新研究顛覆地函流動理論

原文網址：www.sciencedaily.com/releases/2016/07/160706174225.htm

New study upends a theory of how Earth's mantle flows

新研究顛覆地函流動理論

Small-scale processes may have big effects

尺度微小的作用可能具有重大影響

A new study carried out on the floor of Pacific Ocean provides the most detailed view yet of how the earth's mantle flows beneath the ocean's tectonic plates. The findings, published in the journal Nature, appear to upend a common belief that the strongest deformation in the mantle is controlled by large-scale movement of the plates. Instead, the highest resolution imaging yet reveals smaller-scale processes at work that have more powerful effects.

於太平洋海床進行的新研究給出了關於地函在海洋板塊下方如何流動至今為主最詳細的觀察結果。出版在期刊《自然》(Nature)的這篇研究似乎顛覆了廣為接受的理論中，認為地函內部最劇烈的變形活動受控於大尺度板塊運動。反之，迄今解析度最高的影像透露出正在運行的小尺度作用具有更大的影響力。

By developing a better picture of the underlying engine of plate tectonics, scientists hope to gain a better understanding of the mechanisms that drive plate movement and influence related process, including those involving earthquakes and volcanoes.

科學家藉由發展出可以詳細觀測位於板塊運動下方的引擎如何運作，盼望能更加瞭解驅使板塊運動的機制，及其他像火山和地震的相關作用如何受其影響。

When we look out at the earth, we see its rigid crust, a relatively thin layer of rock that makes up the continents and the ocean floor. The crust sits on tectonic plates that move slowly over time in a layer called the lithosphere. At the bottom of the plates, some 80 to 100 kilometers below the surface, the asthenosphere begins. Earth's interior flows more easily in the asthenosphere, and convection here is believed to help drive plate tectonics, but how exactly that happens and what the boundary between the lithosphere and asthenosphere looks like isn't clear.

當我們注視地球時看到的是她堅硬的外殼，這個相對來說薄薄的一層岩石組成了陸地和海床。地殼坐落於「岩石圈」(lithosphere)中會隨著時間緩緩移動的板塊之上。而在板塊底部，位於地表之下大致深80至100公里處則會開始進入「軟流圈」(asthenosphere)。地球內部的岩石於軟流圈可以輕易地產生流動，一般據信此處發生的對流作用有助於驅動板塊構造作用進行。然而，確切來說這如何發生以及岩石圈跟軟流圈之間的邊界為何，仍然不太清楚。

To take a closer look at these processes, a team led by scientists from Columbia University's Lamont-Doherty Earth Observatory installed an array of seismometers on the floor of the Pacific Ocean, near the center of the Pacific Plate. By recording seismic waves generated by earthquakes, they were able to look deep inside the earth and create images of the mantle's flow, similar to the way a doctor images a broken bone.

為了更仔細地觀測這些作用如何進行，由哥倫比亞大學拉蒙特-多爾蒂地球觀測所( Lamont-Doherty Earth Observatory)的科學家領導的研究團隊，在接近太平洋板塊中心處的太平洋底安裝了一組地震儀觀測網。透過記錄地震產生的震波，他們可以看進地球深處並且繪製出地函流動的圖像，這跟醫生對骨折部位製作顯影的方法頗為相似。

Seismic waves move faster through flowing rock because the pressure deforms the crystals of olivine, a mineral common in the mantle, and stretches them in the same direction. By looking for faster seismic wave movement, scientists can map where the mantle is flowing today and where it has flowed in the past.

地震波在流動的岩石中會以較快的速度傳播，這是因為壓力會使橄欖石(olivine)這種地函中常見的礦物變形，並將它們往同樣方向延展。經由尋找地震波傳播速度較快的地方，科學家可以標示出今日地函有哪些部分正在流動，而哪些地方則在從前曾經流動過。

Three basic forces are believed to drive oceanic plate movement: plates are "pushed" away from mid-ocean ridges as new sea floor forms; plates are "pulled" as the oldest parts of the plate dive back into the earth at subduction zones; and convection within the asthenosphere helps ferry the plates along. If the dominant flow in the asthenosphere resulted solely from "ridge push" or "plate pull," then the crystals just below the plate should align with the plate's movement. The study finds, however, that the direction of the crystals doesn't correlate with the apparent plate motion at any depth in the asthenosphere. Instead, the alignment of the crystals is strongest near the top of the lithosphere where new sea floor forms, weakest near the base of the plate, and then peaks in strength again about 250 kilometers below the surface, deep in the asthenosphere.

一般認為有三種基本作用力驅動了板塊運動：首先是中洋脊產生新生海床時會把板塊往外「推離」；接著是板塊最古老部分在隱沒帶潛回地球內部時會「拉扯」板塊；而軟流圈中的對流作用則能協助這條輸送帶運作。如果軟流圈內部的主要流向僅由「洋脊推動」(ridge push)以及「板塊隱沒拉力」(plate pull)決定，那板塊正下方的晶體應該會順著板塊運動方向排列。然而，此篇研究卻發現不管在軟流圈中哪個深度，晶體排列方向跟現在的板塊運動方向之間皆沒有相關性。反之，晶體排列在岩石圈表層新生海床形成處附近最為一致，而在板塊最底部則最為凌亂，接著在軟流圈底部距地表約250公里深處，晶體排列一致度又達到高峰。

"If the main flow were the mantle being sheared by the plate above it, where the plate is just dragging everything with it, we would predict a fast direction that's different than what we see," said coauthor James Gaherty, a geophysicist at Lamont-Doherty. "Our data suggest that there are two other processes in the mantle that are stronger: one, the asthenosphere is clearly flowing on its own, but it's deeper and smaller scale; and, two, seafloor spreading at the ridge produces a very strong lithospheric fabric that cannot be ignored." Shearing probably does happen at the plate boundary, Gaherty said, but it is substantially weaker.

「假設地函流動的主要方向是由上方板塊整個拖曳時產生的剪力決定，基於此理論預測出的波速較快方向跟我們觀測到的並不一致。」共同作者，拉蒙特-多爾蒂的地球物理學家 James Gaherty說。「我們的數據顯示在地函內部還有兩種更加強勢的作用。第一，我們清楚看見軟流圈有自己的獨立流動模式，但這發生在軟流圈較深處且規模較小。第二，於中洋脊海床擴張時會產生不容小覷，相當明顯的岩石圈結構。」在板塊跟軟流圈邊界確實會發生剪動作用，但大體上其影響力並不足，Gaherty 說。

Donald Forsyth, a marine geophysicist at Brown University who was not involved in the new study, said, "These new results will force reconsideration of prevailing models of flow in the oceanic mantle."

並未參與此研究的布朗大學海洋地球物理學家Donald Forsyth說：「這些新發表的成果將會迫使我們重新審視現行的海洋地函流動模型。」

Looking at the entire upper mantle, the scientists found that the most powerful process causing rocks to flow happens in the upper part of the lithosphere as new sea floor is created at a mid-ocean ridge. As molten rock rises, only a fraction of the flowing rock squeezes up to the ridge. On either side, the pressure bends the excess rock 90 degrees so it pushes into the lithosphere parallel to the bottom of the crust. The flow solidifies as it cools, creating a record of sea floor spreading over millions of years.

綜觀整個上部地函，科學家發現使岩石圈上層岩石流動最有力的作用，來自於中洋脊生成新生海洋板塊時。當熔融岩石上湧時，流動的岩石中只有一部分會被擠出中洋脊。其餘岩石在壓力作用下會往兩側彎折90度，而被推進岩石圈內部並平行於地殼底部流動。這些流動的岩石會逐漸冷卻而凝固，形成經過數百萬年後仍能留存的海底擴張紀錄。

This "corner flow" process was known, but the study brings it into greater focus, showing that it deforms the rock crystals to a depth of at least 50 kilometers into the lithosphere.

這種「角落流」(corner flow)作用雖然早已為人所知，但此篇研究對它著墨甚多，認為這種使岩石圈的岩石晶體發生變形的作用範圍至少可以深達50公里處。

In the asthenosphere, the patterns suggest two potential flow scenarios, both providing evidence of convection channels that bottom out about 250 to 300 kilometers below the earth's surface. In one scenario, differences in pressure drive the flow like squeezing toothpaste from a tube, causing rocks to flow east-to-west or west-to-east within the channel. The pressure difference could be caused by hot, partially molten rock piled up beneath mid-ocean ridges or beneath the cooling plates diving into the earth at subduction zones, the authors write. Another possible scenario is that small-scale convection is taking place within the channel as chunks of mantle cool and sink. High-resolution gravity measurements show changes over relatively small distances that could reflect small-scale convection.

在流軟圈中的波速分布模式顯示有兩種可能的流動模式，都可以解釋軟流圈底層距地表深約250至300公里處的地函對流。在第一種模式中，壓力差導致流動發生的方式就像將牙膏擠出軟管一樣，造成岩石往西至東或往東至西流動。作者寫說壓力差會產生是因為熾熱的部分熔融岩石堆積在中洋脊之下，或者是累積在隱沒帶沉入地球深處的冷卻板塊下方。另一種模式則是當地函塊體冷卻並下沉時產生的小尺度對流。高解析度重力探測影像顯示相對短距離中發生的重力變化反映了小尺度對流的發生。

"The fact that we observe smaller-scale processes that dominate upper-mantle deformation, that's a big step forward. But it still leaves uncertain what those flow processes are. We need a wider set of observations from other regions," Gaherty said.

「我們觀察到小尺度作用主導了上部地函的變形作用，這可說是一大邁進。但詳細的流動過程仍然有不清楚的地方。我們還需要更多從其他地區得到的觀測資料。」Gaherty 說。

The study is part of the NoMelt project, which was designed to explore the lithosphere-asthenosphere boundary at the center of an oceanic plate, far from the influence of melting at the ridge. The scientists believe the findings here are representative of the Pacific Basin and likely ocean basins around the world.

這項研究是 NoMelt計畫的一部分，其目的是要於海洋板塊中央，遠離洋脊融熔物質影響的環境中來探索岩石圈—軟流圈的交界。科學家認為這裡的發現可以代表整個太平洋海盆以及世界各處的其他相似海盆。

NoMelt is unique because of its location. Most studies use land-based seismometers at edge of the ocean that tend to highlight the motion of the plates over the asthenosphere because of its large scale and miss the smaller-scale processes. NoMelt's ocean bottom seismometer array, with the assistance of Lamont's seismic research ship the Marcus G. Langseth, recorded data from earthquakes and other seismic sources from the middle of the plate over the span of a year.

NoMelt的獨特之處在於它的研究地點。多數研究利用架設在海洋邊緣的陸上地震儀觀測網得到的資訊，通常因為軟流圈驅動的板塊運動尺度較大而著重於此，但卻忽略了小尺度作用。NoMelt的海底地震儀觀測網在拉蒙特-多爾蒂的研究船 Marcus G. Langseth號的協助下，可以記錄數年之間發生在板塊中央的地震以及其他震波來源產生的數據。

引用自：Lamont-Doherty Earth Observatory, Columbia University. "New study upends a theory of how Earth's mantle flows: Small-scale processes may have big effects." ScienceDaily. ScienceDaily, 6 July 2016.