为何马里亚纳海沟成为了地球上最深的地方

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•卡梅隆登上各大媒体的资讯头条，他乘坐单人潜艇“深海挑战者号”成功潜入海底6.8英里(即11千米)处，到达马里亚纳海沟的最深处。曾成功抵达这一地球最深处的仅有三人，著名好莱坞导演卡梅隆就是其中之一，而且是唯一逗留时间足够长以便能四处察看的人。尽管卡梅隆及其团队希望发现更多有关马里亚纳海沟深处生态环境的情况，但对于马里亚纳海沟是如何形成的以及它何以成为地球最深处，其实地质学家们已了解颇多。

三大因素成就地球最深之处

马里亚纳海沟并非真如“沟”这个词所指的是那种又深又窄的垄沟，更确切地说，这条海渊是(大洋板块)俯冲带所在之处。当大洋板块的一部分冲到另一大洋板块的下面时，就形成了俯冲带。就马里亚纳海沟这个例子而言，就是太平洋板块冲到了菲律宾板块下面。地球强大的构造作用力最后会让太平洋板块几乎垂直地插入地球内部，但在海床水平面上，板块下沉的角度还算相对平缓。

得克萨斯大学的地球物理学家罗伯特•斯特恩说：“马里亚纳海沟如此深的原因之一就在于西太平洋板块拥有一些地球上最古老的海床——大约有1.8亿年之久。熔岩在洋中脊涌出后形成了海床。新涌出的熔岩温度相对较高，不断往上涌，高高地盖过了下面的地幔。但随着时间的推移，熔岩从涌出点往四周扩散，慢慢冷却下来，变得越来越稠密，使它能够沉入更低处——马里亚纳海沟就是这样形成的。”

还有其他两个因素的共同作用才让长达1,580英里(即2,550千米)的马里亚纳海沟有着惊人的深度。原因之一是马里亚纳海沟距离几个主要的大陆板块甚远，这也就意味着它远离了带有大量泥沙的河流入海口。“很多深海沟都有更多的沉积物，”俄勒冈州立大学的教授克里斯•戈德芬格解释道。“但马里亚纳海沟却不是。”此外，附近的断层线在马里亚纳海沟这个地方将太平洋板块切成狭长的舌状地带，使得板块向下的弯度比在其他俯冲带更为陡直。

卡梅隆完成“登月”般的壮举

虽然卡梅隆成功完成了创纪录的深潜行动，但我们还是不可能知道俯冲带里真正发生的一切，因为大部分活动发生在地球表面420英里(即700千米)之下。“那就像一座冰山，”斯特恩说，“卡梅隆甚至还没接触到它的一角，海沟深度700千米，他才潜到了11千米处。马里亚纳海沟是人类经历极限与人类无法亲身体验现实之间的交界处。”

即便如此，人类随后的深潜行动有望还能带回一些岩石样本，进入比之前曾发现生命之处更深的地方去寻找生命。卡梅隆评价道：“当你意识到在这广袤、黑暗、未知的海底世界中自己是有多么渺小的时候，那才是真正意义上的一种与世隔绝感，比其他任何感觉都要强烈……我将这次行动看作一个开端。这并不是一次浅尝辄止的经历，而是一个探索新领域的开端。”斯特恩深表赞同，他将卡梅隆的深潜之旅喻为“如人类登月般的壮举。”