解放军文职招聘考试Plate Tectonics and Sea-floor Spreading

  Plate   Tectonics   and   Sea-floor Spreading

The theory of plate tectonics describes the motions of the lithosphere, the

comparatively rigid outer layer of the Earth  that includes all the  crust 

and  part  of the underlying  mantle. The lithosphere is divided into a few

dozen    plates of various sizes and shapes, in general the plates are in

motion with respect to one another. A mid-ocean    ridge is a boundary

between plates where new lithospheric material is injected from belows. As

the plates diverge     from a mid-ocean ridge they slide on a more yielding

layer at the base of the lithosphere.

Since  the size of the Earth is essentially constant, 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 subduction zone. There one plate dives under the

edge of another and is reincorporated 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 actually preceded the theory of plate

tectonics. In its original version, in the              early 1960's, it described the

creation and destruction of the ocean floor, but it did not specify rigid 

lithospheric plates. The hypothesis was substantiated soon afterward by the

discovery that periodic reversals of     the Earth's magnetic field are

recorded in the oceanic crust. As  magma  rises  under  the  mid-ocean 

ridge,    ferromagnetic  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 field are preserved in the magnetized

volcanic rock.     Reversals of the field give rise to a series of magnetic

stripes running parallel to the axis of the rift. The oceanic crust thus

serves as a magnetic tape recording of the history of the geomagnetic field

that can be dated      independently; the width of the stripes indicates the

rate of the sea-floor spreading.

板块结构与海床扩展

板块结构理论描述岩石圈的运动。  岩石圈是相对坚硬的地球外层，包括全部地壳和部 分地幔。 

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。