原文網址：www.sciencedaily.com/releases/2016/12/161215085909.htm

改寫板塊厚度的定義？研究橄欖石對如何測量地球表面提供了新資訊

板塊構造學說意指地球表面是由好幾塊會彼此遠離與靠近的板塊組成。自1960年代開始，科學家便利用這項學說來解釋火山和地震為何會在某些地點發生。

一個知名的例子便是太平洋火環(Pacific Ring of Fire)。根據美國國家海洋與大氣總署(NOAA)，這條25,000英哩長的環太平洋帶狀區域，因為含有成串的海底火山 (大約450座)和地震發生帶而為人所知。

在北美的太平洋海岸地區，火環經過從加拿大西岸延伸至美國西岸的卡斯卡迪亞(Cascadia)隱沒帶。據悉，大部分的地震都是沿著隱沒帶或是板塊內部的斷層發生。

然而，板塊實際上的定義為何以及板塊究竟有多厚，仍然是項爭議性十足的議題。這是因為雖然科學家知道板塊的頂部就是地球表面，但要界定板塊的底部卻相當困難。

近日由德拉瓦大學的Jessica Warren，和牛津大學與明尼蘇達大學雙城分校的科學家進行的研究，提出了全新的一組數據有助於科學家了解這項問題。

「知道板塊厚度為何對於暸解板塊運動的過程相當關鍵。板塊運動包括了板塊在中洋脊形成，以及隨後這些物質經由像卡斯卡迪亞、安地斯、日本及印尼附近的隱沒帶回到地球內部的過程。」德拉瓦大學地球、海洋與環境學院的地質科學系助理教授Warren表示。

「這也有助於科學家模擬並預測地震以及火山活動未來會造成的危害。依據模擬結果他們可以知道地震以及火山爆發可能的發生位置以及造成的破壞程度有多大。」

橄欖石：地球內部的穩健模型

由於要親臨現場研究地球內部是件不可能的事，因此當科學家企圖了解地球內部究竟發生何種事物時，他們必須另闢蹊徑。

科學家轉而研究地震波通過地球內部的過程，並將接收到的訊號反轉回去，以回推發生在地球內部的事物。另外，因為科學家知道地球內部比表層的地殼還要高溫，所以他們也能模擬岩石的熱學性質，包括地球內部發生溫度變化的位置。

「科學告訴我們對地球內部溫度變化的預測結果，應該要與地震波呈現給我們的一致。問題就是這兩種模型彼此之間並不吻合。」研究岩石來源和形成過程的岩石學專家Warren表示。

一項歷時已久的爭議是由地震波性質變化而辨識出的古氏不連續面(Gutenberg discontinuity)，是否可以代表板塊的底部。

為了探討這個問題，Warren和她的同僚在實驗室對橄欖石進行了試驗，這種礦物是在[岩石圈]地函(地球最上層250英哩厚的區域)中發現的主要礦物。橄欖石同時也是組成橄欖岩(peridotite)的主要礦物，科學家視橄欖岩為地球內部成分的穩健模型。

研究人員取用橄欖石並添加熔融物質(又稱玄武岩)，來模擬在中洋脊，新生板塊如何形成。團隊接著將橄欖石-熔融物質的混合物在高溫高壓之下扭轉，以測定熔融物質對橄欖岩晶體的排列方向會有什麼樣的影響。然後他們利用實驗結果，來推測地震波經過這類岩石後產生的訊號，並跟古氏不連續面的地震波訊號互相比較。

團隊的研究結果顯示古氏不連續面無法定義為板塊的底部。反之，古氏不連續面是因板塊內部含有橄欖石-融熔物質的混合物而產生。

「我已經研究橄欖岩中的橄欖石是如何排列10年以上，因為它們的流動模式可以做為一種歷史紀錄，從中可以得知這些從地函來的岩石如何隨著歲月移動及變形。」

研究人員的結果提出模擬板塊厚度的最佳方式，是依據溫度剖面，以及隨著板塊年齡增長由傳導造成的冷卻程度。

「我們認為板塊底部比溫度剖面中發生降溫之處還要更深。在地函內部有層岩石具有融熔物質或者在此凝固，改變了岩石的地震波性質，因而形成了我們看到的特殊層面，」她說。「經由我們的估算，這意味著海洋板塊的厚度大約為100公里，或是62英哩。」

Warren接著說明團隊得到的數據也可以用來說明古氏不連續面的成因，其等同於岩石在中洋脊融化之後，產生的熔融物質被包覆在岩石當中或者於岩石內部凝固的位置。當地震波經過此處時性質便會改變。

Tectonic shift? Study of olivine provides new data for measuring Earth's surface

Plate tectonics, the idea that the surface of the Earth is made up of plates that move apart and come back together, has been used to explain the locations of volcanoes and earthquakes since the 1960s.

One well-known example of this is the Pacific Ring of Fire, a 25,000-mile stretch of the Pacific Ocean known for its string of underwater volcanoes (nearly 450 of them) and earthquake sites, according to the National Oceanic and Atmospheric Administration (NOAA).

On the Pacific Coast, this area sits along the subduction zone known as the Cascadia plate, which runs down the west coast of Canada to the west coast of the United States. Most earthquakes are said to occur at subduction zones or along faults in tectonic plates.

What actually defines a tectonic plate and how thick plates are, however, has remained a hotly debated topic. This is because while scientists know that the top of the plate is the surface of the Earth, defining the plate's bottom boundary has been challenging.

A recent study by the University of Delaware's Jessica Warren and colleagues at the University of Oxford and the University of Minnesota, Twin Cities, provides a new data set that scientists can use to understand this problem.

「Understanding the thickness of the plate is important to understanding how plates move around, both when they form at mid-ocean ridges and later on when the material goes back down into the Earth through subduction zones such as those in Cascadia, the Andes, Japan and Indonesia," said Warren, assistant professor in the Department of Geological Sciences in the College of Earth, Ocean, and Environment.

"It also can help scientists model and predict future earthquake and volcanic hazards, where they might occur and how deep the devastation might be depending on what the models show."

Olivine a robust model of Earth's interior

To understand what's happening inside the Earth, scientists must be creative because studying the interior of the Earth in situ is impossible.

Instead, scientists study how seismic waves pass through the Earth and then invert the signal that is received to reverse engineer what's happening. They also model the thermal properties of the rock, including where temperature changes occur, because they know that the interior of the Earth is hotter than the surface crust.

"Science has been telling us that what we predict for temperature changes within the Earth should agree with what the seismic waves are telling us. The problem has been that these two models don't agree," said Warren, a petrology expert who studies the origin of rocks and how they formed.

One longstanding argument has been whether the Gutenberg discontinuity -- the identification of a change in seismic properties -- represents the bottom of the plate.

To investigate this problem, Warren and her colleagues performed laboratory experiments on olivine, the main mineral found in the Earth's mantle (the upper ~250 miles of the planet). Olivine also is the main mineral in peridotite rock, which is considered to be a robust model of the interior of the Earth's composition.

The researchers took olivine and added melt (also known as basalt) to mimic how a new plate is created at a mid-ocean ridge. The team then twisted the olivine-melt mixture under high temperatures and high pressure to determine the influence of melt on the alignment of olivine crystals. They then used these experiments to predict the seismic signature of this rock and compared it to the seismic signature associated with the Gutenberg discontinuity.

The team's results showed that the Gutenberg discontinuity does not define the bottom of the plate, but instead is caused by the presence of olivine-melt mixtures within tectonic plates.

"I've spent over a decade studying how olivine minerals are oriented in peridotite rocks because the flow patterns provide a historical record of how these rocks from the mantle have changed and deformed over time," says Warren.

The research team's results suggest the best way to model the plate thickness is based on the thermal profile and the conductive cooling that occurs as a plate ages.

"We think that the bottom of the plate is below where you have a cooling in the temperature profile. It is a layer that is associated with melt being trapped or frozen in the rock and changing the seismic properties in the rock that subsequently produced the layer that we're imaging," she said. "By our estimates, this would mean that the tectonic plates in the ocean are approximately 100 kilometers or about 62 miles thick. "

The team's data also offers an explanation for the Guttenberg discontinuity, Warren continued, saying that it corresponds to melt that was trapped or frozen in the rock after melting at mid-ocean ridges, which produced a change in how the seismic waves pass through the rock.

原始論文：Lars N. Hansen, Chao Qi, Jessica M. Warren. Olivine anisotropy suggests Gutenberg discontinuity is not the base of the lithosphere. Proceedings of the National Academy of Sciences, 2016; 113 (38): 10503 DOI: 10.1073/pnas.1608269113

引用自：University of Delaware. "Tectonic shift? Study of olivine provides new data for measuring Earth's surface." ScienceDaily. ScienceDaily, 15 December 2016.