冰河沉積物潤滑了板塊構造運動的引擎
新的研究強調了遍及全世界的冰河作用、沉積物、板塊構造運動三者之間有所關聯
地球最外層是由巨大的板塊拼接而成。板塊彼此擠壓、錯動，或是其中一方下潛到另一方之下的時候會造成地震和火山活動。板塊也會在洋盆的中心遠離彼此，造成熔岩滾滾冒出，形成海底山脊。
這幅大峽谷的圖片呈現了「大不整合面」(Great
Unconformity)，此交界顯示出相當於十億年歷史的沉積物從地質紀錄中消失了。大不整合面大概位在圖片中央，下方是年代較老、塊狀且稜角眾多的岩石，上面則是年代較近的水平岩層。新的研究認為覆蓋全世界的「雪球地球」期間，冰河刮除了大量陸地，使得十億年的地質紀錄因而消失。約在6.35億年前雪球地球結束之後， 這些沉積物被沖到海洋當中，潤滑了隱沒帶的斷層，促成了當代的板塊構造運動。圖片來源：USGS/Alex
Demas
但板塊構造運動並非一直存在。在地球歷史的早期，地球最外層是一塊完整的岩石外殼，上面點綴著火山――跟今日的金星表面相去不遠。隨著地球逐漸冷卻下來，岩石外殼也開始折曲並碎裂，最後形成了地球的板塊構造系統。
新的研究指出地球開始轉變成具有板塊構造運動，是因為有沉積物帶來的潤滑作用協助。這些沉積物由冰河從地球最初的陸地上刮落下來。當它們匯聚在地球初生的海岸附近，可以幫助新形成的隱沒帶斷層更快滑動，使較薄的海洋板塊滑到較厚的大陸板塊之下。
這篇於2019年6月6日發表在期刊《自然》(Nature)的新研究，首度提出全球板塊構造運動的源起和演變過程中，沉積物具有相當重要的地位。論文作者為馬里蘭大學的地質學教授Michael
Brown，以及德國波茨坦地球科學研究中心的地球動力學教授Stephan
Sobolev。
這項發現提出沉積物的潤滑效果控制了地殼彼此之間的擠壓和翻攪速率。Sobolev和Brown發現兩次擴及全世界的大型冰河期從陸地上刮下了許多沉積物，可能是之後全球板塊構造運動速率提升的原因。
此類事件最近一次發生在大約6.35億年前的「雪球地球」結束之後，塑造出當代地球的板塊構造運動系統。
「地球並非一直以來都有板塊構造運動，它的運作步調也不是一成不變。」Brown表示，「板塊構造運動至少有兩次加速進行的時期，其他證據指出它也曾經遲緩下來將近十億年。我們發現這些事件和冰河沉積物的相對增加或減少都有關聯。」
就像機器需要上油來維持零件順暢運作，板塊構造運動塗上潤滑劑之後也可以進行得更有效率。雖然一堆由黏土、粉砂、粗砂和礫石混和而成的粗糙砂土，不太容易會和滑膩的油脂搞混，但在板塊交會的海溝內部，尺度放大到陸塊等級時，它們起到的作用便相差無幾。
「這其中的機制跟我們鑽入地殼時必須要用泥漿來潤滑一樣。因為單用水和油的效果都不好，所以要把水和油跟非常細的黏土混和成泥漿。」Brown表示，「泥漿中的顆粒可以降低鑽頭的摩擦力。我們的結果顯示板塊也需要這類潤滑劑才能持續運作。」
之前對於南美西岸的研究首度證實了隱沒帶斷層上的摩擦力和沉積物的潤滑作用有關。在智利北部的外海由於海溝裡的斷層缺少沉積物，使得海洋板塊(納斯卡板塊)隱沒到大陸板塊(南美板塊)下方的時候產生了很大的摩擦力，造成大陸板塊持續受到擠壓變形，將安地斯山脈中部的最高峰不停往上推高。
反之，更往南的海溝中則裝有較多的沉積物，使得摩擦力變低。大陸板塊的變形幅度因此較小，形成了的山峰較為平緩。但這不過是從單獨一個地理區得到的結果。
Sobolev和Brown在他們的研究中運用了板塊構造的地球動力學模型，模擬沉積物潤滑作用對隱沒速率的影響。為了證實他們的假設，他們也檢視了影響範圍相當廣闊的已知冰河期，以及前人發表海洋和海溝中含有多少陸地沉積物的數據，兩者之間是否有對應關係。在探討後者時，Sobolev和Brown主要的依據有兩個：第一個是海洋化學成分中可以顯示陸地沉積物影響的訊號；第二個則是和今日環繞太平洋的「火環」一樣，跟隱沒作用有關的火山中顯示受到沉積物汙染的化學指標。
Sobolev和Brown的分析結果顯示地球的板塊構造運動可能在30億至25億年前出現，大約是地球第一批陸地開始形成的時候。此外，地球首次涵蓋全球的冰河期也發生在這段期間。
22億至18億年前板塊運動的速率大幅增加。在此之前發生了另外一次全球冰河期，使得冰河刮下大量沉積物，堆積到大陸邊緣的海溝斷層內部。
接下來的17.5億至7.5億年前，10億年來全球板塊運動的速率都慢了下來。由於這段期間在地球歷史上相對而言是相當平靜的時期，故被地質學家戲稱為「無聊的十億年」。
接著，造成地球變成「雪球地球」的冰河期在6.35億年前左右結束之後，可能發生了地球歷史中規模最大的地表侵蝕事件，刮除了陸地表面厚度超過半哩(約600公尺)的物質。Sobolev和Brown認為這些沉積物進到海洋之後，促成了當代相當活躍的板塊構造運動。
Glacial sediments greased the gears of plate
tectonics
New research highlights the
connection between worldwide glaciations, sediments and plate tectonics
Earth’s outer layer is composed of giant
plates that grind together, sliding past or dipping beneath one another, giving
rise to earthquakes and volcanoes. These plates also separate at undersea
mountain ridges, where molten rock spreads from the centers of ocean basins.
But this was not always the case. Early in Earth’s
history, the planet was covered by a single shell dotted with volcanoes—much
like the surface of Venus today. As Earth cooled, this shell began to fold and
crack, eventually creating Earth’s system of plate tectonics.
According to new research, the transition to plate
tectonics started with the help of lubricating sediments, scraped by glaciers
from the slopes of Earth’s first continents. As these sediments collected along
the world’s young coastlines, they helped to accelerate the motion of newly
formed subduction faults, where a thinner oceanic plate dips beneath a thicker
continental plate.
The new study, published June 6, 2019 in the journal Nature, is the first to suggest a role
for sediments in the emergence and evolution of global plate tectonics. Michael
Brown, a professor of geology at the University of Maryland, co-authored the
research paper with Stephan Sobolev, a professor of geodynamics at the GFZ
German Research Centre for Geosciences in Potsdam.
The findings suggest that sediment lubrication
controls the rate at which Earth’s crust grinds and churns. Sobolev and Brown
found that two major periods of worldwide glaciation, which resulted in massive
deposits of glacier-scrubbed sediment, each likely caused a subsequent boost in
the global rate of plate tectonics.
The most recent such episode followed the “snowball
Earth” that ended sometime around 635 million years ago, resulting in Earth’s
modern plate tectonic system.
“Earth hasn’t always had plate tectonics and it
hasn’t always progressed at the same pace,” Brown said. “It’s gone through at
least two periods of acceleration. There’s evidence to suggest that tectonics
also slowed to a relative crawl for nearly a billion years. In each case, we
found a connection with the relative abundance—or scarcity—of glacial
sediments.”
Just as a machine needs grease to keep its parts
moving freely, plate tectonics operates more efficiently with lubrication.
While it may be hard to confuse the gritty consistency of clay, silt, sand and
gravel with a slippery grease, the effect is largely the same at the
continental scale, in the ocean trenches where tectonic plates meet.
“The same dynamic exists when drilling Earth’s crust.
You have to use mud—a very fine clay mixed with water or oil—because water or
oil alone won’t work as well,” Brown said. “The mud particles help reduce
friction on the drill bit. Our results suggest that tectonic plates also need
this type of lubrication to keep moving.”
Previous research on the western coast of South
America was the first to identify a relationship between sediment lubrication
and friction along a subduction fault. Off the coast of northern Chile, a
relative lack of sediment in the fault trench creates high friction as the
oceanic Nazca plate dips beneath the continental South America plate. This
friction helped to push the highest peaks of the central Andes Mountains
skyward as the continental plate squashed and deformed.
In contrast, further south there is a higher sediment
load in the trench, resulting in less friction. This caused less deformation of
the continental plate and, consequently, created smaller mountain peaks. But
these findings were limited to one geographic area.
For their study, Sobolev and Brown used a geodynamic
model of plate tectonics to simulate the effect of sediment lubrication on the
rate of subduction. To verify their hypothesis, they checked for correlations
between known periods of widespread glaciation and previously published data
that indicate the presence of continental sediment in the oceans and trenches.
For this step, Sobolev and Brown relied on two primary lines of evidence: the
chemical signature of the influence of continental sediments on the chemistry
of the oceans and indicators of sediment contamination in subduction-related volcanoes,
much like those that make up today’s “ring of fire” around the Pacific Ocean.
According to Sobolev and Brown’s analysis, plate
tectonics likely emerged on Earth between 3 and 2.5 billion years ago, around
the time when Earth’s first continents began to form. This time frame also coincides
with the planet’s first continental glaciation.
A major boost in plate tectonics then occurred
between 2.2 to 1.8 billion years ago, following another global ice age that
scrubbed massive amounts of sediments into the fault trenches at the edges of
the continents.
The next billion years, from 1.75 billion to 750
million years ago, saw a global reduction in the rate of plate tectonics. This
stage of Earth’s history was so sedate, comparatively speaking, that it earned
the nickname “the boring billion” among geologists.
Later, following the global “snowball Earth”
glaciation that ended roughly 635 million years ago, the largest surface
erosion event in Earth’s history may have scrubbed more than a vertical mile of
thickness from the surface of the continents. According to Sobolev and Brown,
when these sediments reached the oceans, they kick-started the modern phase of
active plate tectonics.
原始論文：Stephan V.
Sobolev, Michael Brown. Surface erosion events controlled the evolution
of plate tectonics on Earth. Nature, 2019; 570 (7759): 52
DOI: 10.1038/s41586-019-1258-4
引用自：University of Maryland. "Glacial sediments
greased the gears of plate tectonics.”
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