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Plate  Tectonics   and   Sea-floor Spreading

　　The theory of plate tectonics describes the motions of thelithosphere, the comparatively rigid outer layer of the Earth  thatincludes all the  crust  
　　and  part  of the underlying  mantle.The lithosphere is divided into a few dozen plates of various sizes andshapes, in general the plates are in motion with respect to one another. Amid-ocean    ridge is a boundary between plates where new lithosphericmaterial is injected from belows. As the plates diverge from a mid-oceanridge they slide on a more yielding 
　　layer at the base of the lithosphere.Since  thesize of the Earth is essentially constant, new lithosphere can be created at the mid-ocean ridges only if an equal amount of lithospheric material isconsumed elsewhere. The site of this destruction is another kind of plateboundary: a subduction zone. There one plate dives under the edge of anotherand is reincorporated into the mantle. Both kinds of plate boundary areassociated with fault systems, earthquakes and  volcanism, but thekinds  of geologic activity observed at the two boundaries are quitedifferent.
　　The idea of sea-floor spreading actually preceded thetheory of plate tectonics. In its original version, in the early 1960's, itdescribed the creation and destruction of the ocean floor, but it did notspecify rigid lithospheric plates. The hypothesis was substantiated soonafterward by the discovery that periodic reversals of the Earth's magneticfield are recorded in the oceanic crust. As  magma  rises under  the  mid-ocean  ridge,   ferromagnetic  minerals  in  the  magma  becomemagnetized in the direction of the geomagnetic field. When the magma coolsand solidifies, the direction and the polarity of the field are preserved inthe magnetized volcanic rock. Reversals of the field give rise to a seriesof magnetic 
　　stripes running parallel to the axis of the rift. Theoceanic crust thus serves as a magnetic tape recording of the history of the geomagneticfield that can be dated independently; the width of the stripes indicatesthe rate of the sea-floor spreading.
　　全文翻譯：板塊結構與海床擴展
　　板塊結構理論描述巖石圈的運動。巖石圈是相對堅硬的地球外層，包括全部地殼和部分地幔。巖石圈被劃分為幾十個大小不同形狀各異的板塊，一般而言這些板塊都處于相對 運動之中。 一道中海脊是板塊之間的邊界，在那里新的巖石圈的物質從下部注入。當板塊從中海脊脫離時，它們滑向在巖石圈基部較易變形的地層上。因為地球的大小本質上是不變的，只有同等數量的巖石圈物質在其它地方被吞沒，新的巖石圈才能生成。銷毀舊巖石圈的地方形成另外一種板塊邊界：一塊潛沒的區域。在這里，一塊板塊潛沒到另一板塊的邊緣之下并結合入地幔之中。  
　　兩種板塊邊界均與地層系統、地震以及火山活動有關，但在兩種邊界處觀察到的諸般地質活動卻迥然不同。海床擴展說實際上早于板塊結構理論。在20世紀60年代它的理論雛形中，描述了海底的生成和毀滅，但沒有詳細介紹堅硬的巖石圈板塊。這個假定不久之后為發現所證實。 該發現表明地球磁場周期性的逆轉被記錄在海洋地殼中。當巖漿從中海脊下涌起的時候，巖漿中的磁鐵礦物質按地磁場的方向被磁化。巖漿冷卻并凝固下來后，地磁場的方向和磁極被保留在磁化了的火山巖中。磁場的逆轉形成一系列與斷層軸線平行的條形磁區。這樣海洋殼就扮演了磁帶的角色，記錄下可以鑒定時間的地磁場的歷史。條形磁區的寬度表明了海底擴展的速度。

 

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