地質學家發現第八大陸：西蘭大陸

原文網址：http://www.nature.com/news/geologists-spy-an-eighth-continent-zealandia-1.21503

地質學家發現第八大陸：西蘭大陸

研究人員主張這座幾乎沉於海中的世界應該跟非洲、澳洲和其他大陸齊名

原作：Alexandra Witze

在西南太平洋的波濤之下坐落著一塊大多區域不為人所見的陸地――稱為「西蘭大陸」――地質學家表示其有資格成為一座大陸。

紐西蘭、澳洲和新喀里多尼亞的科學家組成的團隊在三月/四月號的《今日美國地質學會》(GSA Today)中提出，這個廣達500萬平方公里，涵蓋紐西蘭以及新喀里多尼亞的區域，經由地球物理數據顯示其為一片完整的大陸地殼，且從地質角度上來說跟澳洲有所區別。

「如果可以拔掉全球海洋底下的塞子，那麼許久以前應該就會承認西蘭大陸的地位了。」主持研究團隊，紐西蘭達尼丁地質與核子科學研究所的地質學家Nick Mortimer說。

然而，國際上並沒有一個組織負責正式公告大陸是哪幾座，因此研究人員得冀望他們的同僚中有夠多人同樣認可這座陸塊。否則他們的提案只能流於理論層面的願景，無法從根本改寫每個孩子在地理課上都會學到的內容。

「研究結果促使我們重新思考在地質上大陸地塊的現有定義，應該可以適用的範圍有多廣。」紐西蘭地質與核子科學研究所下哈特分部的礦物地質學家Patricia Durance表示。

並非混雜而成

Mortimer和他的同事十多年來在演講、科普文章和書籍上推廣西蘭大陸存在的證據，這篇最新論文則是彙整了他們目前為止在學術上的研究成果。在此他們發表西蘭大陸大約是從1億年前開始從岡瓦那超大陸分裂出來。

這次分裂導致西蘭大陸獨立出來，卻也讓它的地殼受到拉張而變薄，造成西蘭大陸逐漸下沉，最終使其大部分區域皆位於水中。今日，西蘭大陸位在水面以上的部分僅占總面積的6%，像是紐西蘭和新喀里多尼亞。

Mortimer表示由地球重力場繪製而成的地圖清楚顯示西蘭大陸在地理上為一個整體，從澳洲東北外海一路往東南方延伸並經過紐西蘭諸島。海床岩石的採樣指出西蘭大陸是由密度低的大陸地殼組成，而非組成附近海底高原的暗色火山岩。西蘭大陸在結構上看起來相當完整，並不是由不同大陸地殼碎片拼湊而成。

目前對於「大陸」還沒有一個廣為接受的定義，地理學家和地質學家在此問題上也有所分歧 (在地理上，歐洲和亞洲被視為不同的大陸；但地質學家卻把它們視作同一塊歐亞大陸) 。「這篇文章帶來的主要效益之一是，吸引人們去關注像『大陸』如此基本的用語卻有許多反覆且不一致的使用方式。」加拿大安蒂崗尼希，聖弗朗西斯澤維爾大學的地質學家Brendan Murphy說。

要讓西蘭大陸跟歐亞、非洲、南極洲、澳洲、南美和北美齊名，成為大眾認可的名字勢必會面臨一場苦戰。「主張西蘭大陸是座新大陸跟集郵有幾分相似，」澳洲墨爾本，蒙納許大學的地質學家Peter Cawood說，「有這麼重要嗎？」

Mortimer表示無論如何，西蘭大陸的研究應能幫助生物地理學家更加瞭解紐西蘭的動植物特有種如何出現，也可以促進地質學家去理解大陸地殼的重塑過程。

Geologists spy an eighth continent: Zealandia

This mostly submerged world should be recognized alongside Africa, Australia and others, argue some researchers.

Beneath the waves in the southwest Pacific Ocean lies a mostly hidden realm — dubbed Zealandia — that deserves to be called a continent, geologists say.

Geophysical data suggest that a region spanning 5 million square kilometres, which includes New Zealand and New Caledonia, is a single, intact piece of continental crust and is geologically separate from Australia, a team of scientists from New Zealand, Australia and New Caledonia argue in the March/April issue of GSA Today¹. (see 'Hidden crust')

“If you could pull the plug on the world’s oceans, then Zealandia would probably long ago have been recognized as a continent,” says team leader Nick Mortimer, a geologist at GNS Science in Dunedin, New Zealand.

However, there is no international body in charge of designating official continents, and so the researchers must hope that enough of their colleagues agree to recognize the landmass. Otherwise, their proposal could remain more of a theoretical wish than a radical reshaping of what every child has to learn in geography class.

“The results are pushing us to rethink how broadly we can or should apply the established definition of geological continental landmasses,” says Patricia Durance, a mineral geologist at the GNS Science office in Lower Hutt, New Zealand.

Not a mash-up

Mortimer and his colleagues have been making the case for Zealandia for more than a decade, in talks, popular articles and books; the latest paper is their most technical synthesis yet. In it, they report that Zealandia began to peel away from the supercontinent of Gondwana starting about 100 million years ago.

The rift gave Zealandia its independence, but it also pulled and thinned the crust, causing the area to sink, and dooming most of it to a watery existence. Today, only about 6% of it remains above water, as New Zealand and New Caledonia.

Satellite maps made using Earth’s gravitational field clearly show that Zealandia is a coherent geographical feature stretching from near Australia’s northeastern coast well past the islands of New Zealand, Mortimer says. Sea-floor samples reveal that Zealandia consists of light continental crust and not the dark volcanic rocks that make up nearby underwater plateaus. The area seems to be structurally intact, rather than a mash-up of different continental-crust fragments.

There is no widely accepted definition of a continent, and geographers and geologists differ on the question. (Geographically, Europe and Asia are considered separate continents, whereas geologists consider them the single landmass of Eurasia.) “One of the main benefits of this article is that it draws attention to the arbitrary and inconsistent use of such a fundamental term as continent,” says Brendan Murphy, a geologist at St. Francis Xavier University in Antigonish, Canada.

Zealandia will face an uphill battle in garnering the same popular name recognition as Eurasia, Africa, Antarctica, Australia and North and South America. “Claiming that Zealandia is a continent is a bit like stamp collecting,” says Peter Cawood, a geologist at Monash University in Melbourne, Australia. “So what?”

Whatever it is called, Mortimer says, studies of Zealandia should help biogeographers to better understand how New Zealand’s endemic plants and animals arose — and give geologists a boost in learning how continental crust can be reshaped.

引用自：Nature doi:10.1038/nature.2017.21503