The theory of plate tectonics① describes the motions of the lithosphere which includes all the crust and part of the underlying mantle. Lithosphere is a comparatively rigid outer layer of the Earth.
The lithosphere is divided into a few dozen plates of various sizes and shapes. Generally， the plates are in motion with respect to one another. A mid-ocean ridge is a boundary between plates where new lithospheric② material is injected from below.
As the plates diverge from③ a mid-ocean ridge， they slide on a more yielding layer on the basic part of the lithosphere. Since the size of the Earth is essentially unchanged， new lithosphere can be created at the mid-ocean ridges only if an equal amount of lithospheric material is consumed elsewhere. The site of this destruction is another kind of plate boundary： a subduction④ zone. Here one plate dives under the edge of another and is reincorporated⑤ into the mantle.
Both kinds of plate boundary are associated with fault systems， earthquakes and volcanism， but the kinds of geologic activity observed at the two boundaries are quite different.
The idea of sea-floor spreading is actually prier to the theory of plate tectonics. In its original version， in the early 1960s， it described the creation and destruction of the ocean floor， but it did not specify rigid lithospheric plates.
The hypothesis⑥ was substantiated soon afterward by the discovery that periodic reversals of the Earth’s magnetic field are recorded in the oceanic crust. As magma⑦ rises under the mid-ocean ridge， ferromagnetic⑧ minerals in the magma become magnetized in the direction of the geomagnetic field. When the magma cools and solidifies， the direction and the polarity of the magnetized field are preserved in the volcanic rock. Reversals of the magnetized field give rise to a series of magnetic stripes running parallel to the axis of the rift.
Therefore， the oceanic crust plays a role of magnetic tape recording of the history of the geomagnetic field that can show the time； the width of the stripes indicates the rate of the sea-floor spreading.
① tectonicn. ［地］大地构造学
② lithosphericadj. 岩石圈的
③ diverge from离开，背道而驰
④ subductionn. ［地］潜没(地壳的板块沉到另一板块下)
⑤ reincorporatev. 把……重新（再）合并，使并入
⑥ hypothesisn. 假说，前提［C］
⑦ magman. ［地］岩浆
⑧ ferromagneticadj. ［物］强磁性的
板块结构与海床扩展
板块结构理论是描述岩石圈的运动的理论。岩石圈包括全部地壳和部分地幔，是相对坚硬的地球外层。
岩石圈被划分为几十个大小不同形状各异的板块，一般而言这些板块都处于相对运动之中。一道中海脊是板块之间的边界，在那里新的岩石圈的物质从下部注入。
当板块从中海脊脱离时，它们滑向在岩石圈基部较易变形的地层上。因为地球的大小本质上是不变的，只有同等数量的岩石圈物质在其它地方被吞没，新的岩石圈才能生成。销毁旧岩石圈的地方形成另外一种板块边界：一块潜没的区域。在这里，一块板块潜没到另一板块的边缘之下并结合入地幔之中。
两种板块边界均与地层系统、地震以及火山活动有关，但在两种边界处观察到的诸般地质活动却迥然不同。
海床扩展说实际上早于板块结构理论。在20世纪60年代，它的理论雏形中，描述了海底的生成和毁灭，但没有详细介绍坚硬的岩石圈板块。
这个假定不久之后为发现所证实。该发现表明地球磁场周期性的逆转被记录在海洋地壳中。当岩浆从中海脊下涌起的时候，岩浆中的磁铁矿物质按地磁场的方向被磁化。岩浆冷却并凝固下来后，地磁场的方向和磁极被保留在磁化了的火山岩中。磁场的逆转形成一系列与断层轴线平行的条形磁区。
这样海洋壳就扮演了磁带的角色，记录下可以鉴定时间的地磁场的历史。 条形磁区的宽度表明了海底扩展的速度。
