陳蕙雅思教材 - 雅思閱讀精選 (Plate Tectonics and Sea-floor Spreading)

Plate   Tectonics  and   Sea-floor Spreading

The theory of platetectonics describes the motions of the lithosphere, the

comparatively rigid outerlayer of the      Earth  that includes all the  crust  

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

dozen plates of various sizes and shapes, in generalthe plates are in

motion with respect toone another. A mid-ocean    ridge is aboundary

between plates where newlithospheric material is injected from belows. As

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

layer at the base of thelithosphere.

Since  the size of the Earth is essentiallyconstant, new lithosphere can

be created  at the mid-ocean ridges only if an equal amountof lithospheric

material is consumedelsewhere. The site of this destruction is another

kind of          plate boundary: a subduction zone.There one plate dives under the

edge of another and isreincorporated into the    mantle. Bothkinds of plate

boundary are associatedwith fault systems, earthquakes and volcanism, but

the kinds  of geologic activity observed at the twoboundaries are quite

different.

The idea of sea-floorspreading actually preceded the theory of plate

tectonics. In itsoriginal version, in the               early1960's, it described the

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

lithospheric plates. Thehypothesis was substantiated soon afterward by the

discovery that periodicreversals of         the Earth's magneticfield are

recorded in the oceaniccrust. As  magma  rises under  the  mid-ocean  

ridge,    ferromagnetic  minerals in  the  magma become magnetized in the

direction of thegeomagnetic field. When the magma    coolsand solidifies,

the direction and the polarityof the field are preserved in the magnetized

volcanic rock.          Reversals of the field give rise to aseries of magnetic

stripes running parallelto the axis of the rift. The oceanic crust thus

serves as a magnetic taperecording of the history of the geomagnetic field

that can be dated      independently; the width of the stripesindicates the

rate of the sea-floorspreading.

板塊結構與海床擴展

板塊結構理論描述岩石圈的運動。  岩石圈是相對堅硬的地球外層，包括全部地殼和部 分地幔。  

岩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於相對 運動之中。 一道中海脊是板塊之間的邊界，在那裡新的岩石圈的物質從下部注入。

當板塊 從中海脊脫離時，它們滑向在岩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，只有同等數量的岩石圈物質在其它地方被吞沒，新的岩石圈才能生成。  銷毀舊岩石

圈的地方形成另外一種板塊邊界：一塊潛沒的區域。  在這裡，一塊板塊潛沒到另一板塊的 邊緣之下並結合入地幔之中。  

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於板塊結構理論。 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岩石圈 板塊。 這個假定不久之後為發現所證實。

該發現表明地球磁場週期性的逆轉被記錄在海洋 地殼中。 當岩漿從中海脊下湧起的時候，岩漿中的磁鐵礦物質按地磁場的方向被磁化。 岩

漿冷卻並凝固下來後，地磁場的方向和磁極被保留在磁化了的火山岩中。  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。