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

發現出露於地表的斷層中最大者

原文網址：www.sciencedaily.com/releases/2016/11/161128132928.htm

發現出露於地表的斷層中最大者

地質學家首度能觀察並描述印尼東部的班達滑脫斷層(Banda Detachment fault)，並研究它的形成過程。

來自澳洲國立大學的研究第一作者Jonathan Pownall博士表示，這項發現有助於研究人員預測此區將來受到海嘯侵襲的風險高低。此區為火環(Ring of Fire)的一部分，也就是太平洋海盆周遭為人所知地震與火山爆發頻繁發生的地區。

「人們知道這處深淵的存在已有90年之久，但直到現在才有人可以解釋它的深度為何如此驚人。」Pownall博士說。

「我們的研究發現印尼東岸班達海下方7公里深的深淵形成原因，是海底沿著可能為地球上出露於地表的最大已知斷層面伸張而造成。」

國立澳洲大學和倫敦大學哈洛威學院的地質學家分析了班達海床的高解析度海底地形圖，發現位於海床的岩石被數百條互相平行的切痕劃開。

這些傷口顯示必定有塊比塔斯馬尼亞或比利時還大的地殼片段，沿著一道低角度的破裂面，也就是滑脫斷層(detachment fault)伸張了120公里而撕裂成現今所見的海底低地。

Pownall博士表示這條班達滑脫斷層出露在海床上的裂痕超過了60,000平方公里。

「這項發現有助於解釋地球海洋最深的區域之一何以達到如此驚人的深度。」他說。

同樣來自國立澳洲大學地球科學研究院的Gordon Lister教授表示，這是班達斷層首度能被研究人員觀察並且做出詳細紀錄。

「我們根據海底地形資料以及對此區域地質的相關知識，為這條斷層的存在做出詳細論述並命名為班達滑脫斷層。」Lister教授說。

Pownall博士表示在七月於印尼東部的航行中，他注意到此處特別顯著的地形跟地殼沿著斷層線伸張時會出現的地形之間彼此相符。

「我很震驚可以親眼見到我們假說中的斷層面，這次不是在電腦螢幕上，而是直接從海上探測出來。」Pownall博士說。

他說緊鄰斷層底部的岩石包含了源自於下方地函的岩石。

「這意味著必然發生了程度極大的拉張作用，使得海洋地殼變薄，在某些地方甚至被完全拉斷。」他說。

Pownall博士表示班達滑脫斷層的發現有助於科學家評估未來發生海嘯和地震的風險。

「班達滑脫斷層這類大型斷層滑移時會造成大地震。在海嘯發生風險極高的區域，對這些大型斷層有更深的認識是評估區域構造災害時的重要基礎。」他說。

Biggest exposed fault on Earth discovered

Geologists have for the first time seen and documented the Banda Detachment fault in eastern Indonesia and worked out how it formed.

Lead researcher Dr Jonathan Pownall from The Australian National University (ANU) said the find will help researchers assess dangers of future tsunamis in the area, which is part of the Ring of Fire -- an area around the Pacific Ocean basin known for earthquakes and volcanic eruptions.

"The abyss has been known for 90 years but until now no one has been able to explain how it got so deep," Dr Pownall said.

"Our research found that a 7 km-deep abyss beneath the Banda Sea off eastern Indonesia was formed by extension along what might be Earth's largest-identified exposed fault plane."

By analysing high-resolution maps of the Banda Sea floor, geologists from ANU and Royal Holloway University of London found the rocks flooring the seas are cut by hundreds of straight parallel scars.

These wounds show that a piece of crust bigger than Belgium or Tasmania must have been ripped apart by 120 km of extension along a low-angle crack, or detachment fault, to form the present-day ocean-floor depression.

Dr Pownall said this fault, the Banda Detachment, represents a rip in the ocean floor exposed over 60,000 square kilometres.

The discovery will help explain how one of Earth's deepest sea areas became so deep," he said.

Professor Gordon Lister also from the ANU Research School of Earth Sciences said this was the first time the fault has been seen and documented by researchers.

"We had made a good argument for the existence of this fault we named the Banda Detachment based on the bathymetry data and on knowledge of the regional geology," said Professor Lister.

Dr Pownall said he was on a boat journey in eastern Indonesia in July when he noticed the prominent landforms consistent with surface extensions of the fault line.

"I was stunned to see the hypothesised fault plane, this time not on a computer screen, but poking above the waves," said Dr Pownall.

He said rocks immediately below the fault include those brought up from the mantle.

"This demonstrates the extreme amount of extension that must have taken place as the oceanic crust was thinned, in some places to zero," he said.

Dr Pownall also said the discovery of the Banda Detachment fault would help assesses dangers of future tsunamis and earthquakes.

"In a region of extreme tsunami risk, knowledge of major faults such as the Banda Detachment, which could make big earthquakes when they slip, is fundamental to being able to properly assess tectonic hazards," he said.

原始論文：Jonathan M. Pownall, Robert Hall, Gordon S. Lister. Rolling open Earth’s deepest forearc basin. Geology, 2016; 44 (11): 947 DOI: 10.1130/G38051.1

引用自：Australian National University. "Biggest exposed fault on Earth discovered." ScienceDaily. ScienceDaily, 28 November 2016. 

地質學家發現板塊如何下沉

原始網址：www.sciencedaily.com/releases/2016/11/161114124949.htm

地質學家發現板塊如何下沉

在刊登於美國國家科學院院刊(Proceedings of the National Academy of Sciences, PNAS)的論文中，聖路易斯大學的研究人員發表了關於哪些因素會造成地球板塊下沉的新說法。

聖路易斯大學的地球與大氣科學博士John Encarnacion及研究生Timothy Keenan是地球構造和硬岩地質學的專家。他們同時利用地球化學、地質年代學，結合實地野外考察來研究板塊構造的運動。

「就定義來說，板塊是具有剛性(rigidity)的。意味著其相當堅硬並且會以整體為單位來運動。因此，我們正跟腳下踩的北美板塊以每年約一英吋的速度一同往大略西邊的方向移動。」Encarnacion解釋。「但當思考是什麼原因造成了板塊移動，我把板塊比喻成浸在池水中的濕毛巾。大部分的板塊會移動是因為它們正往地球內部下沉，就像是鋪在水面的毛巾開始沉沒時會將其餘部分拖往水中一樣。」

板塊移動的平均速度大約是每年1至2英吋。最快的板塊以每年4英吋左右的速度移動，而最慢的則幾乎文風不動。由於板塊運動是地震發生的主因，因此地震學家和地質學家皆在研究板塊運動的每分細節，以對未來可能發生的地震作出更精準的預測。

「每當科學家得出意料之外的事物實際上真的有可能發生時，就能讓我們更加貼近地球運作的真實方式。 」 Encarnacion表示。「而我們對地球大尺度作用的圖像刻畫得越精細，就能幫助我們更加了解地震和火山作用的運行。此外，由於大尺度板塊運動會生成並影響礦脈的分布，因此這也能讓我們知道礦脈的起源與所在地。」

板塊運動還會以其他方式影響我們的生活：近日有人發表由於板塊運動的緣故，澳洲地圖必須要重新繪製。由於澳洲往北移動的速度相對來說快上許多，因此經過數十年後它會位移數十公分，造成GPS定位結果嚴重失準。

隱沒作用(subduction)係指板塊沉入地函的作用。它是地球板塊運動的基礎作用力之一，同時也是板塊發生移動的主要成因；然而，新生隱沒帶會如何以及在哪形成，仍然是項眾說紛紜的議題。

聖路易斯大學的地質學家於野外研究岩石，並採取樣本回實驗室進行更全面的分析來進行這項研究。

他們的工作包括了繪製地質圖：觀察並辨認岩石後，將它們給繪製在地圖上以得出這些岩石如何形成，在形成之後又遭受了何種變故。研究人員定年這些岩石樣品並探討其化學性質，以詳細得知這些遠古岩石形成時的環境條件，像是某座火成岩體是形成於夏威夷這類的火山島，亦或是深海海床。

在此研究中，Keenan和Encarnacion前往菲律賓研究此處的板塊。他們發現在兩個板塊互相遠離的分離板塊邊界(divergent plate boundary)，會迅速地因外力轉變成聚合板塊邊界(convergent boundary)，使得其中一座板塊終將開始隱沒。

這個結果令他們相當驚訝，因為分離型邊界的板塊組成物質較為軟弱，且浮力也較大使它們較不易發生隱沒作用。這項研究的發現主張，於分離板塊邊界浮力較大且軟弱的板塊物質能被外力作用而往彼此聚合，直到較古老且密度較大的板塊物質終於進入初生隱沒帶之後，後續過程便能不倚靠外力而繼續維持運作。

「我們認為我們研究的隱沒帶實際上最初是因為印度碰撞亞洲而連帶形成。印度過去曾經跟亞洲分隔兩地，但它慢慢地往北漂移最後終於跟亞洲發生碰撞。此次碰撞事件將一大塊亞洲往東南方擠出。我們認為這股推力一路傳到海洋並讓新的隱沒帶於焉形成。」

他們的發現有助於建立用來瞭解板塊如何開始下沉的模型：「在板塊互相遠離之處，板塊可以被推往彼此而讓隱沒作用發生。」

聖路易斯大學的研究人員現在想要得知他們的模型是否能適用在其他的板塊之中。

「我們認為發生在菲律賓這種由外力啟動的隱沒帶有多常見？」Encarnacion提出。「我想看看其他對古代隱沒帶的研究成果，來觀察我們的模型是否也能適用於這些隱沒帶。」

這項研究的其他研究人員包括Robert Buchwaldt, Dan Fernandez, James Mattinson, Christine Rasoazanamparany和P. Benjamin Luetkemeyer。

聖路易斯大學地球與大氣科學系結合豐富的教學資源以及實地考察經驗，在物質科學的不同領域中，包括地震學、水文學、地球化學、氣象學、環境科學，以及當代與古代氣候變遷上的研究皆享譽國際。學生同樣擁有機會直接參與教職員的研究工作，且隨著跟公家與私人機關的合作網路日趨密切，學生也能尋求在不同機關的實習機會。

研究中心包括地震中心、降水系統合作研究所、全球地球動力學計畫、環境科學中心以及量子氣象站。結合專業課程與世界級的研究讓學生有絕佳的機會去探索自身興趣，並讓他們預先瞭解畢業後可能從事的各種工作。

Geologists discover how a tectonic plate sank

In a paper published in Proceedings of the National Academy of Sciences (PNAS) Saint Louis University researchers report new information about conditions that can cause Earth's tectonic plates to sink.

John Encarnacion, Ph.D., professor of earth and atmospheric sciences at SLU, and Timothy Keenan, a graduate student, are experts in tectonics and hard rock geology, and use geochemistry and geochronology coupled with field observations to study tectonic plate movement

"A plate, by definition, has a rigidity to it. It is stiff and behaves as a unit. We are on the North American Plate and so we're moving roughly westward together about an inch a year," Encarnacion said. "But when I think about what causes most plates to move, I think about a wet towel in a pool. Most plates are moving because they are sinking into Earth like a towel laid down on a pool will start to sink dragging the rest of the towel down into the water."

Plates move, on average, an inch or two a year. The fastest plate moves at about four inches a year and the slowest isn't moving much at all. Plate motions are the main cause of earthquakes, and seismologists and geologists study the details of plate motions to make more accurate predictions of their likelihood.

"Whenever scientists can show how something that is unexpected might have actually happened, it helps to paint a more accurate picture of how Earth behaves," Encarnacion said. "And a more accurate picture of large-scale Earth processes can help us better understand earthquakes and volcanoes, as well as the origin and locations of mineral deposits, many of which are the effects and products of large-scale plate motions."

Plate movement affects our lives in other ways, too: It recently was reported that Australia needs to redraw its maps due to plate motion. Australia is moving relatively quickly northwards, and so over many decades it has traveled several feet, causing GPS locations to be significantly misaligned.

Subduction, the process by which tectonic plates sink into Earth's mantle, is a fundamental tectonic process on earth, and yet the question of where and how new subduction zones form remains a matter of debate. Subduction is the main reason tectonic plates move.

The SLU geologists' research takes them out into the field to study rocks and sample them before taking them back to the lab to be studied in more detail.

Their work involves geological mapping: looking at rocks, identifying them, plotting them on a map and figuring out how they formed and what has happened to them after they form. Researchers date rock samples and look at their chemistry to learn about the specific conditions where an ancient rock formed, such as if a volcanic rock formed in a volcanic island like Hawaii or on the deep ocean floor.

In this study, Keenan and Encarnacion traveled to the Philippines to study plates in that region. They found that a divergent plate boundary, where two plates move apart, was forcefully and rapidly turned into a convergent boundary where one plate eventually began subducting.

This is surprising because although the plate material at a divergent boundary is weak, it is also buoyant and resists subduction. The research findings suggest that buoyant but weak plate material at a divergent boundary can be forced to converge until eventually older and denser plate material enters the nascent subduction zone, which then becomes self-sustaining.

"We think that the subduction zone we studied was actually forced to start because of the collision of India with Asia. India was once separated from Asia, but it slowly drifted northwards eventually colliding with Asia. The collision pushed out large chunks of Asia to the southeast. That push, we think, pushed all the way out into the ocean and triggered the start of a new subduction zone."

Their finding supports a new model for how plates can begin to sink: "Places where plates move apart can be pushed together to start subduction."

The SLU researchers now want to learn if their model applies to other tectonic plates.

"How common was this forced initiation of a subduction zone that we think happened in the Philippines?" Encarnacion said. "I would like to see work on other ancient subduction zones to see whether our model applies to them as well."

Other researchers on the study include Robert Buchwaldt, Dan Fernandez, James Mattinson, Christine Rasoazanamparany, and P. Benjamin Luetkemeyer.

Saint Louis University's Department of Earth and Atmospheric Sciences, combines strong classroom and field-based instruction with internationally recognized research across a broad spectrum of the physical sciences, including seismology, hydrology, geochemistry, meteorology, environmental science, and the study of modern and ancient climate change. Students also have the opportunity to work directly with faculty on their research and pursue internships through a growing network of contacts in the public and private sector.

Research centers include the Earthquake Center, the Cooperative Institute for Precipitation Systems, the Global Geodynamics Program, the Center for Environmental Sciences, and Quantum WeatherTM. The fusion of academic programs with world-class research provides students with an unparalleled opportunity to explore their interests and prepare for a wide variety of careers after graduation.

原始論文：Timothy E. Keenan, John Encarnación, Robert Buchwaldt, Dan Fernandez, James Mattinson, Christine Rasoazanamparany, P. Benjamin Luetkemeyer. Rapid conversion of an oceanic spreading center to a subduction zone inferred from high-precision geochronology. Proceedings of the National Academy of Sciences, 2016; 201609999 DOI: 10.1073/pnas.1609999113

引用自：Saint Louis University Medical Center. "Geologists discover how a tectonic plate sank." ScienceDaily. ScienceDaily, 14 November 2016.