南安普敦大學的科學家解開了關於板塊運動的一大謎題：為什麼某些陸地的「穩定」部分可以逐漸抬升成地球最雄偉的地形特徵？又是透過什麼樣的作用？
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從Sentinel Hub Earth Observation Browser下載非洲南部大斷崖的衛星影像。該影像取自哨兵2號(Sentinel-2)的L1C資料庫。圖片來源： Tom Gernon教授，南安普敦大學。
他們發現板塊分裂的時候會在地球內部引發強烈的波動，使得陸地表面的抬升高度超過一公里。
他們的發現有助於解開一道歷時已久的謎題：稱為「斷崖」與「高原」的廣袤地形特徵是地球最壯觀的地景之一，它們對於氣候與生物都有深遠的影響。但是塑造出它們的動力是什麼，兩者之間又有何種關連？。
這篇新研究由南安普敦大學的科學家主持，他們探討了從數億年的時間尺度來看，全球構造作用力對於地形演育會有什麼樣的影響。成果發表於期刊《自然》(Nature)。
南安普敦大學的地球科學教授Tom
Gernon是研究主要作者，他說：「大斷崖是種高達一公里以上的陡峭地形特徵，環繞南非的一連串斷崖便是經典的例子。科學家長久以來猜測它們是在陸地張裂、最終分開的時候形成。然而，離這些斷崖相當遠的陸地內部為什麼也會抬升並遭到侵蝕，卻是非常難以解釋。其中的過程到底跟這些高聳的斷崖形成有沒有關係？簡單來說，我們一無所知。」
這些陸地中的穩定區域——稱為「穩定地塊」的垂直運動，長久以來都是板塊構造學說中理解最不完全的部分之一。
包括Thea
Hincks博士、Derek
Keir博士、Alice
Cunningham的南安普敦大學團隊，與亥姆霍茲波茨坦中心—GFZ德國地球科學研究中心(GFZ
Potsdam)以及伯明罕大學的同仁合作之下試著解決這道基本的問題。
他們的結果有助於解釋為什麼以往認為是「穩定」的部分陸地會遭到大幅抬升與侵蝕，以及這些作用如何朝內陸傳遞數百甚至數千公里，造成一整片區域往上抬起而形成高原，就像是南非的中央高原一樣。
鑽石與地形演育的關聯
南非龍山山脈的斷崖。圖片來源：Jean
Braun教授，GFZ
Potsdam
他們去年發表於《自然》的研究找出了會將鑽石噴出的火山爆發與陸地分裂之間的關係。以此為基礎，團隊運用了先進的電腦模型與統計方法，調查大陸板塊的分裂過程長久下來會讓地表產生什麼樣的反應。
他們發現當陸地分裂開來，大陸地殼伸張會翻攪地函(位在地殼與地核之間的巨大岩石層)的內部。
Sascha Brune教授是GFZ
Potsdam地球動力學模擬部門的主任，他說：「這道過程就像是往陸地內部席捲的一道浪潮，擾動了它們的根源。」
同樣任職於GFZ的Brune教授與Anne
Glerum博士進行模擬以探討這道過程如何開始。他們注意到一個非常有趣的模式：在模擬當中，這些地函波在陸地下方的行進速度，以及古代岡瓦納超大陸分裂之後，席捲南非地表的大型侵蝕事件的傳遞速度，兩者之間非常接近。
這群科學家在拼湊各方證據之後，提出大斷崖一開始形成於古代的裂谷邊緣，就像今日的東非大裂谷兩側是相當陡峭的岩壁一樣。於此同時，張裂事件也會引發「深層地函波」，它會沿著大陸底部以每百萬年15到20公里速度的行進。
他們認為這股地函波能夠以對流的方式剝除陸地根部的岩層。
「就像卸掉熱氣球上的重物來讓它飛得更高一樣，陸地的組成消失一部分之後也會使陸地抬升——此作用稱為『地殼均衡』，」Brune教授表示。
根據此原理，團隊模擬了這種由地函驅動的抬升會讓地形有什麼樣的變化。他們發現地函擾動的傳遞會引起一陣持續數千萬年的地表侵蝕事件，並以相似的速度在地表擴散開來。這些強烈的侵蝕作用會移除大量的岩石，使得地表進一步向上抬升，形成聳立的高原地形。
「我們的地形演育模型顯示了跟張裂有關的一連串事件，可以造成斷崖以及穩定且平坦的高原，即使在過程中有數千公尺厚的岩石遭到了侵蝕，」GFZ
Potsdam與波茨坦大學的教授，專長為地表作用模擬的Jean
Braun表示。
陸地邊緣通常是抬升作用進行的地方，因此離該處相當遠的穩定陸塊為什麼會發生垂直運動，長久以來都是個謎團。此團隊的研究結果提出新的方法來加以解釋。
伯明罕大學地球系統科學的副教授Steve
Jones博士補充：「我們在此提出了一個可信的論點，主張張裂作用在特定情況下，可以直接在上部地函產生持續許久、規模有陸地大小的對流胞，而這些由張裂引發的對流系統對於地表的地形、侵蝕、沉積與自然資源的分布來說都具有深遠的影響。」
團隊的結論中表示一連串的地函擾動也會讓鑽石從地球內部深處快速上升到地表，並且從基礎層面上改變陸地的地形而對許多方面產生影響，像是區域性氣候、生物多樣性，以至於人類的居住模式。
Gernon教授從慈善機構WoodNext基金會(由Greater
Houston Community Foundation管理)獲得許多經費來研究全球冷化。他解釋陸地分裂不只可以對地球深處造成擾動，其影響也會迴盪至先前認為是穩定的陸地區域的表面。
「陸地的核心部分變得不穩定勢必也會對古代的氣候造成衝擊，」Gernon教授總結。
Scientists
uncover hidden forces causing continents to rise
Scientists at the University of
Southampton have answered one of the most puzzling questions in plate tectonics:
how and why ‘stable’ parts of continents gradually rise to form some of the
planet’s greatest topographic features.
They have found that when tectonic plates break
apart, powerful waves are triggered deep within the Earth that can cause
continental surfaces to rise by over a kilometre.
Their findings help resolve a long-standing mystery
about the dynamic forces that shape and connect some of the Earth’s most
dramatic landforms – expansive topographic features called ‘escarpments’ and
‘plateaus’ that profoundly influence climate and biology.
The new research, led by the University of
Southampton, examined the effects of global tectonic forces on landscape
evolution over hundreds of millions of years. The findings are published in the
journal Nature .
Tom Gernon , Professor of Earth Science at the
University of Southampton and lead author of the study said: “Scientists have
long suspected that steep kilometre-high topographic features called Great
Escarpments — like the classic example encircling South Africa — are formed
when continents rift and eventually split apart. However, explaining why the
inner parts of continents, far from such escarpments, rise and become eroded
has proven much more challenging. Is this process even linked to the formation
of these towering escarpments? Put simply, we didn’t know.”
The vertical motions of the stable parts of
continents, called cratons, remain one of the least understood aspects of plate
tectonics.
The team from the University of Southampton,
including Dr Thea Hincks , Dr Derek Keir , and Alice Cunningham, collaborated
with colleagues from the Helmholtz Centre Potsdam – GFZ German Research Centre
for Geosciences and the University of Birmingham to address this fundamental
question.
Their results help explain why parts of the
continents previously thought of as ‘stable’ experience substantial uplift and
erosion, and how such processes can migrate hundreds or even thousands of
kilometres inland, forming sweeping elevated regions known as plateaus, like
the Central Plateau of South Africa.
Linking diamonds
with landscape evolution
Building on their study linking diamond eruptions to
continental breakup, published last year in Nature
, the team used advanced computer models and statistical methods to interrogate
how the Earth’s surface has responded to the breakup of continental plates
through time.
They discovered that when continents split apart, the
stretching of the continental crust causes stirring movements in Earth’s mantle
(the voluminous layer between the crust and the core).
Professor Sascha Brune, who leads the Geodynamic
Modelling Section at GFZ Potsdam, said: “This process can be compared to a
sweeping motion that moves towards the continents and disturbs their deep
foundations.”
Professor Brune and Dr Anne Glerum, also based at
Potsdam, ran simulations to investigate how this process unfolds. The team
noticed an interesting pattern: the speed of the mantle ‘waves’ moving under
the continents in their simulations closely match the speed of major erosion
events that swept across the landscape in Southern Africa following the breakup
of the ancient supercontinent Gondwana.
The scientists pieced together evidence to propose
that the Great Escarpments originate at the edges of ancient rift valleys, much
like the steep walls seen at the margins of the East African Rift today. Meanwhile,
the rifting event also sets about a ‘deep mantle wave’ that travels along the
continent’s base at about 15-20 kilometres per million years.
They believe that this wave convectively removes
layers of rock from the continental roots.
“Much like how a hot-air balloon sheds weight to rise
higher, this loss of continental material causes the continents to rise – a
process called isostasy,” said Professor Brune.
Building on this, the team modelled how landscapes
respond to this mantle-driven uplift. They found that migrating mantle
instabilities give rise to a wave of surface erosion that lasts tens of
millions of years and moves across the continent at a similar speed. This
intense erosion removes a huge weight of rock that causes the land surface to
rise further, forming elevated plateaus.
“Our landscape evolution models show how a sequence
of events linked to rifting can result in an escarpment as well as a stable,
flat plateau, even though a layer of several thousands of meters of rocks has
been eroded away”, explained Jean Braun, Professor of Earth Surface Process
Modelling at GFZ Potsdam, also based at the University of Potsdam.
The team’s study provides a new explanation for the
puzzling vertical movements of cratons far from the edges of continents, where
uplift is more common.
Dr Steve Jones, Associate Professor in Earth Systems
at the University of Birmingham, added: “What we have here is a compelling
argument that rifting can, in certain circumstances, directly generate
long-lived continental scale upper mantle convection cells, and these
rift-initiated convective systems have a profound effect on Earth’s surface
topography, erosion, sedimentation and the distribution of natural resources.”
The team has concluded that the same chain of mantle
disturbances that trigger diamonds to quickly rise from Earth’s deep interior
also fundamentally shape continental landscapes, influencing a host of factors
from regional climates and biodiversity to human settlement patterns.
Professor Gernon, who was awarded a major
philanthropic grant from the WoodNext Foundation, administered by Greater
Houston Community Foundation, to study global cooling, explained that
continental breakup disturbs not only the deep layers of the Earth but also has
effects that reverberate across the surface of the continents, previously
thought to be stable.
“Destabilising the cores of the continents must have
impacted ancient climates too,” concluded Professor Gernon.
原始論文：Thomas M.
Gernon, Thea K. Hincks, Sascha Brune, Jean Braun, Stephen M. Jones, Derek Keir,
Alice Cunningham, Anne Glerum. Coevolution of craton margins and
interiors during continental break-up. Nature, 2024; 632
(8024): 327 DOI: 10.1038/s41586-024-07717-1
引用至：University of Southampton. "Scientists
uncover hidden forces causing continents to rise."
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