新东方背诵文选 L36.Plate Tectonics and Sea-floor Spreading

（单词翻译）

The 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 few dozen plates of 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 lithosphere.
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 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 geologic¹⁰ 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 specify¹¹ rigid lithospheric plates. The hypothesis was substantiated¹² soon afterward¹³ 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, 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 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 independently; the width of the stripes indicates the rate of the sea-floor spreading.