新東方優(yōu)美背誦短文50篇 Unit36:Plate Tectonics and Sea-floor Spreading

36.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(n.[地]巖石圈)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 below. 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 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.

板塊結(jié)構(gòu)與海床擴(kuò)展
板塊結(jié)構(gòu)理論描述巖石圈的運(yùn)動(dòng)。巖石圈是相對(duì)堅(jiān)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(gè)大小不同形狀各異的板塊，一般而言這些板塊都處于相對(duì)運(yùn)動(dòng)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(zhì)從下部注入。
當(dāng)板塊從中海脊脫離時(shí)，它們滑向在巖石圈基部較易變形的地層上。因?yàn)榈厍虻拇笮”举|(zhì)上是不
變的，只有同等數(shù)量的巖石圈物質(zhì)在其它地方被吞沒(méi)，新的巖石圈才能生成。銷(xiāo)毀舊巖石
圈的地方形成另外一種板塊邊界：一塊潛沒(méi)的區(qū)域。在這里，一塊板塊潛沒(méi)到另一板塊的邊緣之下并結(jié)合入地幔之中。
兩種板塊邊界均與地層系統(tǒng)、地震以及火山活動(dòng)有關(guān)，但在兩種邊界處觀察到的諸般地質(zhì)活動(dòng)卻迥然不同。海床擴(kuò)展說(shuō)實(shí)際上早于板塊結(jié)構(gòu)理論。在
20世紀(jì)60年代它的理論雛形中，描述了海底的生成和毀滅，但沒(méi)有詳細(xì)介紹堅(jiān)硬的巖石圈板塊。這個(gè)假定不久之后為發(fā)現(xiàn)所證實(shí)。
該發(fā)現(xiàn)表明地球磁場(chǎng)周期性的逆轉(zhuǎn)被記錄在海洋地殼中。當(dāng)巖漿從中海脊下涌起的時(shí)候，巖漿中的磁鐵礦物質(zhì)按地磁場(chǎng)的方向被磁化。巖
漿冷卻并凝固下來(lái)后，地磁場(chǎng)的方向和磁極被保留在磁化了的火山巖中。磁場(chǎng)的逆轉(zhuǎn)形成一系列與斷層軸線平行的條形磁區(qū)。
這樣海洋殼就扮演了磁帶的角色，記錄下可以鑒定時(shí)間的地磁場(chǎng)的歷史。條形磁區(qū)的寬度表明了海底擴(kuò)展的速度。