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 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 seduction 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 1960s, 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 Earths 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.
快速英语写作的三步法
说明薪金的待遇
英语写作的要诀
英语写作绝招与万能公式
应试写作学与练批驳篇
英语常用同义词的辨析WXYZ
英语常用同义词的辨析B
英语常用的同义词辨析L
图表作文写作的攻略
段落展开法II
英语常用的同义词辨析I
英语考试作文开头和结尾技巧
医学写作形状与特征表示法
应试写作学与练原因篇
应试写作学与练比较篇
应试写作学与练评论篇
应试写作学与练理由篇
英语常用同义词的辨析D
段落展开法I
九类英语写作实用的句型
应试写作学与练分析篇
英语常用同义词的辨析S
英语常用同义词的辨析C
说明教育的程度
医学写作结构表示法
英语写作好的句子1
Speak, Talk, Say与Tell
医学写作位置表示法
英语写作好的句子2
句子类型
不限 |
英语教案 |
英语课件 |
英语试题 |
不限 |
不限 |
上册 |
下册 |
不限 |