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地球科學 1912

《海陸的起源》

阿爾弗雷德·韋格納

各大陸能像撕開的地圖般拼合——它們曾是一整塊。

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In depth · the introduction

看一眼世界地圖:南美洲東側的隆起,恰好嵌進非洲西側的凹陷,活像一頁紙被撕成的兩半。韋格納說,這絕非偶然——它們曾經是一體。

把這個想法拆開看

韋格納提出,各大陸並非被釘死在原處。很久很久以前,它們曾緊緊擠作一整塊巨大的陸地;此後,便緩緩漂移,散到了今天的位置。

他並不只把論據押在海岸線上——那可能只是碰巧——而是押在大西洋兩岸的岩石與化石如何彼此「接續」上:就好像你把一頁印好的報紙撕成兩半,卻發現那些字行仍筆直地跨過裂口、連成一氣。

一個「越界」的氣象學家

韋格納是氣象學家、是極地探險家,而非地質學家。1912 年 1 月 6 日,在法蘭克福,他站到地質學會面前,主張大陸會移動——並在同年發表了這一想法。其後二十年,他不斷搜集證據,把它擴充成一本書《海陸的起源》,前後出了四版。多數地質學家拒絕了它:一來,他說不出究竟是什麼力,能把一塊大陸推過整個地球;二來,他是個「外行」。1930 年,他在格陵蘭冰蓋上辭世,學說仍未獲平反。

它為何重要

韋格納是對的,而幾乎所有人都錯了——這在科學史上,是難得而乾淨的一例。他看見了那個能把海岸線、山脈、化石與遠古氣候統統串起來的單一事實(大陸會移動),而且比任何人能解釋「如何移動」早了整整四十年。當解釋終於到來——洋底自身在擴張——它印證了他,並把整個地質學,圍繞「移動的板塊」重新組織起來。你腳下的大地,正是一塊每年滑移幾公分的巨板的一部分,那速度,約莫和你指甲生長的速度相當。

撕開的報紙

這是韋格納自己的比喻:想像把一張報紙撕成兩半。要判斷兩片碎紙是否曾經相連,靠的不只是對齊那參差的撕邊,更要看那些印刷的字行,是否筆直地跨過裂口連成一線。海岸線,就是那參差的撕邊;而岩層與化石層,就是那一行行的字——它們對得上。

兩塊陸地——左為南美洲、右為非洲——隔著海洋。一個滑桿把非洲拖回大西洋對岸;兩條相對的海岸線被畫成同一道吻合的撕裂邊緣,缺口閉合時便彼此咬合。三條彩色帶分別標示相互對應的山脈帶、舌羊齒植物群與中龍,它們在兩側大陸邊緣處於相同緯度;接縫合攏時,連接線把它們橋接起來,於是這些帶便連續地跨過接合處。

之前與之後

大西洋兩岸的吻合,幾個世紀以來早有人留意——製圖家亞伯拉罕·奧特柳斯(Abraham Ortelius)早在 1596 年就提出,美洲是從歐洲與非洲「撕扯」開來的——但這種相似,要麼被一笑置之,要麼被一座座「恰好」沉沒的陸橋所搪塞。韋格納之後,缺失的引擎終於補上:1960 年代,海底擴張與洋底的磁條帶,把「大陸漂移」變成了「板塊構造」——如今,地震、火山與山脈,都由這同一套框架來解釋。

The original document
Original source text
Alfred Wegener (1880–1930) · first proposed in lectures and papers of 1912; developed in Die Entstehung der Kontinente und Ozeane (Vieweg, 1915; 4th ed. 1929)
The fit of the coasts
Wegener opens from a fact anyone can see on a globe: the Atlantic coastlines of South America and Africa run nearly parallel — the eastern bulge of Brazil matches the bight of the Gulf of Guinea. He insists this congruence is a clue to be tested, not a proof in itself.
It is just as if we were to refit the torn pieces of a newspaper by matching their edges and then check whether the lines of print run smoothly across. If they do, there is nothing left but to conclude that the pieces were in fact joined in this way.
(From the opening chapter, in John Biram's English translation of the fourth German edition.) The “lines of print” are the geological structures and fossils that must continue from one continent to the other if the two were once joined.
Pangaea
Reassembling the southern continents, Wegener reconstructs a single primeval landmass — in the 1920 edition he names it Pangaea, “all the Earth” — surrounded by one universal ocean, which begins to break apart from the Mesozoic onward as the fragments drift to their present places.
The lines of print: geology, fossils, climate
The evidence he assembles runs across the seam. Mountain belts and Precambrian shield rocks align on both Atlantic shores; the seed-fern Glossopteris is found across all the southern continents; Mesosaurus, a small freshwater reptile that could not cross an ocean, occurs only in Brazil and southern Africa. Late-Palaeozoic glacial deposits now lie in today's tropics while tropical coals lie near the poles — anomalies that resolve once the continents are reassembled around an ancient south-polar ice cap.
The unanswered question
Wegener proposed that the light continental blocks (which he called sial) drift through a denser substratum (sima), driven by a pole-fleeing force and by tidal attraction. He knew the forces he could name were weak, and conceded that the problem of the driving mechanism was still unsolved — the gap his critics would seize upon.
[ … ]
Marburg, 1912 — Greenland, 1930