James Cameron made headlines in March by successfully diving 6.8 miles (11 kilometers) to the deepest part of the Mariana Trench in a one-person submersible[2] called the Deepsea Challenger. Best known as a Hollywood director, Cameron is one of just three people to dive to Earth’s deepest point, and the only one to be able to stay long enough to look around. Though Cameron and his team are hoping to discover more about the biology of the Mariana depths, geologists already know a lot about how the Mariana Trench formed—and why it’s Earth’s deepest spot.
Three Major Factors that Made Earth’s Deepest Spot
The Mariana Trench isn’t really the deep, narrow furrow that the word 「trench」 implies. Rather, the abyss marks the location of a subduction zone.[3] Subduction zones occur where one part of the seabed—in this case the Pacific plate—dives beneath another, the Philippine plate. Tectonic forces eventually make the Pacific plate dive into the Earth’s interior almost vertically, but at seabed level the plate dips at a relatively gentle angle.[4]
Robert Stern, a geophysicist at the University of Texas, said, 「One reason the Mariana Trench is so deep is because the western Pacific is home to some of the oldest seafloor in the world—about 180 million years old. Seafloor is formed as lava at mid-ocean ridges.[5] When it’s fresh, lava is comparatively warm and buoyant, riding high on the underlying mantle.[6] But as lava ages and spreads away from its source, it slowly cools and becomes increasingly dense, causing it to settle ever lower—as is the case with the Mariana Trench.」
Two other factors conspire to make the 1,580-mile-long (2,550-kilometer-long) Mariana Trench staggeringly deep.[7] For one, the trench lies far from any major landmass, which means it’s remote from the mouths of muddy rivers. 「Many other deep trenches are more filled with sediment,」 Chris Goldfinger, professor of Oregon State University, explained. 「This one isn’t.」 In addition, nearby fault lines cut the Pacific plate into a narrow tongue at the site of the trench, allowing the plate to bend more steeply downward than at other subduction zones.[8]
Cameron Dive a 「Man on the Moon」 Feat
Despite Cameron’s record dive, it’s impossible to know what really happens in the subduction zone, since most of the action occurs up to 420 miles (700 kilometers) below Earth’s surface. 「That’s the iceberg,」 Stern said. 「Cameron wasn’t even at the tip of it—11 kilometers out of 700. The trench is the interface between the limits of human experience and the reality humans can’t experience.」
Even so, with subsequent dives, there’s the prospect of retrieving rock samples and looking for life deeper than it’s ever been found before. Cameron commented, 「It’s really the sense of isolation, more than anything, realizing how tiny you are down in this big vast black unknown and unexplored place... I see this as the beginning. It’s not a one-time deal and then moving on. This is the beginning of opening up this new frontier.」 Stern agreed, calling Cameron’s trip 「kind of a man-on-the-moon thing.」
今年三月份，詹姆斯•卡梅隆登上各大媒體的新聞頭條，他乘坐單人潛艇「深海挑戰者號」成功潛入海底6.8英裡（即11千米）處，到達馬裡亞納海溝的最深處。曾成功抵達這一地球最深處的僅有三人，著名好萊塢導演卡梅隆就是其中之一，而且是唯一逗留時間足夠長以便能四處察看的人。儘管卡梅隆及其團隊希望發現更多有關馬裡亞納海溝深處生態環境的情況，但對於馬裡亞納海溝是如何形成的以及它何以成為地球最深處，其實地質學家們已了解頗多。
三大因素成就地球最深之處
馬裡亞納海溝並非真如「溝」這個詞所指的是那種又深又窄的壟溝，更確切地說，這條海淵是（大洋板塊）俯衝帶所在之處。當大洋板塊的一部分衝到另一大洋板塊的下面時，就形成了俯衝帶。就馬裡亞納海溝這個例子而言，就是太平洋板塊衝到了菲律賓板塊下面。地球強大的構造作用力最後會讓太平洋板塊幾乎垂直地插入地球內部，但在海床水平面上，板塊下沉的角度還算相對平緩。
德克薩斯大學的地球物理學家羅伯特•斯特恩說：「馬裡亞納海溝如此深的原因之一就在於西太平洋板塊擁有一些地球上最古老的海床——大約有1.8億年之久。熔巖在洋中脊湧出後形成了海床。新湧出的熔巖溫度相對較高，不斷往上湧，高高地蓋過了下面的地幔。但隨著時間的推移，熔巖從湧出點往四周擴散，慢慢冷卻下來，變得越來越稠密，使它能夠沉入更低處——馬裡亞納海溝就是這樣形成的。」
還有其他兩個因素的共同作用才讓長達1,580英裡（即2,550千米）的馬裡亞納海溝有著驚人的深度。原因之一是馬裡亞納海溝距離幾個主要的大陸板塊甚遠，這也就意味著它遠離了帶有大量泥沙的河流入海口。「很多深海溝都有更多的沉積物，」俄勒岡州立大學的教授克裡斯•戈德芬格解釋道。「但馬裡亞納海溝卻不是。」此外，附近的斷層線在馬裡亞納海溝這個地方將太平洋板塊切成狹長的舌狀地帶，使得板塊向下的彎度比在其他俯衝帶更為陡直。
卡梅隆完成「登月」般的壯舉
雖然卡梅隆成功完成了創紀錄的深潛行動，但我們還是不可能知道俯衝帶裡真正發生的一切，因為大部分活動發生在地球表面420英裡（即700千米）之下。「那就像一座冰山，」斯特恩說，「卡梅隆甚至還沒接觸到它的一角，海溝深度700千米，他才潛到了11千米處。馬裡亞納海溝是人類經歷極限與人類無法親身體驗現實之間的交界處。」
即便如此，人類隨後的深潛行動有望還能帶回一些巖石樣本，進入比之前曾發現生命之處更深的地方去尋找生命。卡梅隆評價道：「當你意識到在這廣袤、黑暗、未知的海底世界中自己是有多麼渺小的時候，那才是真正意義上的一種與世隔絕感，比其他任何感覺都要強烈……我將這次行動看作一個開端。這並不是一次淺嘗輒止的經歷，而是一個探索新領域的開端。」斯特恩深表贊同，他將卡梅隆的深潛之旅喻為「如人類登月般的壯舉。」
（來源：英語學習雜誌 編輯：中國日報網英語點津 陳丹妮）