科學家首度對地函的內部分界提出解釋

原文網址：www.sciencedaily.com/releases/2015/12/151210144707.htm

First explanations for boundary within Earth's mantle

科學家首度對地函的內部分界提出解釋

Observed physical transition hundreds of miles below Earth's surface

觀測顯示地表數百哩之下的物理性質發生轉變

Earth's mantle, the large zone of slow-flowing rock that lies between the crust and the planet's core, powers every earthquake and volcanic eruption on the planet's surface. Evidence suggests that the mantle behaves differently below 1 megameter (1,000 kilometers, or 621 miles) in depth, but so far seismologists have not been able to explain why this boundary exists.

地函位於地殼和地核之間，是一片由緩緩流動的岩石組成的廣大區域，並驅使了所有發生在地表的地震與火山噴發。證據顯示地函的行為在深於1000公里(621哩)後會跟淺處有所差異，但至今為止地震學家仍無法解釋為何會有這個分界存在。

Two new studies co-authored by University of Maryland geologists provide different, though not necessarily incompatible, explanations. One study suggests that the mantle below 1 megameter is more viscous--meaning it flows more slowly--than the section above the boundary. The other study proposes that the section below the boundary is denser--meaning its molecules are more tightly packed--than the section above it, due to a shift in rock composition.

兩篇最新的研究各自提出了一套解釋，但兩者之間未必互相牴觸。其中一篇認為地函在1000公里之下較上方更為黏滯，意即地函在這深度以下更加難以流動；另一篇則表示由於岩石成分改變，使得在此分界下方的密度較上方要高，代表地函物質在這深度以下分子的排列更加緊密。這兩篇研究的共同作者皆包括了馬里蘭大學的地質學家。

Taken together, the studies provide the first detailed look at why large-scale geologic features within the mantle behave differently on either side of the megameter divide. The papers were published on December 11, 2015, in the journals Science and Science Advances.

總和而言，這兩篇研究為科學家首度詳細驗證以深度1000公里為界，兩側地函會出現行為差異的大尺度地質構造是如何出現。它們將會於2015年12月11日分別刊登在期刊《科學》和《科學前緣》。

"The existence of the megameter boundary has been suspected and inferred for a while," said Vedran Lekic, an assistant professor of geology at UMD and co-author of the Science paper that addresses mantle viscosity. "These papers are the first published attempts at a detailed explanation and it's possible that both explanations are correct."

「早已有人猜測且提出證據在1000公里深處有道分界存在於地函當中。」馬里蘭大學地質科學系的助理教授Vedran Lekic說。他參與在內刊登於《科學》的研究引述了地函黏滯性理論。「這兩篇研究為科學家首次發表他們試著對此現象提出的詳細解釋，而這兩者都有可能都是對的。」

Although the mantle is mostly solid, it flows very slowly in the context of geologic time. Two main sources of evidence suggest the existence of the megameter boundary and thus inspired the current studies.

雖然地函絕大部分是由固體組成，但從地質時間尺度來看它仍會緩緩流動。有兩項主要證據顯示1000公里分界確實存在，並啟發了目前正在進行的諸多研究。

First, many huge slabs of ocean crust that have been dragged down, or subducted, into the mantle can still be seen in the deep Earth. These slabs slowly sink downward toward the bottom of the mantle. A large number of these slabs have stalled out and appear to float just above the megameter boundary, indicating a notable change in physical properties below the boundary.

首先，許多巨大的海洋板塊在被拖入(隱沒)至地函後，我們仍然能看到它們存在於地球深處。這些隱沒板塊會緩緩往地函底部下沉，但其中有許多會在停滯在1000公里分界上，這讓它們看起來像在此處漂浮。這種現象意味著在此分界之下地函的物理性質勢必發生了顯著改變。

Second, large plumes of hot rock rise from the deepest reaches of the mantle, and the outlines of these structures can be seen in the deep Earth as well. As the rock in these mantle plumes flows upward, many of the plumes are deflected sideways as they pass the megameter boundary. This, too, indicates a fundamental difference in physical properties on either side of the boundary.

其次，在地函最深處會有由熾熱岩石形成的地函柱往上湧動，而我們同樣能看到這種地球深處構造的外觀。當這些地函柱中的岩石往上流動，可以發現有許多地函柱在經過1000公里分界後流動方向會偏折。這一樣代表著分界兩側的物理性質一定有本質上的差異。

"Learning about the anatomy of the mantle tells us more about how the deep interior of Earth works and what mechanisms are behind mantle convection," said Nicholas Schmerr, an assistant professor of geology at UMD and co-author of the Science Advances paper that addresses mantle density and composition. "Mantle convection is the heat engine that drives plate tectonics at the surface and ultimately leads to things like volcanoes and earthquakes that affect people living on the surface."

「認識地函的基本性質可以告訴我們地球深處如何運作，以及地函的對流機制為何」馬里蘭大學地質科學系的助理教授Nicholas Schmerr說。他參與在內發表於《科學前緣》的論文則是以地函密度和成分差異來解釋1000公里分界。「地函對流是驅動地表板塊構造運動的動力來源，這最終會引發火山爆發和地震之類的種種事物而影響到生活在地表的人們。」

The physics of the deep Earth are complicated, so establishing the mantle's basic physical properties, such as density and viscosity, is an important step. Density refers to the packing of molecules within any substance (gas, liquid or solid), while viscosity is commonly described as the thickness of a fluid or semi-solid. Sometimes density and viscosity correlate with each other, while sometimes they are at odds. For example, honey is both more viscous and dense than water. Oil, on the other hand, is more viscous than water but less dense.

在地球深處進行的物理作用相當複雜，因此確立地函的基本物理性質，像是密度和黏滯性，對於我們要了解它會是相當重要的一步。任何物質(氣體、液體、固體)的密度代表了它內部分子排列的緊密程度，而黏滯性則通常用來描述液體或半液體的濃稠程度。有時密度與黏滯性會有關聯，但有時卻會互相衝突。比方說，蜂蜜跟水相比，它的黏滯性與密度都較高；另一方面，石油雖然比水更為濃稠，然而它的密度卻較低。

In their study, Schmerr, lead author Maxim Ballmer (Tokyo Institute of Technology and the University of Hawaii at Manoa) and two colleagues used a computer model of a simplified Earth. Each run of the model began with a slightly different chemical composition--and thus a different range of densities--in the mantle at various depths. The researchers then used the model to investigate how slabs of ocean crust would behave as they travel down toward the lower mantle.

在Schmerr、Maxim Ballmer(第一作者，任職於東京工業大學和夏威夷大學馬諾分校)以及另外兩名科學家同僚的研究中，他們利用了簡化的地球模型。每一次的模擬開始時，他們都會先微調地函不同深度的化學成分，因此密度也會跟著改變。接著研究人員利用模型來探討海洋隱沒板塊在往下潛至下部地函的過程中，會展現出何種行為模式。

In the real world, slabs are observed to behave in one of three ways: The slabs either stall at around 600 kilometers, stall out at the megameter boundary, or continue sinking all the way to the lower mantle. Of the many scenarios for mantle chemical composition the researchers tested, one most closely resembled the real world and included the possibility that slabs can stall at the megameter boundary. This scenario included an increased amount of dense, silicon-rich basalt rock in the lower mantle, below the megameter boundary.

現實世界中，隱沒板塊呈現出的行為模式會是下列三者其一：堆積在600公里深附近、停滯在1000公里分界、或者一路下沉至下部地函。研究人員反覆以不同地函成分測試，其中有一種呈現出來的情境與真實世界世界最為相似，且包含了隱沒板塊能停留在1000公里分界的可能性。在此情境中，1000公里分界之下的下部地函會擁有更多密度較高、富含矽的玄武岩質岩石。

Lekic, lead author Max Rudolph (Portland State University) and another colleague took a different approach, starting instead with whole-Earth satellite measurements. The team then subtracted surface features--such as mountain ranges and valleys--to better see slight differences in Earth's basic shape caused by local differences in gravity. (Imagine a slightly misshapen basketball with its outer cover removed.)

在Lekic、Max Rudolph(研究第一作者，任職於波特蘭州立大學)和另一名科學家同僚的研究中，他們則採取了不同方法。研究團隊先取得衛星對整個地球的觀測影像，接著他們減去地表特徵，像是山脈和深谷造成的影響，以更詳細的觀察因為各地重力不同，而對地球原本的外形造成的輕微起伏(想像一個外皮剝掉後稍稍變形的籃球)。

The team mapped these slight differences in Earth's idealized shape onto known shapes and locations of mantle plumes and integrated the data into a model that helped them relate the idealized shape to differences in viscosity between the layers of the mantle. Their results pointed to less viscous, more free-flowing mantle rock above the megameter boundary, transitioning to highly viscous rock below the boundary. Their results help to explain why mantle plumes are frequently deflected sideways as they extend upward beyond the megameter boundary.

研究團隊得到這些與地球理想外形的細微差異後，再將其疊合在標有已知地函柱的形狀和位置圖上。接著研究團隊把這些資料整合至模型當中，以幫助他們瞭解地球理想外型和現實的些微出入，跟地函不同層的黏滯性差異之間有何關聯。他們的結果指出原本黏滯性較低，更容易流動的地函岩石，在經過1000公里分界後會變成黏滯性較高的岩石。他們的結論有助於解釋為何地函柱往上流動經過1000公里分界後，時常會往他處偏折。

"While explaining one mystery--the behavior of rising plumes and sinking slabs--our results lead to a new conundrum," Lekic said. "What causes the rocks below the megameter boundary to become more resistant to flow? There are no obvious candidates for what is causing this change, so there is a potential for learning something fundamentally new about the materials that make up Earth."

「在解開為何上湧地函柱和隱沒板塊會有這些行為的謎題時，我們的結果卻又製造了一道新難題。」Lekic說。「是什麼造成1000公里分界下方的岩石更難以流動？目前並無有力的假說可以解答是什麼造成了這種改變，但也因此在解決這項謎題的過程中，我們可能相當有機會可以學習到一些攸關地球組成本質的新事物。」

Lekic and Schmerr plan to collaborate to see if the results of both studies are consistent with one another--in effect, whether the lower mantle is both dense and viscous, like honey, when compared with the mantle above the megameter boundary.

Lekic和Schmerr計畫要合作以觀察他們倆個團隊的研究成果是否可以彼此相容，實際上就是要確認下部地函與1000公里分界之上的地函互相比較，是否前者真的比後者更加緻密且濃稠，就像蜂蜜和水的關係一樣。

"This work can tell us a lot about where Earth has been and where it is going, in terms of heat and tectonics," Schmerr said. "When we look around our solar system, we see lots of planets at various stages of evolution. But Earth is unique, so learning what is going on deep inside its mantle is very important."

「這項工作可以告訴我們許多事物，讓我們了解地球的熱力學和構造運動一直以來是如何運作，而未來的走向又是如何。」Schmerr說。「當我們環顧我們所處的太陽系，我們可以看到處在不同演化階段的各式星體。但地球是獨一無二的，因此了解它的地函深處運作方式是件至關重要的事。」

引用自：University of Maryland. "First explanations for boundary within Earth's mantle: Observed physical transition hundreds of miles below Earth's surface." ScienceDaily. ScienceDaily, 10 December 2015.