是什麼造成了壯觀的夏威夷-天皇海山鍊(Hawaiian-Emperor seamount chain)變得如此彎折？

原始網址：www.sciencedaily.com/releases/2016/05/160511142351.htm

How the spectacular Hawaiian-Emperor seamount chain became so bendy

是什麼造成了壯觀的夏威夷-天皇海山鍊(Hawaiian-Emperor seamount chain)變得如此彎折？

The physical mechanism causing the unique, sharp bend in the Hawaiian-Emperor seamount chain has been uncovered in a collaboration between the University of Sydney and the California Institute of Technology (Caltech).

在雪梨大學和加州理工學院的合作之下，終於找出了可以用來解釋夏威夷-天皇海山鍊中獨一無二的急轉彎是如何形成的物理機制。

Led by a PhD candidate at the University of Sydney's School of Geosciences, researchers used the Southern Hemisphere's most highly integrated supercomputer to reveal flow patterns deep in the Earth's mantle -- just above the core -- over the past 100 million years. The flow patterns explain how the enigmatic bend in the Hawaiian-Emperor seamount chain arose.

在雪梨大學地質科學院的博士候選人領導之下，研究人員利用南半球效能最強的超級電腦展現出緊鄰地核正上方的深層地函過去1億年來的流動模式。這可以用來解釋夏威夷-天皇海山鍊中謎樣的轉折究竟是如何形成。

True to the old adage -- as above, so below -- the Sydney-US collaboration found the shape of volcanic seamount chains (chains of mostly extinct volcanoes), including Hawaii, is intimately linked to motion near the Earth's core.

如同一句古老格言所說：「如其在上，如其在下(As above, so below)。」雪梨跟美國大學的合作成果發現了包括夏威夷在內的海山鍊(排成鍊狀的火山，其中大都為死火山)，其形態跟地核附近的地函運動息息相關。

The findings of PhD candidate Rakib Hassan and fellow researchers including Professor Dietmar Müller from the University's EarthByte Group, are being published in Nature.

此篇刊登於《自然》的研究由博士候選人Rakib Hassan 進行，共同研究人員包括了雪梨大學地球位元團隊的成員之一， Dietmar Müller教授。

Mr Hassan explained: "Until now, scientists believed the spectacular 60° bend in the Hawaiian seamount chain -- not found in any other seamount chains -- was related to a change in plate motion combined with a change in flow direction in the shallow mantle, the layer of thick rock between the Earth's crust and its core.

Hassan解釋：「目前為止科學家仍然認為出現在夏威夷海山鍊中十分引人注目的60度彎曲，這項未在其他海山鍊中出現的特徵，跟板塊運動方向以及淺層地函流向的改變有關。地函是位在地球地核跟地殼中間，厚度相當厚的岩石。」

"These findings suggest the shape of volcanic seamount chains record motion in the deepest mantle, near the Earth's core. The more coherent and rapid the motion deep in the mantle, the more acute its effects are on the shape of seamount chains above," he said.

「這項發現也告訴我們海底火山鍊的型態紀錄了地核附近的深層地函是如何運動。地函深部的運動越一致且迅速，對上方海山鍊型態造成的影響就會越劇烈。」他說。

Although solid, the mantle is in a state of continuous flow, observable only over geological timescales. Vertical columns of hot and buoyant rock rising through the mantle from near the core are known as mantle plumes. Volcanic seamount chains such as Hawaii were created from magma produced near the surface by mantle plumes. Moving tectonic plates sit above the mantle and carry newly formed seamounts away from the plume underneath -- the oldest seamounts in a chain are therefore furthest away from the plume.

地函雖為固體，但它其實處於持續緩緩流動的狀態，不過這樣的運動只能從地質時間尺度來看才能觀察到。稱作地函柱(mantle plume)的高熱且上湧的柱狀岩石會從將近地核處往上升並穿過地函，它在接近地表處產生的岩漿會形成像夏威夷這類的海底火山鍊。在地函上方移動的板塊會將新形成的海山帶離下方的地函柱，造成海山鍊中最老的海山會離地函柱最遠。

"We had an intuition that, since the north Pacific experienced a prolonged phase where large, cold tectonic plates uninterruptedly sank into the mantle, the flow in the deepest mantle there would be very different compared to other regions of the Earth," Mr Hassan said.

「既然北太平洋很長一段時間都處於低溫的大型板塊不斷往地函下沉的環境，直覺告訴我們在這裡地函最深處的運動必然會跟地球上其他地區大不相同。」 Hassan說。

One of the most contentious debates in geoscience has centred on whether piles of rock in the deep mantle -- to which plumes are anchored -- have remained stationary, unaffected by mantle flow over hundreds of millions of years.

地質科學中最眾說紛紜的爭論之一便聚焦於地函深處的岩石，也就是地函柱的根基所在，是否在數億年的時光中能固定在同一個地方，不受地函流動影響。

The new research shows the shapes of these piles have changed through time and their shapes can be strongly dependent on rapid, coherent flow in the deep mantle.

這項新研究顯示這些岩石的形狀會隨著時間變化，且跟深部地函一致且迅速的流動密切相關。

Between 50-100 million years ago, the edge of the pile under the north Pacific was pushed rapidly southward, along with the base of Hawaii's volcanic plume, causing it to tilt. The plume became vertical again once the motion of its base stopped; this dramatic start-stop motion resulted in the seamount chain's sharp bend.

在5000萬年到1億年前，在北太平洋之下這些岩石的邊界迅速地往南推進，夏威夷火山地函柱的根基也連帶著移動，使得整根地函柱傾斜。然而，一旦這個運動停止下來，地函柱便會再次回復直立的狀態。這種劇烈地停止及運動的交互發生，造成海山鍊發生急遽的轉彎。

Using Australia's National Computational Infrastructure's supercomputer Raijin, the team created high-resolution three-dimensional simulations of mantle evolution over the past 200 million years to understand the coupling between convection in the deep Earth and volcanism.

研究團隊利用澳洲國家計算基礎建設中心的超級電腦「雷神(Raijin)」創建了高解析度的三維模型，可以顯現過去2億年來地函的演化過程，並從中了解地球深部的對流跟火山活動之間的關聯。

Mr Hassan said the simulations were guided by surface observations -- similar to meteorologists applying past measurements to predict the weather.

Hassan先生說這項模擬是以地表的觀測結果為基礎來進行，這跟大氣學家利用過往累積的觀測結果來預報天氣之間有異曲同工之處。

"These simulations required millions of central processing unit (CPU) hours on the supercomputer over the course of the project," he said.

「在整個計畫執行的過程中，光是進行模擬就得花上超級電腦的中央處理器數百萬個小時。」他說。

Professor Müller concluded: "Our results help resolve a major enigma of why volcanic seamount chains on the same tectonic plate can have very different shapes.

Müller教授總結說：「我們的結果可以幫助科學家解答這個謎題：為什麼在同一座板塊中，不同海底火山鍊之間的型態可以如此截然不同。」

"It is now clear that we first need to understand the dynamics of the deepest 'Underworld', right above the core, to unravel the history of volcanism at Earth's surface," said Professor Müller.

「我們現在相當明瞭首先要了解『地底世界』的最深處，也就是緊鄰地核處發生了何種動力過程，才能闡明地表火山活動為何會擁有這樣的歷史。」 Müller教授說。

Watch the animation here https://youtu.be/Xy5kHjAHXec

連結此網址來觀看動畫https://youtu.be/Xy5kHjAHXec

引用自：University of Sydney. "How the spectacular Hawaiian-Emperor seamount chain became so bendy." ScienceDaily. ScienceDaily, 11 May 2016.