by Matthew
Carroll
一類古老、遼闊、稱為穩定地塊的大陸地殼幫助地球的陸地數十億年來，即便經歷了陸塊漂移、山脈隆起與海洋形成等作用，仍然處在相當穩定的狀態。距今大約30億年前穩定地塊是透過什麼作用形成，在地球歷史的相關研究中是個歷時已久的問題。最近賓州州立大學的科學家提出的新機制或許能夠加以解釋。
這些稱作片麻岩的古老變質岩發現於北極海海岸。現今出露在地表的這些岩石，過去曾是陸地的根部。科學家表示穿插在這類岩石當中的沉積岩，就像熱機一樣具有讓陸地穩定下來的功能。圖片來源：Jesse Reimink
他們今日(5/8)發表在期刊《自然》(Nature)的文章提出大陸從海洋誕生時或許並非穩定的地體，也就是沒有富含花崗岩的上部地殼這道特徵。反之，他們認為距今大約30億年前，新生岩石跟風雨接觸引發了一連串的地質作用最後讓地殼穩定下來，使得它們可以存活數十億年而不被摧毀或重置。
科學家表示這項發現或許可以為類似地球、或許可以居住的行星是如何演化帶來新的理解。
「要形成類似地球的行星需要有大陸地殼，而且還要讓它們穩定下來，」研究作者之一Jesse
Reimink表示。他是賓州州立大學地球科學系的助理教授。「科學家一直以來認為這兩者是同一件事，也就是大陸穩定之後接著便浮出海面。但我們要說的是這兩個過程其實是分開的。」
穩定地塊可以從地球表面往下延伸超過150公里(約93英里)，直到上部地函。科學家表示它們就像船隻的龍骨一樣，可以讓陸地在長遠的地質時間當中一直漂浮在海平面的高度附近。
科學家表示風化作用的結果或許可以把產生熱能的元素，像是鈾、釷、鉀集中到地殼淺處，使得地殼深部的溫度降低下來而硬化。這道機制可以創造出厚實且堅固的岩層，也許能保護陸地底部不受之後的變形作用摧毀——此即穩定地塊的標準特徵。
「形成大陸地殼並使其穩定的步驟之一，是把產生熱能的元素集中到非常靠近地表的地方——你可以把這些元素想像成是一堆微小的熱機，」研究作者之一Andrew
Smye表示。他是賓州州立大球地球科學系的副教授。「要這麼做的原因是每當一顆鈾、釷或鉀原子衰變的時候，就會產生熱能使地殼的溫度提高。地殼在高溫下會變得不穩定，容易受到變形而且無法固定在原處。」
科學家表示隨著風雨及化學反應把早期陸地上的岩石分解，沉積物和黏土礦物會被沖到溪流並帶往海洋，接著堆積成頁岩這類富含鈾、釷、鉀的沉積岩。
構造板塊間的碰撞把這些沉積岩埋藏到地殼深部，在此頁岩會放出輻射熱而讓下部地殼熔融。由於熔融物質的密度較低因此會湧回上部地殼，使得可以產生熱能的元素集中在此處的岩石(如：花崗岩)，也讓下部地殼可以降溫並硬化。
一般認為穩定地塊形成於距今30億至25億年前，此時的放射性同位素(像是鈾)大概是以目前兩倍的速度衰變，因此釋放出的熱也是兩倍。
Reimink表示成果凸顯了穩定地塊在早中期地球形成的時機，對於可以讓它們穩定下來的作用來說也特別適合。
「我們可以用行星演化的角度來思考這道問題，」Reimink表示。「要讓一顆行星類似地球的關鍵要素之一，可能是在生涯相對較早的時候就要形成陸地。因為這樣才能形成非常高溫的放射性沉積物，進而產生廣大且相當穩定的大陸地殼。由於它們的高度一直保持在海平面附近，因此能提供絕佳的環境讓生命長久地繁衍下去。」
研究人員分析了數百個太古宙，也就是穩定地塊形成時的岩石樣品中的鈾、釷、鉀含量。根據實際的岩石成分，他們評估了放射熱的產量，接著再用這些數值來建立穩定地塊如何形成的熱力學模型。
「過往科學家已經探討並考量了放射性熱產量隨時間變化帶來的效應，」Smye表示。「但我們的研究把岩石產生的熱跟陸地誕生、沉積物形成與大陸地殼的分化這三者連結了起來。」
左邊的岩石年代較老，受過許多次的變形與置換作用。與之並置接觸的是右邊太古宙的花崗岩，其為讓大陸地殼穩固下來的熔融產物。圖片來源：Matt Scott
穩定地塊一般發現在大陸內部，其含有地球上最古老的某些岩石，但是研究起來還是有諸多困難。比方說在構造活躍的地區，造山帶形成可能會把埋在地底深處的岩石帶到地表。
但是穩定地塊的起源卻一直待在地底深處而無法讓我們取得。科學家表示他們未來的工作包含從穩定地塊的古老內部採集樣品，或許還會加上鑽取岩芯來驗證他們的模型。
「這些沉積岩遭到變質、熔融後產生了富含鈾與釷的花崗岩，它們就像飛機上的黑盒子一樣記錄了過去所處的溫度壓力，」Smye表示。「如果我們可以解開這些檔案，就能驗證我們的模型對於大陸地殼『飛行路線』的預測是否正確。」
Rock steady:
Study reveals new mechanism to explain how continents stabilized
Ancient, expansive tracts of continental
crust called cratons have helped keep Earth’s continents stable for billions of
years, even as landmasses shift, mountains rise and oceans form. A new
mechanism proposed by Penn State scientists may explain how the cratons formed
some 3 billion years ago, an enduring question in the study of Earth’s history.
The scientists reported today (May 8) in the journal Nature that the continents may not have
emerged from Earth’s oceans as stable landmasses, the hallmark of which is an
upper crust enriched in granite. Rather, the exposure of fresh rock to wind and
rain about 3 billion years ago triggered a series of geological processes that
ultimately stabilized the crust — enabling the crust to survive for billions of
years without being destroyed or reset.
The findings may represent a new understanding of how
potentially habitable, Earth-like planets evolve, the scientists said.
“To make a planet like Earth you need to make
continental crust, and you need to stabilize that crust,” said Jesse Reimink,
assistant professor of geosciences at Penn State and an author of the study.
“Scientists have thought of these as the same thing — the continents became
stable and then emerged above sea level. But what we are saying is that those
processes are separate.”
Cratons extend more than 150 kilometers, or 93 miles,
from the Earth’s surface to the upper mantle — where they act like the keel of
a boat, keeping the continents floating at or near sea level across geological
time, the scientists said.
Weathering may have ultimately concentrated
heat-producing elements like uranium, thorium and potassium in the shallow
crust, allowing the deeper crust to cool and harden. This mechanism created a
thick, hard layer of rock that may have protected the bottoms of the continents
from being deformed later — a characteristic feature of cratons, the scientists
said.
“The recipe for making and stabilizing continental
crust involves concentrating these heat-producing elements — which can be
thought of as little heat engines — very close to the surface,” said Andrew
Smye, associate professor of geosciences at Penn State and an author of the
study. “You have to do that because each time an atom of uranium, thorium or
potassium decays, it releases heat that can increase the temperature of the
crust. Hot crust is unstable — it’s prone to being deformed and won’t stick
around.”
As wind, rain and chemical reactions broke down rocks
on the early continents, sediments and clay minerals were washed into streams
and rivers and carried to the sea where they created sedimentary deposits like
shales that were high in concentrations of uranium, thorium and potassium, the
scientists said.
Collisions between tectonic plates buried these
sedimentary rocks deep in the Earth’s crust where radiogenic heat released by
the shale triggered melting of the lower crust. The melts were buoyant and
ascended back to the upper crust, trapping the heat-producing elements there in
rocks like granite and allowing the lower crust to cool and harden.
Cratons are believed to have formed between 3 and 2.5
billion years ago — a time when radioactive elements like uranium would have
decayed at a rate about twice as fast and released twice as much heat as today.
The work highlights that the time when the cratons
formed on the early middle Earth was uniquely suited for the processes that may
have led them to becoming stable, Reimink said.
“We can think of this as a planetary evolution
question,” Reimink said. “One of the key ingredients you need to make a planet
like Earth might be the emergence of continents relatively early on in its
lifespan. Because you’re going to create radioactive sediments that are very
hot and that produce a really stable tract of continental crust that lives
right around sea level and is a great environment for propagating life.”
The researchers analyzed uranium, thorium and
potassium concentrations from hundreds of samples of rocks from the Archean
period, when the cratons formed, to assess the radiogenic heat productivity
based on actual rock compositions. They used these values to create thermal
models of craton formation.
“Previously people have looked at and considered the
effects of changing radiogenic heat production through time,” Smye said. “But
our study links rock-based heat production to the emergence of continents, the
generation of sediments and the differentiation of continental crust.”
Typically found in the interior of continents,
cratons contain some of the oldest rocks on Earth, but remain challenging to
study. In tectonically active areas, mountain belt formation might bring rocks
that had once been buried deep underground to the surface.
But the origins of the cratons remain deep
underground and are inaccessible. The scientists said future work will involve
sampling ancient interiors of cratons and, perhaps, drilling core samples to
test their model.
“These metamorphosed sedimentary rocks that have
melted and produced granites that concentrate uranium and thorium are like
black box flight recorders that record pressure and temperature,” Smye said.
“And if we can unlock that archive, we can test our model’s predictions for the
flight path of the continental crust.”
Penn State and the U.S. National Science Foundation
provided funding for this work.
研究經費由賓州州立大學與美國國家科學基金會提供。
原始論文：Jesse R.
Reimink, Andrew J. Smye. Subaerial weathering drove stabilization of
continents. Nature, 2024; DOI: 10.1038/s41586-024-07307-1
引用自：Penn State. "Rock steady: Study reveals
new mechanism to explain how continents stabilized."
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