原文網址：https://msutoday.msu.edu/news/2020/hawaii-pancake-discovery
By Emilie Lorditch
密西根州立大學(MSU)的地球物理學家表示，他們在西伯利亞東部800公里深的地下，首次觀察到有海底高原出現在下部地函，這項發現讓夏威夷的形成時間可以往前推到距今一億年前。
夏威夷「鬆餅」的旅程：從地函柱形成，接著滑入太平洋板塊下方，最後沉到地函深處。圖片來源：密西根州立大學
得到這項發現的是密西根州立大學地球與環境科學系的地質科學副教授Songqiao
“Shawn” Wei。他運用一項創新技術進行研究之後，在數據裡注意到某些不尋常。這項成果11月20日發表在期刊《科學》(Science)。
地函大部分都是固態的，但在中洋脊，地函會融化而在兩個板塊(像是太平洋板塊)之間形成新的海洋地殼。Wei說這些新生成的太平洋地殼厚度通常很一致，都是四英里。
熱地函柱是從地函內部緩慢上升的固態岩石。隨著上方的板塊持續移動，它會造成板塊融化而形成夏威夷群島這樣的火山群。地函柱具有像蘑菇一樣的廣大頭部，寬度可達數千英里，不過細長的尾部寬度卻只有數百公里。
Wei說蘑菇頭一接觸地表的海洋便會開始延伸並且變扁，同時造成上方的板塊融化而形成狀似鬆餅，厚度有20英里的海底高原。這道過程會隨著更多地函來到地表而持續下去，同時上方的板塊還是會繼續移動。長時間下來，留下來的便是點綴在海上的島鏈。
「正常來說，應該會看到蘑菇頭形成的鬆餅狀海底高原，後方接著蘑菇梗形成的一點一點的島鏈，」Wei說。「夏威夷群島是梗的末端，但它們像是鬆餅的頭部在哪？」
是否每個地函柱最初都會形成「鬆餅」，以及這些鬆餅狀的海底高原最終跑到哪去了，都還有許多爭議。包含這些古老的海底高原在內，想要找到古代的海洋地殼並不容易，因為它們可能已經隱沒、滑入海溝的裡面或者下方，因而消失在地球表面。
雖然科學家一般認為海洋地殼隱沒之後仍會留存在地函裡面，但是它們的厚度通常太薄，難以用震波層析成像等傳統技術來觀察。Wei一直以來認為夏威夷的「鬆餅」也是如此，直到在數據裡找出了令他感到驚訝的訊號。
「我在地下500英里深的地方看到了一塊特別厚的海洋地殼，」他說。「雖然這塊海洋地殼的厚度是它可以被辨識出來的原因，但它其實還是很薄而且位在相當深的地方，因此非常難發現。」
Wei和他的團隊針對某種類型的震波圖彙整了至今最大的資料庫，接著利用MSU網路運用研究所的高效能運算叢集系統進行大數據分析以及數值模擬。Wei的合作對象包括斯克里普斯海洋研究所的Peter M. Shearer、加州大學洛杉磯分校的Carolina Lithgow-Bertelloni和Lars Stixrude、密西根州立大學的Dongdong Tian。
團隊也結合了震波層析成像、反射震測和礦物物理學的長處。他們運用先前發表的震波層析成像研究，創造出地函的立體影像，從中可以依稀看出古代的太平洋板塊；接著利用這項研究最核心的觀測方法――反射震測的結果來找出地函極為深處這塊較厚的地殼；最後透過礦物物理學證實他們偵測到的訊號確實是一片海底高原。
板塊重建模型則幫助研究人員找出這塊新發現的海底高原和夏威夷的「鬆餅」有關，大約是在一億年前夏威夷熱點形成時產生的。
有個假說認為夏威夷的「鬆餅」分裂成兩塊。
其中一塊是依邪那岐板塊的一部份，整個板塊大約在七千萬到八千萬年前就已經隱沒到阿留申海溝而消失無蹤。另一塊則是太平洋板塊的一部份，它在兩千萬到三千萬年前進入堪察加海溝之後，較重的海洋地殼便一直往地函深處下沉，直到現在才被Wei和團隊給找出來。
這項發現不只提供了跟夏威夷歷史早期有關的線索，也闡明了其他熱點、海底山和海底高原的演化過程。研究人員計畫利用這項結合震波層析成像、反射震測和礦物物理學的新技術，找出其他「消失的鬆餅」，並且持續尋找古老的海洋地殼碎片存在於地球深部的線索。
Researchers discover
'missing' piece of Hawaii's formation
An oceanic plateau has been observed for
the first time in Earth’s lower mantle, 800 kilometers deep underneath Eastern
Siberia, pushing Hawaii’s birthplace back to 100 million years, says a Michigan
State University geophysicist.
The discovery came when Songqiao “Shawn” Wei, an
Endowed Assistant Professor of Geological Sciences in MSU’s Department of Earth
and Environmental Sciences, noticed something unusual in his data using
groundbreaking techniques. Wei’s research will be published on Nov. 20 in the
journal Science.
Earth’s mantle is mostly solid, but at a mid-ocean
ridge it melts creating new oceanic crust between two tectonic plates such as
the Pacific Plate. Typically, this new Pacific Ocean crust has a uniform
thickness of four miles, Wei said.
As the plates continue to move, a hot plume of solid
rocks slowly rises in the mantle melting the tectonic plate to create volcanoes
like the Hawaiian Islands. The mantle plume has a mushroom-like shape with a
wide head that is thousands of miles across and a thin tail that is only of a
few hundred miles across.
Wei said once this mushroom head reaches Earth’s
surface in the ocean, it stretches and flattens out, while it melts the
overriding tectonic plate to form a pancake-shaped 20-mile-thick oceanic
plateau. This process continues as more of the mantle reaches the surface and
the overriding plate continues to move. Over time, what remains is a dotted
trail of islands.
“Normally, you would
see a pancake-shaped oceanic plateau created by the mushroom's head followed by
a dotted chain of islands created by the mushroom’s tail,” Wei said.
“The Hawaiian Islands are the end of the tail but where is Hawaii’s
pancake head?”
There are still
debates on whether every mantle plume creates a “pancake”
during its earliest history, and the ultimate destination of these
pancake-shaped oceanic plateaus. Trying to find ancient oceanic crust,
including old oceanic plateaus, is difficult because the
crust might have subducted or slid into or
underneath an oceanic trench and disappeared from Earth’s
surface.
Although scientists
generally believe the oceanic crust is preserved in Earth’s
mantle after subduction, it is usually too thin to be observed using
conventional technology, such as seismic tomography. Up until now, this is what
Wei thought happened to Hawaii’s “pancake” until he detected
a surprising signal in the data.
“I spotted an
unusually thick chunk of oceanic crust about 500 miles beneath Earth’s
surface,” he said. “The thickness of this piece of crust made it
distinguishable, but it was still too thin and too deep to be
easily found.”
Wei and his team
compiled the largest dataset of a specific type of
seismograms and conducted big data analysis and numerical simulations
on the High-Performance Computing Cluster managed by the MSU
Institute for Cyber-Enabled Research. His collaborators
include: Peter M. Shearer from Scripps Institute of
Oceanography; Carolina Lithgow-Bertelloni and Lars Stixrude from
the University of California, Los Angeles; and Dongdong Tian from
MSU.
The team also
combined the strengths of seismic tomography, seismic reflection and mineral
physics. Seismic tomography from previously published work creates a 3-D image
which revealed a vague image of the ancient Pacific Plate in the mantle.
Seismic reflection results —the core observation of this work—helped the
researchers find the thick crust at great depths. Mineral physics was
used by the team to prove that the detected signal indicates a piece
of oceanic plateau.
Plate reconstruction
modeling helped the researchers link the newly found oceanic plateau to the
Hawaiian “pancake” that was created during the formation of the
Hawaii hotspot approximately 100 million years ago.
One hypothesis is that
the Hawaii “pancake” broke into two pieces.
One piece was part of
the Izanagi Plate which subducted into the Aleutian Trench and disappeared
about 70-80 million years ago. The other piece was part of the Pacific Plate
and after it entered the Kamchatka Trench 20-30 million years ago, the heavy
oceanic crust sunk deep into Earth’s mantle later until Wei and his
team spotted it.
This discovery
not only provides clues of Hawaii’s early history, but also sheds light on
the evolution of other hotspots, seamounts and oceanic plates. The researchers
plan to use this new technique combining seismic tomography, seismic reflection
and mineral physics to find other “missing pancakes” and to continue looking
for evidence of older pieces of Earth’s oceanic crust in the deep Earth.
原始論文：Songqiao S. Wei, Peter M. Shearer, Carolina Lithgow-Bertelloni,
Lars Stixrude, Dongdong Tian. Oceanic
plateau of the Hawaiian mantle plume head subducted to the uppermost lower
mantle. Science, 2020 DOI:
10.1126/science.abd0312
引用自：Michigan State University. “Researchers discover 'missing' piece
of Hawaii's formation”
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