新概念英语优美短文Unit36：PlateTectonicsandSea-floorSpre

1、新概念英语优美短文 Unit36 ： Plate Tectonics andSea-floor SpreThe theory of plate tectonics describes the motions of the lithosphere1, the comparatively rigid2 outer layer of the Earth that includes all the crust and part of the underlying3 mantle4. The lithosphere（n.地 岩石圈 ）is divided into a fewdozen plates o

2、f various sizes and shapes, in general the plates are in motion with respect to one another. A mid-ocean ridge5 is a boundary between plates where new lithospheric6 material is injected from below. As the plates diverge7 from a mid-ocean ridge they slide on a more yielding layer at the base of the l

3、ithosphere.Since the size of the Earth is essentially8 constant, new lithosphere can be created at the mid-ocean ridges9 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 u

4、nder 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 geologic10 activity observed at the two boundaries are quite different.The idea of sea-floor spreading actually preceded the the

5、ory 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 specify11 rigid lithospheric plates. The hypothesis was substantiated12 soon afterward13 by the discovery that periodic reversals of the Earth's

6、magnetic field are recorded in the oceanic crust. As magma rises under the mid-ocean ridge, ferromagnetic14 minerals in the magma become magnetized in the direction of the magma become magnetized in the direction of the geomagnetic field. When the magma cools and solidifies15, the direction and the

7、polarity of the field are preserved in the magnetized volcanic16 rock. Reversals of the field give rise to a series of magnetic stripes running parallel to the axis17 of the rift18. The oceanic crust thus serves as a magnetic tape recording19 of the history of the geomagnetic field that can be dated

8、 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，一般来说这些板块 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

9、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、地震以及火山活动相关，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，记录下能够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。1 lithospheren. 陆界，岩石圈参考例句：The hydrosphere and the lithosphere together form the Earth's surface. 水圈与岩石圈一起形成地球的表面。They slide on a more yielding layer at the base oflithosphere. 它们在岩石圈底部更柔软的一层滑动。2 rigidadj. 严格的，死板的；刚硬的，僵硬的参考例句：She became