A very little of this had been known for some time. People laying ocean-floor cables in the nineteenth century had realized that there was some kind of mountainous intrusion in the mid-Atlantic from the way the cables ran, but the continuous nature and overall scale of the chain was a stunning surprise. Moreover, it contained physical anomalies that couldn't be explained. Down the middle of the mid-Atlantic ridge was a canyon—a rift—up to a dozen miles wide for its entire 12,000-mile length. This seemed to suggest that the Earth was splitting apart at the seams, like a nut bursting out of its shell. It was an absurd and unnerving notion, but the evidence couldn't be denied.
在一段时间里，人们对这些知之甚少。19世纪铺设海底电缆的人已经发现，大西洋中部有山脉妨碍电缆的走向，但山脉的连贯性质和整体范围完全出乎人们意料。而且，它的形状很不规则，难以解释。在大西洋中部那座山冈的中段，下面有个峡谷──一条裂缝──宽达20公里，全长19000公里。这似乎表明，地球在沿着裂缝裂成两半，就像果仁爆裂出壳那样。这种看法荒诞不经而又扰乱人心，但那种迹象是不可否认的。

Then in 1960 core samples showed that the ocean floor was quite young at the mid-Atlantic ridge but grew progressively older as you moved away from it to the east or west. Harry Hess considered the matter and realized that this could mean only one thing: new ocean crust was being formed on either side of the central rift, then being pushed away from it as new crust came along behind. The Atlantic floor was effectively two large conveyor belts, one carrying crust toward North America, the other carrying crust toward Europe. The process became known as seafloor spreading.
接着，1960年，岩心样品显示，大西洋中部海底的山脊还相当年轻，但由此向东或由此向西，却变得越来越古老。经过考虑，哈里·赫斯觉得那种情况只有一种意思：新的海底地壳正在中央裂缝的两侧形成，然后被后面随即产生的更新的地壳向外推开。大西洋洋底实际上是两条大的传送带，一条把地壳传向北美洲，一条把地壳传向欧洲。这个过程后来被称之为海底扩展。
When the crust reached the end of its journey at the boundary with continents, it plunged back into the Earth in a process known as subduction.
地壳抵达与大陆交界处的终点以后，又突然折回地球内部，这个过程称之为潜没。