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地球科学 1963

《洋脊上空的磁异常》

弗雷德里克·瓦因 与 德拉蒙德·马修斯

洋底是一台磁带录音机——它对称的磁条带,证明了大陆在移动。

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In depth · the introduction

原来洋底一直在写日记——而中洋脊两侧的记录互为镜像,执笔的,是地球那不断翻转的磁场。

把这个想法拆开看

到 1963 年,一些地质学家已经怀疑洋底在扩张:新岩石在中洋脊处诞生,向两侧滑开。可这要怎么证明呢?瓦因与马修斯,在磁性里找到了证据。

每隔几百万年,地球磁场就会翻转一次——南北对调。在洋脊处喷出的熔岩冷却时,会把当时磁场所指的方向锁进岩石,像一枚冻住的指南针。洋底一扩张,便把这份记录向外带走。于是洋底变成了条纹状:一条带朝这边磁化,下一条朝那边,再翻回来——这是每一次翻转的磁带录音。而由于新岩石同时被送往两侧,左边的条带就必定与右边的互为镜像。

一个博士生、他的导师,与一位被退稿的对手

弗雷德·瓦因是剑桥的年轻博士生;德拉蒙德·马修斯是他的导师,从一艘横越印度洋卡尔斯伯格洋脊的船上带回了磁测数据。瓦因看出了那些条带的含义,与马修斯一起写成一则短文,投给《自然》。他们并不知道,一位名叫劳伦斯·莫利的加拿大地球物理学家想到了完全相同的念头——可他的论文两度被退,一位审稿人嗤之以鼻,说这种东西适合在鸡尾酒会上聊,而非见刊。如今,这一想法由三人共同冠名。起初连专家都不信;真实数据太乱。直到 1966 年,一艘调查船横越一道太平洋洋脊带回的记录对称得无懈可击,争论就这样结束了。

它为何重要

半个世纪前,魏格纳主张大陆会漂移,却因说不出「如何漂」而被轰下台。赫斯补上了机制——一片扩张的洋底——但用他自己的话说,那仍是「地质诗」。瓦因与马修斯把诗变成了证明:一个单一的预言,条带的镜像对称,大洋要么显示、要么不显示。它显示了。这扭转了整个地球科学界,几年之内,现代板块构造学说便告建立。今天我们关于地震、火山与山脉的几乎一切理解,都系于这一转折。

一条行星自己打印的条形码

把洋脊想成一台朝两个方向同时运行的标签打印机,把地球磁场想成那种隔一阵就换色的墨水。每一条新岩石出来时,都盖着当下那一刻的颜色,随后被下一条挤到旁边。从洋脊向外读这条形码,就是在倒着读历史;把左侧贴到右侧,两条形码一行对一行地吻合。这种吻合,就是全部的证明。

一道中洋脊纵贯画面中央。其下,磁条带——深色为正向、浅色为反向——以脊为轴对称排布;其上,一条起伏的磁力仪曲线在正向条带上方隆起、在反向条带上方下凹。一个滑块设定扩张速率,把每条条带与每个峰都拉宽;一个按钮把左侧剖面以脊轴翻折到右侧,二者重合,显出镜像对称。

之前与之后

魏格纳(1912)说大陆会动;赫斯(1962)说洋底扩张、载着它们动;古地磁学家则证明了磁场会倒转。瓦因与马修斯(1963),连同莫利,把这些线索拧成了一道大洋通过了的检验。到 1968 年,刚性板块的框架——板块构造学说——宣告完成,而磁条带成了为每一个大洋盆地定年的时钟。在本馆里,正是这篇论文,让海底扩张不再是一个漂亮的猜想,而成了被量出的事实。

The original document
Original source text
F. J. Vine & D. H. Matthews · “Magnetic Anomalies over Oceanic Ridges” · Nature 199 (4897): 947–949 · 7 September 1963 · doi:10.1038/199947a0
The puzzle: stripes on the sea floor
(Paraphrase.) The paper opens from a problem in the new magnetic surveys of the deep ocean. Towed magnetometers crossing the mid-ocean ridges record an anomaly in the total magnetic field that is not random but strongly lineated — long bands of higher-than-average and lower-than-average field running parallel to the ridge crest. Vine and Matthews work from a survey of the Carlsberg Ridge in the north-west Indian Ocean (and refer to the same pattern seen off the western coast of North America).
The hypothesis: a spreading floor that records reversals
(Paraphrase.) They combine two ideas already in the air. First, Hess and Dietz's seafloor spreading: new oceanic crust forms at the ridge crest and moves outward to either side. Second, the established fact that the Earth's magnetic field reverses its polarity from time to time. As molten rock solidifies at the ridge and cools through the Curie temperature, it locks in the direction of the field prevailing at that moment. If the floor then spreads, successive reversals must leave the crust divided into long blocks magnetised alternately in the normal and the reversed direction.
The consequence: the anomalies, and their symmetry
(Paraphrase.) Such alternating blocks would add to, or subtract from, the present field — producing exactly the pattern of parallel positive and negative anomalies that the surveys show, without any need for strips of unusually magnetic rock. The argument requires that a large fraction of the oceanic crust be reversely magnetised. And it makes a sharp prediction: because the blocks are carried away symmetrically on both limbs, the anomaly pattern must be a mirror image of itself across the ridge axis, with block widths set by the spreading rate and the spacing of reversals.
[ … ]
Madingley Rise, Cambridge · 1963