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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.
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1 lithosphere | |
n.陆界,岩石圈 | |
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2 rigid | |
adj.严格的,死板的;刚硬的,僵硬的 | |
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3 underlying | |
adj.在下面的,含蓄的,潜在的 | |
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4 mantle | |
n.斗篷,覆罩之物,罩子;v.罩住,覆盖,脸红 | |
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5 ridge | |
n.山脊;鼻梁;分水岭 | |
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6 lithospheric | |
adj.岩石圈的 | |
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7 diverge | |
v.分叉,分歧,离题,使...岔开,使转向 | |
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8 essentially | |
adv.本质上,实质上,基本上 | |
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9 ridges | |
n.脊( ridge的名词复数 );山脊;脊状突起;大气层的)高压脊 | |
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10 geologic | |
adj.地质的 | |
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11 specify | |
vt.指定,详细说明 | |
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12 substantiated | |
v.用事实支持(某主张、说法等),证明,证实( substantiate的过去式和过去分词 ) | |
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13 afterward | |
adv.后来;以后 | |
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14 ferromagnetic | |
adj.铁磁的, 铁磁体 | |
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15 solidifies | |
(使)成为固体,(使)变硬,(使)变得坚固( solidify的第三人称单数 ); 使团结一致; 充实,巩固; 具体化 | |
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16 volcanic | |
adj.火山的;象火山的;由火山引起的 | |
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17 axis | |
n.轴,轴线,中心线;坐标轴,基准线 | |
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18 rift | |
n.裂口,隙缝,切口;v.裂开,割开,渗入 | |
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19 recording | |
n.录音,记录 | |
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