres high washed over the Kanokupolu peninsula, destroying beach resorts in the process.

"For the big local waves - in order to understand them correctly, I believe you also have to have this atmospheric coupling," she explained. "We had a huge pressure anomaly that on its own would have generated a tsunami. So, when you've already got waves, then you're just adding energy to them."

The second stage, directly above the mountain, was given over to the British robot boat USV Maxlimer. Operated by Sea-Kit International from a control room 16,000km away in Tollesbury, UK, this uncrewed vehicle was able to identify ongoing, albeit relatively subdued, volcanic activity. The boat did this by tracing a persistent layer of glassy ash in the caldera back to its source - a new vent cone some 200m under water.

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