為什麼地球有些地方30億年來幾乎沒有什麼改變？

原文網址：https://pursuit.unimelb.edu.au/articles/why-parts-of-earth-have-barely-changed-in-3-billion-years

為什麼地球有些地方30億年來幾乎沒有什麼改變？

By Dr Daryl Holland, University of Melbourne

關於地球許久之前的過去存在著一道謎題，線索則藏在澳洲沙漠的岩石之中以及地球其他歷史悠久的地方。

對於地球是如何形成，過去一個世紀以來我們目睹了人類在這方面的認知有了飛快進展，並且更加瞭解板塊構造運動是如何推移陸塊，持續形塑我們所見的大地、海洋與山脈。

但是地質學家在某個重要的問題上迄今仍未取得共識：在陸地形成以前的地球是什麼樣子？

發表在期刊《地質》(Geology)的新研究支持的理論認為當時地球的火山活動相當活躍，而從此狀態轉變成板塊構造運動的劇烈過程所留下的證據在今日依然可見。

Adam Beall博士表示：「地質學的根本思想稱為『均變說』(uniformitarianism)，也就是我們可以研究今日正在進行的地質作用，進而瞭解地球在數百萬年的時間尺度下是如何運作。」他在墨爾本大學攻讀博士時的研究主題為早期地球的動力學。

「但是，當我們試著去瞭解早期地球時此方法就會變得不管用，這是因為當時地球的溫度比現今高出許多，且運作的方式也完全不同。要想出地球最古老的陸塊是透過何種我們再也無法觀察到的作用形成，對於地質學家來說是相當艱鉅的任務。」

「主要有兩種觀點，」墨爾本大學地球科學院的教授Louis Moresi表示。

「一種是不存在所謂的『在板塊構造運動之前』。地球最初是一團由塵埃組成的雲氣，隨著地球逐漸成形、結晶，然後成為固體時，他們認為立刻就出現了某種形式的板塊構造運動。」

「另一種理論則認為在地球誕生的最初十億年左右，類似於現今板塊構造運動的作用是完全不存在的。」

地球不斷受到內部的放射性物質加熱，而這些熱能需要宣洩到某些地方。板塊構造運動(有時也稱作大陸漂移)正是地球用來釋放熱能的方法。

Moresi教授表示：「經由板塊構造運動洋盆整體會像輸送帶般地傳動，使得地球表面的低溫物質可以進入地球內部，而裡面的高溫物質則可以來到外面。利用這種方式就可以散去能量。」

科學家一致同意早期地球跟今日相比，溫度要來得更高且放射性物質的量也更多。因此，如果當時沒有板塊構造運動，那這些熱量都去哪裡了？

地質學家從太陽系其他地方尋找可以代替的解釋。

木衛一是木星的69顆衛星之一，它是太陽系火山活動最劇烈的地方。Moresi教授表示木衛一可以當作古代地球的類比。

「在木衛一，內部的熱能基本上是藉由火山活動而來到外部，所以我們才能看見那些永無止盡噴發著的火山。」

這個稱為「熱管地球」(Heat-Pipe Earth)的理論正受到越來越多人的支持。

岩石圈包含了地球的地殼和上部地函。Moresi教授和他的同事建立了一個開源(open source)電腦模型來模擬岩石圈至地下大約200公里深的地方，利用這項工具可以完美地模擬熱管地球是如何結束。

Beall博士在接受Moresi指導的博士後研究期間，跟美國華盛頓州立大學的副教授Katie Cooper合作，運用這個稱為「地底世界」(Underworld)的程式來模擬早期地球轉變成板塊構造運動的過程。他們著重於熱管地球大部分地區表面覆蓋的薄薄一層堅硬岩石，稱為「熱管封蓋」(Heat-Pipe lid)。

藉此他們或許能同時解決第二個地質學問題：為什麼地球有些地方可以不受板塊構造運動的影響？

雖然地殼大部分都會在板塊構造運動的作用之下，持續不斷地被磨碎、熔融、抬升和侵蝕。有些稱為「穩定地塊」(craton)的地區經過了數十億年卻還是沒有多少變化。舉例來說，在澳洲西部、亞馬遜盆地、非洲南部和加拿大部分地區，可以發現這種大致上相當平坦的寬闊地形。

「即使板塊構造運動可以創造出像是喜馬拉雅山之類的雄偉構造，它們最終仍會被侵蝕殆盡。」Moresi教授表示。

「但那些穩定地塊就這樣一直待在那邊。它們沒有受到太大的變形，我們在某些地方仍然可以看見它們一開始就有，年代將近40億年的構造。」

澳洲有些地方就是由這類古老的穩定地塊組成。它們也蘊含了澳洲大多數的礦物資源，包括澳洲西部面積十分廣大的鐵礦床。

Moresi教授表示熱管理論不只解釋了這些岩石最初是如何形成，同時也解釋了為什麼它們是如此強韌。

地質學家已經繪製出穩定陸塊擁有相當厚的火山岩，支持了穩定地塊的地殼最初是由火山爆發形成的說法。但Moresi教授表示這種作用應該只會形成薄薄一層岩石，然而穩定陸塊卻相當厚，超過200公里。

「因此它們形成的時候必定有方法可以讓它們長到非常厚而且相當、相當地堅固。」他說

「第一次有人提出這些古老的穩定地塊擁有相當驚人的厚度是在50多年前，但直到現在仍然沒有人能夠解開它們為何如此厚實。」Beall博士表示。

「跟我們一同進行這項計畫的Katie Cooper副教授在數年前提出了一種假說，認為低溫的地函岩石往下沉的時候造成穩定地塊增厚。」

「這需要非比尋常的大量地函岩石下沉才有可能發生，而我猜想它的引發原因有沒有可能是板塊構造運動開始運作。因為這起事件的規模應該會相當劇烈，而且和穩定地塊的形成或許會在差不多的時間發生。」

研究人員利用電腦模型來驗證此想法。模擬顯示在熱管地球轉變成板塊構造運動的劇烈過程中形成了穩定地塊。

Moresi教授表示：「我們得出了相當簡潔的解答。」

「在轉變成板塊構造運動的途中，地球內部經歷了十分徹底的翻轉。在此之前地球已經儲存了累積十億年左右的大量能量，它們在短時間之內一鼓作氣地宣洩出來。從影片中可以看到十分強烈的翻轉推動了薄層岩石使它們堆積在這些(穩定地塊)地區。」

「地球之後再也沒有發生過這種現象，因為板塊運動一旦開始作用，地球的運作原理就變得截然不同，造成壓力不會再次累積。」

「因此地球一口氣就創造出這些經過數十億年仍然存在，強度驚人的岩石。」

Why parts of earth have barely changed in 3 billion years?

There is a mystery in Earth’s ancient past, and the clues lie in the desert rocks of Australia and other ancient places.

The last century has seen rapid advances in our understanding of how the Earth formed, and how the movement of continents through plate tectonics continues to shape our lands, oceans and mountain ranges.

But geologists are yet to agree on one important question: what was the Earth like before the plates formed?

New research, published in the journal Geology, supports the theory that early Earth was highly volcanically active, and that evidence of the violent transition to plate tectonics can still be seen today.

“Geology is built upon an idea, uniformitarianism, that we can study geological processes occurring today and use these to understand how the Earth works at timescales of millions of years”, says Dr Adam Beall, who studied early Earth dynamics during his PhD project at the University of Melbourne.

“This method breaks down when we try to understand the early Earth, because it was hotter and behaved in a completely different way. Geologists have the difficult task of imagining how the Earth’s oldest continents formed by processes we can no longer observe.”

“There are two views,” says Professor Louis Moresi, from the School of Earth Sciences at the University of Melbourne.

“One is that there was no such thing as ‘before plate tectonics’. There was a cloud of dust, you formed the Earth, and as it crystallises out and becomes solid you immediately get some form of plate tectonics.

“And then the other paradigm is that the first billion or so years of Earth were nothing like modern plate tectonics.”

The Earth is constantly being heated from within by radioactivity, and this heat needs to go somewhere. Plate tectonics, sometimes called continental drift, is the planet’s way of releasing this heat.

“With plate tectonics, the whole ocean basin rolls over, which puts the cold outside stuff into the interior and the hot inside stuff on the surface, which is how you get the energy out,” says Professor Moresi.

Scientists agree that early Earth was hotter and more radioactive than it is today. So if there was no plate tectonics, where did all that heat go?

Geologists looked elsewhere in the Solar System for an alternative explanation.

Io, one of Jupiter’s 69 moons, is the most volcanically active place in the Solar System, and Professor Moresi says this could be a model for the ancient Earth.

“On the moon Io, the inside heat turns itself over to the outside volcanically so you just basically have these endless eruptions of volcanoes.”

This theory, which is gaining in popularity, is called the ‘Heat-Pipe Earth’.

Professor Moresi and his colleagues have built an open source computer model of the lithosphere, which covers the Earth’s crust and upper mantle, to a depth of about 200 kilometres, and this was the perfect tool to model the death of the Heat-Pipe Earth.

Dr Beall, during his PhD work supervised by Professor Moresi and working with Associate Professor Katie Cooper from Washington State University, used this program, called Underworld, to model early Earth’s transition to plate tectonics, focussing on the thin layer of solid rock – called the ‘Heat-Pipe lid’ - that would have covered most of the Heat-Pipe Earth.

And by doing so they may have solved a second geological mystery - why are there parts of the Earth that are not affected by plate tectonics?

While most of the Earth’s crust has been constantly crushed, melted, uplifted and eroded through the actions of plate tectonics, some regions, called cratons, haven’t changed in billions of years. Examples of these large, mostly flat landforms are found in Western Australia, the Amazon basin, Southern Africa and parts of Canada.

“Plate tectonics creates a massive structure like the Himalayas, but eventually it will just erode away,” says Professor Moresi.

“And yet these cratons just sit there, and they don’t deform very much, and we can still see the original, nearly four-billion-year-old structure in some places.”

Parts of Australia are built from these ancient cratons, and they are the source of much of our mineral wealth, including the vast iron ore deposits in Western Australia.

Professor Moresi says the heat pipe theory explains how these rocks originally formed, but not why they are so strong.

Thick layers of volcanic rock have been mapped out, supporting the idea that the cratonic crust initially formed through many volcanic eruptions. But Professor Moresi says this process should form thin layers of rock, while cratons are very thick, more than 200 kilometres.

“So they must have formed in a time where they were able to become very thick, and very very strong,” he says.

“The extreme thickness of these ancient cratons was first proposed more than 50 years ago, but no one has been able to solve the mystery of why they are so thick,” says Dr Beall.

“A few years ago, our colleague on this project, Associate Professor Katie Cooper, came up with a hypothesis that the cratons had thickened as cold mantle rock sunk below.

“An unusually large amount of sinking mantle rock is required and I wondered if this could be triggered by the initiation of plate tectonics, which would have been catastrophic and probably occurred at a similar time to craton formation.”

The researchers tested this idea with the computer model. In the simulations, the cratons are formed during the violent transition from heat-pipe to plate tectonics.

“Our solution is pretty simple,” says Professor Moresi.

“During the transition to plate-tectonics, the Earth goes through this complete overturn. It has stored up a lot of energy for a billion years or so, and then it’s all released in a short period. And you can see in the video that the flat thin rock gets crumpled up into these zones by this very strong, impulsive overturn.

“And that doesn’t repeat, because once plate tectonics starts it’s a different paradigm and you don’t build up that stress again.

“So in one action you create these incredibly strong rocks that then last for billions of years.”

原始論文：A.P. Beall, L. Moresi, C.M. Cooper. Formation of cratonic lithosphere during the initiation of plate tectonics, Geology, 2018. DOI: 10.1130/G39943.1

引用自：University of Melbourne. “Why parts of earth have barely changed in 3 billion years?”