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地球科學 1962

《海洋盆地的歷史》

哈里·哈蒙德·赫斯

洋底是一條輸送帶——在洋脊處誕生,向外擴張,再潛回地球內部。

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In depth · the introduction

大陸之所以讓所有人困惑,是因為它們看起來像在硬生生犁過岩石。赫斯的答案是:它們並非穿過洋底而行——它們馱在洋底上,因為洋底本身就在移動。

把這個想法拆開看

赫斯提出,洋底一直在生成,就沿著那些叫「中洋脊」的海底大山脈。熾熱的岩石從地球深處上湧,在洋脊處凝固成新的洋底,再緩緩被帶向兩側——就像兩條輸送帶,從同一個源頭朝相反方向運行。

那舊的洋底去哪了?向下。在深海溝處,洋底彎折、沉回地球內部,被熔化、回收。於是整片洋底都很年輕:生於洋脊,在旅途中變老,最後被海溝吞下。大陸,不過是馱在它上面隨之而行。

一位從軍艦上「讀」洋底的地質學家

哈里·赫斯是普林斯頓的地質學家,二戰時任美國海軍艦長。橫渡太平洋時,他讓艦上的回聲測深儀一路開著,測繪出幾十座古怪的平頂海山——他稱之為「平頂海山」(guyot)的沒頂島嶼。戰後,聲納與其他調查揭示了中洋脊,也揭示了洋底沉積之薄。1962 年,赫斯把這一切拼到了一起。他知道自己是在證明到來之前作推理,便自嘲地稱這篇論文為「一篇地質詩」。而這首詩,後來證明是對的:沒幾年,洋脊的磁條帶就證實了擴張的洋底,而韋格納那被否定多年的漂移大陸,也終獲平反。

它為何重要

五十年前,韋格納主張大陸會動,卻說不出是什麼推動了它們,於是遭到否定。赫斯補上了那臺缺失的引擎——而且把它安在了洋底,而非大陸身上。僅此一變,便解開了一切:它解釋了洋底為何年輕、地震與火山為何聚在某些地帶、山脈如何隆起。到 1960 年代末,它長成了板塊構造學說——如今整個地質學的根基;加利福尼亞為何地震、大西洋為何變寬、喜馬拉雅為何還在長高,皆源於此。

兩條輸送帶

想像兩條行李輸送帶首尾相接,從中間的一道縫朝相反方向運行。新岩石不斷從那道縫——洋脊——裡冒出來,被兩條帶子以相同的速度向外送。一塊岩石走得越遠,就越老、越冷;到了盡頭,它從帶子上翻落、消失不見——那就是海溝。而一直馱在帶子上、從不沉下去的行李,就是一塊大陸。

一道中洋脊縱貫畫面中央,兩側對稱排布著磁條帶——深色為正向磁性、淺色為反向磁性。一個滑桿設定擴張半速率,速率越大、條帶被拉得越寬;另一個滑桿把一支岩芯探針從洋脊向外移動,讀數給出該處的地殼年齡(距離除以速率)及其所屬的磁極期。同一年齡在鏡像的另一側同樣出現。

之前與之後

亞瑟·霍姆斯早在 1930 年代就設想過對流的地函拖著地殼走,只是缺少洋底的數據為它錨定。羅伯特·迪茨於 1961 年得出同一幅圖景,並為它取名「海底擴張」。隨後,瓦因與馬修斯(1963)在洋脊的磁條帶裡找到了證明;到 1968 年,剛性板塊的框架——板塊構造學說——宣告完成。在本館裡,赫斯正是韋格納(1912)那道無解之問的答案:是什麼搬動了大陸。

The original document
Original source text
Harry H. Hess (1906–1969) · “History of Ocean Basins,” in Petrologic Studies: A Volume in Honor of A. F. Buddington · Geological Society of America, 1962, pp. 599–620
An essay in geopoetry
Hess opens by naming the spirit of the paper. He is reasoning ahead of the data, and says so — a frank disclaimer rare in a scientific paper, and one that became its most quoted line.
Like Umbgrove, I shall consider this paper an essay in geopoetry. In order not to travel any further into the realm of fantasy than is absolutely necessary I shall hold as closely as possible to a uniformitarian approach; even so, at least one great catastrophe will be required early in the Earth's history.
The puzzle of the ocean floor
(Paraphrase.) Hess assembles the wartime and post-war findings about the deep sea: the mid-ocean ridges form a single globe-encircling mountain system; the ocean floor carries surprisingly little sediment and no rocks older than the Mesozoic; heat flow is high over the ridges; and the deep trenches mark belts of earthquakes. The oceans, he argues, are young and active, not the ancient permanent basins geology had assumed.
The mechanism: mantle convection and a spreading floor
(Paraphrase.) His proposal: slow convection cells in the mantle rise beneath the mid-ocean ridges, where hot mantle wells up and freezes into new basaltic ocean floor. That floor is then carried laterally away from the ridge — like a conveyor belt riding on the top of the convecting mantle — and descends again at the trenches, where the cell turns downward. The whole ocean floor is therefore created, transported, and recycled on a timescale of only a few hundred million years, which is why it is so young and so thinly veneered with sediment.
Continents ride passively; guyots record the ride
(Paraphrase.) Crucially, the continents do not plough through the ocean floor — Wegener's fatal difficulty — but ride passively atop the moving mantle, between a rising ridge and a descending trench. Hess uses the flat-topped drowned seamounts he had charted from the deck of USS Cape Johnson during the war (he named them guyots) as evidence: volcanoes built near a ridge, planed flat by waves, then carried down and away as the floor spread and cooled and sank.
[ … ]
Princeton, New Jersey · 1962