地函柱引發板塊運動？

原文網址：www.sciencedaily.com/releases/2015/11/151111143225.htm

Plate tectonics thanks to plumes?

地函柱引發板塊運動？

"Knowing what a chicken looks like and what all the chickens before it looked like doesn't help us to understand the egg," says Taras Gerya. The ETH Professor of Geophysics uses this metaphor to address plate tectonics and the early history of the Earth. The Earth's lithosphere is divided into several plates that are in constant motion, and today's geologists have a good understanding of what drives these plate movements: heavier ocean plates are submerged beneath lighter continental plates along what are known as subduction zones. Once the movement has begun, it is perpetuated due to the weight of the dense subducting plate.

「知道一隻雞以及牠列祖列宗的長相並不能讓我們認識雞蛋的模樣。」Taras Gerya說。這名蘇黎世聯邦理工學院(ETH)地球物理學系的教授以此比喻板塊運動和地球早期的歷史。地球岩石圈分裂成好幾塊會不斷運動的板塊。現今的地質學家對驅動板塊移動的作用力已有相當瞭解：密度較高的海洋板塊沿著隱沒帶沉沒至較輕的大陸板塊之下。一旦此運動開始進行，隱沒板塊的重量便會使之維持下去。

But just as in the past, earth scientists still do not understand what triggered plate tectonics in the first place, nor how the first subduction zone was formed. A weak spot in the Earth's lithosphere was necessary in order for parts of the Earth's crust to begin their descent into the Earth's mantle. Was this weak spot caused by a gigantic meteorite that effectively smashed a hole in the Earth's lithosphere? Or did mantle convection forces shatter the lithosphere into moving parts?

但跟過去的地球科學家一樣，我們現在仍然不瞭解一開始是什麼觸發了板塊運動，而隱沒帶又是如何形成。我們所知的是地球岩石圈必然出現了一個軟弱區域，才能使部分的地球地殼開始沉入地函。這個軟弱區域會是因為一次巨型隕石撞擊，在地球岩石圈重重擊出一個大坑而形成嗎？或者是地函對流將岩石圈碎裂成好幾塊能自由移動的碎片？

Venus as a model

以金星作為模型

Gerya is not satisfied with any of these potential explanations. "It's not trivial to draw conclusions about what set the tectonic movements in motion," he says. The ETH professor therefore set out to find a new, plausible explanation.

Gerya對這些可能的解釋都不甚滿意。「得出是什麼開啟了能夠持續運作的板塊運動可是件至關重要的事。」他說。這位ETH的教授因此計畫要找出一套可行的新解釋。

Among other things, he found inspiration in studies about the surface of the planet Venus, which has never had plate tectonics. Gerya observed (and modelled) huge, crater-like circles (coronae) on Venus that may also have existed on the Earth's surface in the early period (Precambrian) of the Earth's history before plate tectonics even began. These structures could indicate that mantle plumes once rose from Venus' iron core to the outer layer, thus softening and weakening the planet's surface. Plumes form in the deep interior of the planet. They rise up to the lithosphere, bringing with them hot partially molten mantle material that causes the lithosphere to weaken and deform. Halted by the resistance of the hard lithosphere, the material begins to spread, taking on a mushroom-like shape.

除了其他方面之外，他也從關於金星地表的研究中得到了不少啟發。Gerya觀察到(並模擬)金星上有著像隕石坑般的巨大圓形構造(冕狀物，coronae)。由於這顆星球從未發生板塊運動，因此在板塊運動發生前的早期(前寒武紀)地球地表，也可能存在著類似構造。這種構造象徵著有地函柱(plume)從金星的鐵質地核往上升至星球外層，因而軟化並弱化了行星地表。地函柱在金星深處形成，往上移動而與岩石圈接觸。它們攜帶的炙熱部分熔融地函會弱化岩石圈並使其變形。在被堅硬的岩石圈阻擋之後，這些熱物質會往外擴散，而形成蘑菇狀的外觀。

Such plumes also likely existed in the Earth's interior and could have created the weaknesses in the Earth's lithosphere needed to initiate plate tectonics on Earth.

這種地函柱也可能會存在於地球內部，並形成啟動板塊運動所需，存在於岩石圈之內的軟弱區域。

Mantle plumes create weaknesses

地函柱創造軟弱區域

The ETH geophysicist worked with his team to develop new computer models that he then used to investigate this idea for the first time in high resolution and in 3D. The corresponding publication has recently been published in Nature.

ETH的地球物理學家與他的團隊發展出一套新的電腦模型，讓他首度能在3D高解析度系統下探討上述想法。相關結果近日發表於期刊《自然》(Nature)之中。

The simulations show that mantle plumes and the weaknesses they create could have actually initiated the first subduction zones.

模擬結果顯示出地函柱與其造成的軟弱區域真的能夠促使第一個隱沒帶形成。

In the simulations, the plume weakens the overlying lithosphere and forms a circular, thinning weak point with a diameter of several dozen to hundreds of kilometres. This is stretched over time by the supply of hot material from the deep mantle. "In order to make a ring larger, you have to break it," explains the researcher. This also applies to the Earth's surface: the ring-shaped weaknesses can (in the model) only be enlarged and subducted if the margins are torn.

在模擬過程當中，地函柱會弱化上覆的岩石圈，並在其中形成一層薄薄的環形軟弱區域，直徑可從數十至數百公里不等。隨著深部地函不斷提供熱物質，它會開始往外擴展。「若要將一個環拉得更大，你非得扯斷它不可。」研究人員解釋。此觀點也能套用在地球表面：(模擬中)唯有這道環形弱點的邊緣被撕裂後，它才能往外擴張並且發生隱沒。

Water lubricates the plate margin

水分潤滑了板塊邊緣

The tears spread throughout the lithosphere, large slabs of the heavier rigid lithosphere plunge into the soft mantle, and the first plate margins emerge. The tension created by the plunging slabs ultimately sets the plates in motion. They plunge, well lubricated by the buried seawater of the ocean above. Subduction has begun -- and with it, plate tectonics. "Water acts as a lubricant and is an absolute necessity in the initiation of a self-sustaining subduction," says Gerya.

這些裂縫會延伸至岩石圈各處，造成密度較高的堅硬岩石圈斷塊往下沉至柔軟的地函當中。第一道板塊邊界就此誕生。而下沉板塊造成的拉力最後會使得整個板塊跟著移動。在下沉的過程當中，上方海水會隨之埋入而達到潤滑效果，使整個過程進行得更加順利。隱沒作用於焉開始，隨之而來的是板塊運動開始運作。「在啟動可以永續運作的隱沒作用的過程中，水的潤滑絕對是項不可或缺的要素。」

In their simulations, the researchers compare different temperature conditions and lithosphere states. They came to the conclusion that plume-induced plate tectonics could plausibly develop under the conditions that prevailed in the Precambrian around three billion years ago. Back then the Earth's lithosphere was already thick and cool, but the mantle was still very hot, providing enough energy to significantly weaken the lithosphere above the plumes.

研究人員在模擬過程當中，比較不同溫度條件和岩石圈狀態會有何差異。他們得出的結論顯示30億年前前寒物紀時期的地球，可以達到發生由地函柱引發的板塊運動所需的條件。那時地球岩石圈雖然已經達到一定厚度且冷卻下來，然而地函仍然相當熾熱，能夠提供使地函柱之上的岩石圈弱化至一定程度需要的能量。

Had the lithosphere instead being thin and warm, and therefore soft, the simulations show that a ring-shaped rapidly descending structure called drip would simply have formed around the plume head. While this would have steadily sunk into the mantle, it would not have caused the soft lithosphere to subduct and tear and therefore would not have produced plate margins. Likewise, the computer simulations showed that under today's conditions, where there is less temperature difference between lithosphere and plume material, plume-induced subduction is hard to initiate because the lithosphere is already too rigid and the plumes are barely able to weaken it sufficiently.

模擬結果顯示假如岩石圈的厚度較薄、溫度較高，因此較為軟弱， 則僅會在地函柱頭附近形成一種會迅速下沉，稱作drip的環狀構造。由於這種構造會穩定地往下沉入地函，因此它不會撕裂軟弱的岩石圈並造成隱沒作用，板塊邊界也不會形成。同樣地，電腦模擬也顯示在目前岩石圈以及地函柱物質之間溫差較小的狀態下，由於岩石圈已經太過堅硬使得地函柱不足以大幅弱化它，因此難以發生由地函柱引發的隱沒作用。

Dominant mechanism

主要機制

"Our new models explain how plate tectonics came about," says the geophysicist. Plume activity was enough to give rise to today's plate mosaic. He calls the power of the plumes the dominant trigger for global plate tectonics.

「我們的新模型解釋了板塊運動如何開始。」這名地球物理學家說。他聲稱地函柱的力量便是啟動地球板塊運動的主因，也造就了今日我們所見的片片板塊。

The simulations can also explain how so-called triple junctions, i.e. zones in which three plates come together, are nucleated by multi-directional stretching of the lithosphere induced by plumes. One such example of a triple junction can be found in the Horn of Africa where Ethiopia, Eritrea and Djibouti meet.

這項模擬也能解釋三向聯結構造(triple junction，三個板塊交接的地區)是如何因為地函柱造成岩石圈往多重方向拉伸，使得不同板塊交接而形成。在衣索比亞、厄利垂亞、吉布地三國交界的非洲之角即可看到三向聯結構造中的一例。

A possible plume-weakened zone analogous to a starting point for global plate tectonics likely exists in the modern world: the researchers see such a zone in the Caribbean plate. Its shape, location and spread correspond largely to the new model simulations.

今日，在世界上可能還存在著類似地球板塊運動的起始地點，由地函柱產生的軟弱帶。研究人員在加勒比板塊中看到一個這樣的區域，它的形狀、位置以及逐漸擴張都相當符合這個新模型的模擬結果。

Indeed it is arguably impossible to prove how global plate tectonics started on Earth based solely on observations: there is no geophysical and only a small amount of geological data from the Earth's early years, and laboratory experiments are not possible for extremely large-scale and very long-term tectonic processes, says the ETH researcher. "Computer models are therefore the only way we can reproduce and understand the events of the Earth's early history."

我們的確能說幾乎不可能僅靠觀察來證明地球上的板塊運動是如何開始。我們無法得到地球早年歲月時的地球物理資料，而地質證據也是吉光片羽。同樣地，實驗室試驗不可能重現如此大規模且極度長期的構造運動過程。「因此電腦模擬是我們僅剩的唯一方法，可以重現並瞭解地球早期究竟有何事件發生。」這位ETH的研究人員說。

引用自：ETH Zurich. "Plate tectonics thanks to plumes?." ScienceDaily. ScienceDaily, 11 November 2015.