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.