地函似乎具有驅動板塊構造運動的作用

原始網址：www.sciencedaily.com/releases/2016/07/160728143525.htm

Earth's mantle appears to have a driving role in plate tectonics

地函似乎具有驅動板塊構造運動的作用

Deep down below us is a tug of war moving at less than the speed of growing fingernails. Keeping your balance is not a concern, but how the movement happens has been debated among geologists.

在我們腳下正發生一場節奏比指甲生長速度還慢的拔河比賽。要在其上保持平衡並不是什麼問題，但這項運動如何發生卻是地質學家長久以來爭論不休的議題。

New findings from under the Pacific Northwest Coast by University of Oregon and University of Washington scientists now suggest a solution to a mystery that surfaced when the theory of plate tectonics arose: Do the plates move the mantle, or does the mantle move the plates.

奧勒岡大學和華盛頓大學的科學家對太平洋西北岸下方進行了新研究之後，提出了一項理論可以解答自板塊構造學說問世以來就有的一件謎題：到底是板塊使地函流動，或是地函讓板塊移動？

The separation of tectonic plates, the researchers proposed in a paper online ahead of print in the journal Nature Geoscience, is not simply dictating the flow of the gooey, lubricating molten material of the mantle. The mantle, they argue, is actually fighting back, flowing in a manner that drives a reorientation of the direction of the plates.

研究人員在期刊《自然—地質科學》(Nature Geoscience)出刊前的線上版論文中表示，地函的黏滑物質往哪裡流動並非受板塊的分離運動單方面支配。他們主張地函事實上也會有所反抗，它的流動在某種程度上也會重新調整板塊運動的方向。

The new idea is based on seismic imaging of the Endeavor segment of the Juan de Fuca Plate in the Pacific Ocean off Washington and on data from previous research on similar ridges in the mid-Pacific and mid-Atlantic oceans.

這項新理論主要奠基於來自華盛頓外海，太平洋的胡安‧德富卡板塊中(Juan de Fuca Plate)的Endeavor段得到的震測影像，以及過往對太平洋及大西洋中其他類似洋脊的研究成果。

"Comparing seismic measurements of the present mantle flow direction to the recent movements of tectonic plates, we find that the mantle is flowing in a direction that is ahead of recent changes in plate motion," said UO doctoral student Brandon P. VanderBeek, the paper's lead author. "This contradicts the traditional view that plates move the mantle."

「比較震測影像呈現出來的現今地函流動方向以及近期板塊的移動方向，我們發現地函的流動方向竟然早於板塊運動近期發生的改變之前。」此篇論文的第一作者，奧勒岡大學的博士生 Brandon P. VanderBeek表示。「這跟傳統中認為板塊拖曳地函移動的論點相悖。」

While the new conclusion is based on a fraction of such sites under the world's oceans, a consistent pattern was present, VanderBeek said. At the three sites, the mantle's flow is rotated clockwise or counterclockwise rather than in the directions of the separating plates. The mantle's flow, the researchers concluded, may be responsible for past and possibly current changes in plate motion.

VanderBeek 說雖然這項新結論僅根據全世界海洋中上述位置一部分的觀察結果而來，但它們之間確實有相當一致的模式存在著。在這三個地點，地函流動方向跟板塊分離方向之間並不一致，而是往順時針或逆時針偏差了一個角度。研究人員總結，地函流動可能造成過去，甚至是當下正在發生的板塊運動方向的改變。

The research -- funded through National Science Foundation grants to the two institutions -- also explored how the supply of magma varies under mid-ocean ridge volcanoes. The researchers conducted a seismic experiment to see how seismic waves moved through the shallow mantle below the Endeavor segment.

這項研究由美國國家科學基金會資助上述兩個機構來進行，同時也探討了中洋脊火山之下的岩漿供應隨空間變化的情形。研究人員進行震測實驗來觀察地震波在Endeavor段之下的淺層地函中會如何行進。

They found that the middle of the volcanic segment, where the seafloor is shallowest and the inferred volcanic activity greatest, the underlying mantle magma reservoir is relatively small. The ends, however, are much deeper with larger volumes of mantle magma pooling below them because there are no easy routes for it to travel through the material above it.

他們發現火山帶的中間地段，也就是海床最淺且推測火山活動最劇烈處，底下的地函岩漿庫相對而言是比較小的。然而，位於較深處的兩端其下方卻有大量的地函岩漿集中於此，這是因為此處並沒有捷徑可以讓這些岩漿穿越上方的物質。

Traditional thinking had said there would be less magma under the deep ends of such segments, known as discontinuities.

傳統觀點認為在這些段落最深兩端稱作「不連續帶」的地方，其下的岩漿會比較少。

"We found the opposite," VanderBeek said. "The biggest volumes of magma that we believe we have found are located beneath the deepest portions of the ridges, at the segment ends. Under the shallow centers, there is much less melt, about half as much, at this particular ridge that we investigated.

「我們發現的事實恰好相反。」 VanderBeek說。「我們認為我們發現的岩漿庫中體積最大者正好位於洋脊兩端最深的地方。在我們研究的這段洋脊深度較淺的中央地帶，下方岩漿庫的體積頂多只有最大者的一半。」

"Our idea is that the ultimate control on where you have magma beneath these mountain ranges is where you can and cannot take it out," he said. "At the ends, we think, the plate rips apart much more diffusely, so you are not creating pathways for magma to move, build mountains and allow for an eruption."

「我們的想法是，控制這些洋脊下方岩漿庫大小的根本因素是有沒有通路可以讓它們流出來。」他說。「我們認為在洋脊兩端，板塊撕裂造成的傷口較中央更加不平整，因此無法形成可以讓岩漿流動的通路，連帶使岩漿無法在此噴發而形成山脊。」

引用自：University of Oregon. "Earth's mantle appears to have a driving role in plate tectonics." ScienceDaily. ScienceDaily, 28 July 2016.