原文網址：https://www.southampton.ac.uk/news/2021/01/atlantic-plate-tectonics.page
新研究發現從地殼下方深處湧上來的物質可能是把南北美大陸推離歐洲和非洲的原因。
承載著美洲的板塊和承載著歐洲與非洲的板塊正以每年四公分的速度彼此遠離。位在這些陸塊的中間是大西洋中洋脊，新板塊會在這裡形成，而兩側的板塊則分別往西和往東移動；在洋脊之下，往上升的物質會填補板塊分開之後產生的空缺。
科學家普遍認為此過程的動力來自於遠方板塊密度較高的部分，往下沉回地球內部時產生的重力。但是在大西洋周遭的板塊並沒有因密度較高而往下沉，因此大西洋兩側板塊彼此分離的原動力為何仍是一道謎題。
最近南安普敦大學領導的地震學家團隊，發現的證據顯示大西洋中洋脊下方有地函物質(介於地殼與地核之間的部分)從超過600公里深的地方湧上來。這股來自下方的力量可能會推動板塊，使得兩旁的陸地彼此分開。過往一般認為洋脊下方湧上來的物質來源的深度要淺很多，大約為60公里深而已。
這項發表於期刊《自然》(Nature)的發現能讓我們對板塊運動――這個在全世界造成許多天災，像是地震、海嘯與火山爆發的作用――有更深的瞭解。
此研究是PI-LAB
(Passive Imaging of the Lithosphere-Asthenosphere Boundary，岩石圈―軟流圈邊界被動成像)與EURO-LAB(Experiment
to Unearth the Rheological Oceanic Lithosphere-Asthenosphere Boundary，海洋岩石圈―軟流圈邊界流變學實驗)的一部份。團隊在研究船藍賽斯號(RV
Langseth)與發現號(RRV
Discovery)的兩次航程當中於大西洋底設置了39個地震儀，蒐集到的數據首度以高解析度呈現了大西洋中洋脊下方的地函的大尺度影像。這是在海洋少數規模如此浩大的實驗，從結果產生的影像團隊可以看出地函在深度410公里到660公里左右，礦物相在發生劇烈改變的同時構造有什麼變化。他們觀察到的訊號指出地函深處有物質緩緩上升，這在他們的意料之外。
主要作者Matthew
Agius之前是南安普敦大學的博士後研究員，現在則在羅馬第三大學進行研究。他說：「這趟在大西洋海上總計十個禮拜的任務讓我十分難忘。我們的成果令人相當驚奇，從這些前所未有的觀察結果當中，我們對於地球內部和板塊運動的關聯有了全新的見解。」
領導這些實驗的為南安普敦大學的Kate
Rychert 和Nick
Harmon博士以及牛津大學的Mike
Kendall教授，他們也是主持這兩趟航次的科學家。實驗經費來自NERC(英國自然環境研究委員會)與ERC(歐洲研究委員會)。
Harmon博士表示：「北美和歐洲正在漸行漸遠，這並非政治或者理念上的差異所導致，而是因為地函對流！」
除了幫助科學發展更好的模型以及開發對於自然災害的預警系統，板塊運動也會對海平面造成很大的影響，因此評估地質時間尺度上的氣候變遷時也會造成影響。
Rychert博士表示：「這項結果完全出人意料。在了解地球的演化過程以及適居性時這具有很多方面的意涵，也強調出從海洋蒐集新的數據相當重要。那裡還有很多事物等著我們去探索！」
Mike Kendall教授也表示：「這項成果令人為之一振，它否決了長久以來中洋脊只是被動地參與板塊運動的推論。在大西洋中央的這類場所當中，洋脊產生的力道或許在讓新形成的板塊分開時扮演了很重要的腳色。」
Geological phenomenon widening the
Atlantic Ocean
An upsurge of matter from deep beneath
the Earth’s crust could be pushing the continents of North and South America
further apart from Europe and Africa, new research has found.
The plates attached to the Americas are moving apart
from those attached to Europe and Africa by four centimetres per year. In
between these continents lies the Mid-Atlantic Ridge, a site where new plates
are formed and a dividing line between plates moving to the west and those
moving to the east; beneath this ridge, material rises to replace the space
left by the plates as they move apart.
Conventional wisdom is that this process is normally
driven by distant gravity forces as denser parts of the plates sink back into
the Earth. However, the driving force behind the separation of the Atlantic
plates has remained a mystery because the Atlantic ocean is not surrounded by
dense, sinking plates.
Now a team of seismologists, led by the University of
Southampton, have found evidence of an upwelling in the mantle – the material
between the Earth’s crust and its core – from depths of more than 600
kilometres beneath the Mid Atlantic ridge, which could be pushing the plates
from below, causing the continents to move further apart. Upwellings beneath
ridges are typically thought to originate from much shallower depths of around
60 km.
The findings, published in the journal Nature provide a greater understanding
of plate tectonics which causes many natural disasters around the world,
including earthquakes, tsunamis and volcanic eruptions.
Over two research cruises on the RV Langseth and RRV
Discovery, the team deployed 39 seismometers at the bottom of the Atlantic as
part of the PI-LAB (Passive Imaging of the Lithosphere-Asthenosphere Boundary)
experiment and EURO-LAB (Experiment to Unearth the Rheological Oceanic
Lithosphere-Asthenosphere Boundary). The data provides the first large scale
and high-resolution imaging of the mantle beneath the Mid-Atlantic Ridge. This
is one of only a few experiments of this scale ever conducted in the oceans and
allowed the team to image variations in the structure of the Earth’s mantle
near depths of 410 km and 660 km – depths that are associated with abrupt
changes in mineral phases. The observed signal was indicative of a deep,
sluggish and unexpected upwelling from the deeper mantle.
Lead author, Matthew Agius, a former post-doctoral
fellow at the University of Southampton and currently at Università degli studi
Roma Tre said: “This was a memorable mission that took us a total of 10 weeks
at sea in the middle of the Atlantic Ocean. The incredible results shed new
light in our understanding of how the Earth interior is connected with plate
tectonics, with observations not seen before.”
Dr Kate Rychert and Dr Nick Harmon from the
University of Southampton and Professor Mike Kendall from the University of
Oxford led the experiment and were the chief scientists on the cruises. The
experiment was funded by NERC (Natural Environment Research Council, UK) and
the ERC (European Research Council).
Dr Harmon said: “There is a growing distance between
North America and Europe, and it is not driven by political or philosophical
differences - it is caused by mantle convection!”
As well as helping scientists to develop better
models and warning systems for natural disasters, plate tectonics also has an
impact on sea levels, and therefore affects climate change estimates over
geologic times scales.
Dr Rychert said: “This was completely unexpected. It
has broad implications for our understanding of Earth’s evolution and
habitability. It also demonstrates how crucial it is to gather new data from
the oceans. There is so much more to explore!”
Professor Mike Kendall added: “This work is exciting
and that it refutes long held assumptions that mid-ocean ridges might play a
passive role in plate tectonics. It suggests that in places such as the
Mid-Atlantic, forces at the ridge play an important role in driving
newly-formed plates apart.”
原始論文：Matthew R.
Agius, Catherine A. Rychert, Nicholas Harmon, Saikiran Tharimena, J.-Michael
Kendall. A thin mantle transition zone beneath the equatorial
Mid-Atlantic Ridge. Nature, 2021; 589 (7843): 562 DOI: 10.1038/s41586-020-03139-x
引用自：University of Southampton. "Geological
phenomenon widening the Atlantic Ocean."
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